Biological Flora of the British Isles: Urtica dioica L.

BIOLOGICAL FLORA OF THE BRITISH ISLES* No. 256

List Vasc. PI. Br. Isles (1992) no. 36,1,1

Biological Flora of the British Isles: Urtica dioica L.

Kenneth Taylor†

Centre for Ecology & Hydrology Lancaster, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster LA1

4AP, UK

Summary

1. This account presents information on all aspects of the biology ofUrtica dioica that are relevant

to understanding its ecological characteristics and behaviour. The main topics are presented within

the standard framework of the Biological Flora of the British Isles: distribution, habitat, communi-

ties, responses to biotic factors, responses to environment, structure and physiology, phenology,

floral and seed characters, herbivores and disease, history and conservation.

2. Urtica dioica is a tall, usually dioecious, rhizomatous, perennial herb with numerous stinging

hairs, probably native in fens and semi-natural ancient woodlands, but widely naturalized in a range

of habitats and abundant throughout the British Isles. British material is mainly ssp. dioica which

also extends throughout Europe, and locally ssp. galeopsifolia (without stinging hairs) which is also

found in western, central and eastern Europe.

3. Urtica dioica is a moderately shade-tolerant species, which occurs on most moist or damp,

weakly acid or weakly basic, richly fertile soils.

4. A highly competitive ruderal species, Urtica dioica often forms monospecific stands which are

not infrequently the product of a single individual that has spread bymeans of horizontal rhizomes.

5. Urtica dioica has frequently been described as a nitrophile, but there are many soils in which the

supply of inorganic nitrogen is adequate for growth. However, there are other soils in which growth

is checked with symptoms of severe phosphorus deficiency unless soluble phosphate is added to the

soil. This is especially so in the long-established deciduous woodlands, and where there has been no

addition of fertilizers to the soil. The growth responses of U. dioica to the availability, source and

utilization of nitrogen and phosphorus have been examined experimentally in some detail.

6. Urtica dioicawith its stems and leaves densely covered with stinging hairs, which release potential

pain-inducing toxins when brushing contact is made with them, is rarely eaten by cattle and rabbits,

but is palatable to some species of snail. It is the food plant of the larvae of a number of attractive

butterflies and other phytophagous insects.

7. Male clones of U. dioica flower in advance of female clones. The pollen grains are extremely

small. It is usually wind-pollinated, but occasionally insect-pollinated.

8. The low seedmass ofU. dioica enables the production of vast numbers of seeds. Few of the seeds

germinate in the period immediately following dispersal, and the species maintains a seed bank that

changes little in size with season and is large in relation to annual seed production.

Key-words: communities, conservation, ecophysiology, geographical and altitudinal distribu-

tion, germination, herbivory, nutrients, reproductive biology, soils

Urticaceae, Urtica dioica L. (stinging nettle, common nettle)

is a perennial herb with an extensive sympodial system of rhi-

zomes and stolons, rooting at the nodes and giving rise in

spring to aerial shoots up to 1.5–2 m or (rarely) 3 m, or

more. Stems and leaves usually with abundant conspicuous

stinging hairs and relatively coarse, sparse simple non-sting-

*Nomenclature of vascular plants follows Stace (1997) and for

non-British species, Flora Europaea. This account supersedes that of

Urtica dioica byGreig-Smith (1948).

†Correspondence author. Email: [email protected]

Journal of Ecology 2009, 97, 1436–1458 doi: 10.1111/j.1365-2745.2009.01575.x

� 2009 The Author. Journal compilation � 2009 British Ecological Society

ing hairs. Leaves opposite, lower ovate and the upper

ones±lanceolate, both with base±cordate, acuminate,

coarsely serrate; terminal leaf-tooth longer than adjacent lat-

erals; petiole not more than half as long as the lamina; stip-

ules free, four at each node 1–2 (-3) mm wide, lanceolate,

entire. Inflorescences axillary, spike-like, four per node,

many-flowered, so that there are several thousand flowers in

each flowering panicle. Flowers small, greenish and unisex-

ual. Usually the male and female flowers are found on sepa-

rate plants; the males more upright or patent and the females

tending to be pendent; the male with four perianth segments

and four stamens, the female with two smaller and two larger

perianth segments and a one-celled ovary with a sessile tufted

stigma. Fruits single seeded, achenes small, c. 1.3 · 1.0 mm,

with a mean seed mass of 0.2 mg (Thompson & Grime 1979;

Wheeler 1981).

Urtica dioica is a very variable species in size, branching, leaf

and inflorescence form and degree of hairiness. Cultivation

experiments carried out by Pollard & Briggs (1982) with indi-

viduals from nine populations covering a range of habitats

provide evidence that phenotypic plasticity is important in

U. dioica; for example, fewer hairs were produced on plants

grown in the shade than on those in full sun. These experiments

also support the conclusion that much of the variation (includ-

ing the polymorphism in stinging-hair density) is genetically

based and heritable (Pollard & Briggs 1984a). The typical,

widely distributed British form has been classified as ssp. dioica

(Stace 2004). A much more local form ssp. galeopsifolia (Wie-

rzb. ex Opiz) Chrtek., which has finer, denser, non-stinging

hairs, is also recognized by Stace (2004). However, Geltman

(1992, 1993) considered it more appropriate to give both of

these taxa-specific rank, along with the other closely related

European segregates of the U. dioica group (Urtica sondenii

Simmons and U. pubescens Ledeb.). He suggested that the

tetraploid U. dioica could have been formed as a result of

hybridization of diploidU. galeopsifolia andU. sondeniiwhich

occupy similar niches.

1. Urtica dioica ssp. dioica. Lamina with conspicuous sting-

ing hairs at least on the upperside. Non-stinging hairs rel-

atively coarse and sparse. The lowest flowering node of

the inflorescence 7–14th from the base. Throughout Eur-

ope, but only as an introduced weed in some districts. It is

the most abundant subspecies in the British Isles,

although sometimes replaced by subsp. galeopsifolia in

wet woodland.

2. Urtica dioica ssp. galeopsifolia. Lamina without stinging

hairs, densely pubescent at least beneath. Usually with

leaves longer and much narrower than in ssp. dioica (see

Rich & Jermy 1998). The lowest flowering node of the

inflorescence 13th–22nd from the base. It starts to flower

later than ssp. dioica, about mid-July. It occurs in the

European part of Russia to the south of latitude 60�N, in

the central Ukraine, Bulgaria, Czechslovakia, Hungary,

Rumania and in the Netherlands. Known from scattered

sites in England and Wales and from Scotland and

Ireland, but still under-recorded.

Prehistoric macrofossil records (see X) indicate that Urtica

dioica is native in the British Isles. It is also referred to by Ellen-

berg (1988) as a native ruderal plant of the Central European

flora before it felt any influence of man. In Britain, U. dioica

has spread in historical times to become abundant or dominant

in a wide range of terrestrial habitats, including fens, primary

and secondary woodland, scrub, hedgerows, unmanaged

grasslands, banks of rivers and streams, floodplains, unman-

aged roadside verges, and in various maritime habitats. It also

occurs in ruderal sites, sites of former habitation, recently

disturbed sites such as heaps of earth, sand or rubble, areas

enriched with cattle and sheep dung, and in weed communities

of cultivated ground.

I. Geographical and altitudinal distribution

Urtica dioica ssp. dioica is ubiquitous throughout the British

Isles (Fig. 1), whereas ssp. galeopsifolia, although long known

as a distinctive variant from Wicken and Chippenham Fens,

Cambridgeshire (Perring, Sell & Walters 1964), has been

neglected until recently. It is now known from scattered sites in

England andWales, and isolated localities in Scotland and Ire-

land. Stinging nettle, U. dioica is widespread and probably

native throughout Europe and Asia from the arctic regions to

the Mediterranean. Of the other members of the U. dioica

group,U. sondenii occurs in northern Finland,Norway,Russia

and Sweden; the non-stinging U. galeopsifolia is found in wes-

tern, central and eastern Europe, andU. pubescens, confined to

the Volga delta in Russia and lower Dnepr in theUkraine. The

distribution of the members of theU. dioica group in Europe is

shown in Fig. 2, and the distribution in the northern hemi-

sphere is mapped byHulten &Fries (1986) and Srutek&Teck-

lemann (1998). In Canada, U. sondenii (as U. dioica ssp.

gracilis (Aiton) Selander) is a common and widespread native

taxon (Bassett, Crompton & Woodland 1977). Worldwide U.

dioica (ssp. dioica) is alien in other temperate regions in North

and South Africa, China, India, Australia, New Zealand and

North and SouthAmerica, but not found in the tropics (Greig-

Smith 1948).

The altitudinal range ofU. dioica in the British Isles extends

from near sea level, as in the fixed dunes at Holkham N.N.R.

(Pollard&Briggs 1982) and Blakeney Point, Norfolk (Pearson

et al. 2007), to an upper altitudinal limit of c. 850 m on Great

Dun Fell, Cumbria (Halliday 1997; Pearman & Corner 2004).

In continental Europe, U. dioica is mainly a lowland species,

but ascends into the mountains, reaching 2500 m in Spain

(Castroviejo et al. 1993).

II. Habitat

(A ) CL IMATIC AND TOPOGRAPHICAL L IMITATIONS

Urtica dioica is recorded from 2773 (of a maximum possible

2805) of the hectads (10 · 10 km) squares in Britain, 983 (985)

in Ireland and 13 (14) in the Channel Islands. In these squares,

mean annual rainfall is 1102 mm year)1, and the mean

Urtica dioica L. 1437

� 2009 The Author. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 1436–1458

Fig. 2. The distribution of the Urtica dioica

group in Europe. The map includes

U. dioica, U. galeopsifolia, U. pubescens and

U. sondenii. Reproduced from Jalas &

Suominen (eds), Atlas Florae Europaeae 3,

1976, by permission of the Committee for

the mapping of the Flora of Europe and

Societas Biologica Fennica Vanamo.

Fig. 1. The distribution of Urtica dioica in

the British Isles. Each dot represents at least

one record in a 10-km2 of the National Grid.

Native (•) 1970 onwards; (s) pre 1970.

Mapped by Stephanie Ames, Biological

Records Centre, Centre for Ecology and

Hydrology, Wallingford, using Dr A. Mor-

ton’s DMAP software, mainly from records

collected by members of the Botanical Soci-

ety of the British Isles.

1438 K. Taylor


January and July temperatures are 3.6 �C and 14.5 �C respec-

tively (Hill, Preston & Roy 2004). Even at the tetrad

(2 · 2 km) scale, it is virtually ubiquitous in most counties in

England and Wales, such as Dorset (Bowen 2000), Leicester-

shire (Primavesi & Evans 1988) and Monmouthshire (Evans

2007), and it is absent from only the most exposed moorland

tetrads in Cornwall (French, Murphy & Atkinson 1999) and

the most upland squares in Cumbria (Halliday 1997). It has a

more restricted distribution in the tetrads of northern and wes-

tern Scotland, as in Assynt (Evans, Evans & Rothero 2002)

and on the Isle of Rum (Pearman et al. 2008), where it is

invariably closely associated with human or their animals. It is

less frequent at very fine scales, being for example present in

98% of 1 · 1 km2 and the sixth commonest species at this

scale in Berkshire, but present in only 10% and the 22nd most

frequent species in 1 · 1 m2 (Crawley 2005).

U. dioica has been assigned to the Eurosiberian Boreo-tem-

perate element in the British flora; it is widely naturalized out-

side its native range (Preston&Hill 1997).

Although wide-ranging, it is most abundant on level or

moderately sloping ground (Grime, Hodgson & Hunt 2007).

In Scotland, U. dioica has been reported as abundant in the

crevices between boulders on talus slopes at St. Cyrus, Kincar-

dineshire, and on steep grassy slopes dominated byHolcus lan-

atus at Bettyhill, Sutherland (Gimingham 1964).

(B ) SUBSTRATUM

Urtica dioica subsp. galeopsifolia in particular grows in winter-

flooded habitats, whereas subsp. dioica occurs on almost all

soil types, although it prefers moist or damp soils and is absent

from permanently waterlogged soils (Ellenberg value for mois-

ture = 6; Hill, Preston & Roy 2004) and weakly acid or

weakly basic conditions (Ellenberg value for pH 7; Hill, Pres-

ton&Roy 2004).

Urtica dioica occurs in a former water meadow, now a pop-

lar plantation, at Bucklebury, Berkshire, in a humose very cal-

careous sandy silt loam to clay loam (pH 7.3 in the 0–28 cm

surface horizon) with a constant water level during the year,

described by Avery (1990) as a humic calcaric alluvial gley soil.

It is also found in pasture on cut-over ombrotrophic bog, with

applied calcareous mineral material (marl), on Castletown

Moor, County Meath, Ireland, of pH 7.2 in the 0–33 cm sur-

face horizon, described by Avery (1990) as an earthy semi-

fibrous bog soil. In East Anglia,U. dioica is abundant in many

different habitats, including ancient deciduous woodlands on

mildly acid to calcareous pelosols on chalky boulder clay

(Avery 1990).

It is very abundant and rooted where mildly acid, peaty

humus (pH 5.2) has accumulated between granite blocks on

the talus slopes (30–60�) on the island of Ailsa Craig, west of

Girvan, S. Ayrshire, and in the accumulation of guano (pH

7.0) below ledges on sheer cliffs occupied by nesting sea birds

(Gimingham 1964). Al-Mufti et al. (1977) recorded soil pH

values (0–3 cm) ranging between 6.3 and 7.1 in four different

vegetation types in which U. dioica thrives. Gebauer, Rehder

& Wollenweber (1988) recorded pH values in Central Europe

in the soil rooting area of an occasionally cut grassland and on

the slopes of river banks of 5.63±0.42 (SE) and 7.10±1.47

respectively. Falkengren-Grerup (1995) measured a range of

pH values (in 0.2 M KCl) from 3.1 to 4.5, in topsoil (without

the litter layer) from well-drained beech and oak forests where

U. dioica occurs in southern Sweden. In general, Urtica dioica

occurs in topsoils over the pH range 4.5–7.5 (Olsen 1921;

Rackham 2003; Grime,Hodgson&Hunt 2007).

Olsen (1921) investigated the distribution of U. dioica in

Danish woodlands in relation to soil conditions. He confirmed

earlier findings that the plant contained nitrates (NO3)) in its

tissues (using the diphenylamine–sulphuric acid test), in partic-

ular in the aerial stems, rhizomes and roots, but in smaller

quantities in the leaves. On the other hand, in adjacent stands

ofDeschampsia cespitosa, little or no NO3) was detected in the

leaves of the grass. Using an incubation technique, Olsen dem-

onstrated that this difference was related not only to the inten-

sity of nitrification in the soil, but also to a high concentration

of most plant nutrients, including calcium, magnesium, phos-

phate and potassium. His conclusion that the natural distribu-

tion of U. dioica is primarily controlled by a relatively high

supply of nitrogen, especially in the form of nitrate, supported

by the results of a sand culture experiment, has found wide

acceptance (see also VI E). Hence, U. dioica has frequently

been described as a nitrophile or nitrophyte found in highly

rich fertile conditions (Ellenberg value for fertile soils = 8;

Hill, Preston&Roy 2004).

In non-fertilized soils, nitrate is found in plants only if there

is nitrification in the soil. Thus the NO3) content of plants

should be an indicator of the supply of NO3) on different sites.

Falkengren-Grerup (1995) measured the net N mineralization

of topsoils, from beech and oak forests in southern Sweden,

following an 8-week incubation period under constant water

and temperature conditions. The frequency of U. dioica was

highest in soils with the highest NO3) percentage of mineral-

ized N. Utilization of the NO3) by a wide range of species has

been measured using an assay for nitrate reductase activity

(NRA) in the shoots (see VI E).

There are many soils in which the supply of inorganic

nitrogen is adequate for the growth of seedlings of U. dioica.

However, there are other soils in which the seedlings grow

extremely slowly or growth is checked at the first leaf stage,

with seedlings showing the characteristic symptoms of severe

phosphorus deficiency, unless soluble phosphate is added to

the soil. This is especially so in long-established deciduous

woodlands and where there has been no addition of agricul-

tural fertilizers to the soil (Taylor 1963; Pigott & Taylor

1964; Pigott 1964; see also VI E). Holter (1979) analysed soil

samples collected in Denmark from wood margins, waste

places and roadsides. In each locality, a sample was taken

where U. dioica was growing and one 1–6 m away with

seemingly identical conditions except for the absence of the

plant. The values for mineralized N, pH and organic matter

were similar for each pair of samples. The level of inorganic

phosphate was low in the samples from wood margins and

waste places and was limiting the species distribution,

whereas in roadsides it was sufficient and other factors were



limiting. In Hayley Wood, Cambridgeshire, U. dioica occurs

on the site of the former gamekeeper’s garden, where the

concentration of phosphorus (1.2–1.3 mg g)1) is significantly

higher than that in the rest of the wood (0.35–0.9 mg g)1),

probably because of the addition of manure and rubbish

(Martin & Pigott 1990).

III. Communities

Greig-Smith (1948) gave examples of a range of communities

in which U. dioica was abundant or dominant in the British

Isles. The Classification of British plant communities provides

a wider-ranging account of the habitats in which U. dioica

occurs. In the following sections, the equivalent European

phytosociological alliances are given in parenthesis and are

taken fromRodwell et al. (2000).

(A ) SWAMPS AND TALL-HERB FENS

Urtica dioica is a joint constant in the Phragmites australis-

Urtica dioica tall-herb fen (S26; Phragmition australis) that

occurs throughout the lowlands but is particularly well repre-

sented in Broadland, the Fens and around the Shropshire and

Cheshiremeres (Rodwell 1995).

It is frequent in the Phalaridetum arundinacea tall-herb fen

(S28; Phragmition australis) in the Epilobium hirsutum-Urtica

dioica sub-community, widespread throughout the British low-

lands and on uplandmargins.

U. dioica is scarce in a number of swamp communities,

namely the Carex paniculata swamp (Magnocaricion elatae;

S3), Phragmites australis swamp and reed-beds (S4; Phragm-

ition australis),Glyceria maxima swamp (S5; Phragmition aus-

tralis), Carex riparia swamp (S6; Magnocaricion elatae),

Carex acutiformis swamp (S7; Magnocaricion elatae), Typha

latifolia swamp (S12; Phragmition australis), Sparganium erec-

tum swamp (S14; Phragmition australis), Acorus calamus

swamp (S15; Phragmition australis), Phragmites australis-Peu-

cedanum palustre tall-herb fen (S24; Phragmition australis) and

the Phragmites australis-Eupatorium cannabinum tall-herb fen

(S25; Phragmition australis).

(B ) WOODLANDS, SCRUB AND HEDGES

Urtica dioica is a common species in two types of wet wood-

land (Rodwell 1991a). It is the sole constant in the field layer

throughout, sometimes in a virtually continuous cover, in the

Alnus glutinosa-Urtica dioica woodland (W6; Salicion albae).

The community is widespread but local throughout the low-

lands, occurring where active alluvial deposition is taking place

on more mature rivers and on the remnants of undrained

floodplains and eutrophicated mires. In Alnus glutinosa-Fraxi-

nus excelsior-Lysimachia nemorum woodland (W7; Alnion in-

carnae), Urtica dioica sub-community is a constant together

with Chrysosplenium oppositifolium and Ranunculus repens.

This sub-community is often found where flush waters drain

down through colluvium and on to stream-side flats. It is

widely, though locally, distributed throughout the upland

fringes of the north and west, with outlying occurrences in the

wetter parts of southern England, notably theWeald.

Urtica dioica occurs less abundantly in other woodland com-

munities. It occurs in Fraxinus excelsior-Acer campestre-Mer-

curialis perennis woodland (W8; Carpinion betuli), in the

Geranium robertianum, Allium ursinum and Teucrium scorodo-

nia sub-communities, with Acer pseudoplatanus, Ulmus glabra

and Quercus petraea abundant, that have a more north-wes-

terly distribution in the British Isles on light well-drained soils.

The first two sub-communities are especially characteristic of

the Yorkshire and Derbyshire Dales and the Welsh Marches,

whereas the last is much rarer and has been recorded only in

parts of theWye valley andDerbyshire.U. dioica occurs under

more mature canopies in Salix cinerea-Betula pubescens-

Phragmites australis woodland (W2; Salicion cinereae), Alnus

glutinosa-Filipendula ulmaria sub-community, with a field layer

dominated by Phragmites australis occasionally accompanied

or sometimes replaced by Carex acutiformis and a variety of

tall herbs, including Filipendula ulmaria, Eupatorium cannabi-

num and Urtica dioica, that is now largely confined to East

Anglia and some of the Shropshire and Cheshire meres. It is

also in Alnus glutinosa-Carex paniculata woodland (W5;

Alnion glutinosae), which is now fairly local, though wide-

spread, throughout the English lowlands. At least some of the

populations in W2 and W5 represent ssp. galeopsifolia, which

is particularly associated with such vegetation.

Urtica dioica is of common occurrence on drier ground in

Crataegus monogyna-Hedera helix scrub (W21; Berberidion

vulgaris), which includes many hedges, and especially in the

Hedera helix-Urtica dioica sub-community, in which Cratae-

gus monogyna, Rubus fruticosus agg., Hedera helix, Urtica

dioica and Galium aparine are constants, that is widely dis-

tributed through the British lowlands. It is also represented

in the floristically richer Mercurialis perennis sub-community,

found on heavy-textured base-rich soils, especially in areas

with clays and shales. U. dioica occurs in Rubus fruticosus-

Holcus lanatus underscrub (W24; Rubion subatlanticum)

sub-community Arrhenatherum elatius-Heracleum sphondyli-

um that is consistently enriched by a very distinctive group

of preferentials (A. elatius, Festuca rubra, U. dioica,

H. sphondylium, Taraxacum officinale agg. and Galium apar-

ine which are constants, and frequently Anthriscus sylvestris

and Chaerophyllum temulum), and is especially characteristic

of this kind of vegetation which is ubiquitous on suitable

soils throughout the British lowlands. It is also less abundant

in Pteridium aquilinum-Rubus fruticosus underscrub (W25;

Rubion subatlanticum) that is widespread on suitable soils

throughout lowland Britain.

Urtica dioica occurs with lower frequencies in a number of

other woodland and scrub communities, namely Salix pentan-

dra-Carex rostratawoodland (W3; Salicion cinereae), Fraxinus

excelsior-Sorbus aucuparia-Mercurialis perennis woodland

(W9; Alnion incarnae), Quercus robur-Pteridium aquilinum-

Rubus fruticosus woodland (W10; Carpinion betuli), Fagus

sylvatica-Mercurialis perenniswoodland (W12; Fagion sylvati-

cae),Taxus baccatawoodland (W13; Fagion sylvaticae),Fagus

sylvatica-Mercurialis perennis woodland (W14; Fagion

1440 K. Taylor


sylvaticae), Juniperus communis ssp. communis-Oxalis acetosel-

lawoodland (W19;Dicrano-Pinion) andPrunus spinosa-Rubus

fruticosus scrub (W22; Prunion fruticosae).

U. dioica is not found in deciduous woodland on very acid

soils (see II B).

It is absent from beechwoods on acid loams, or podsolized

sands and gravels, in southern England. Neither does it

occur in seral birchwoods on extensive areas of lowland and

upland heath, nor does it occur in dry oakwoods on poorer

well-drained soils that are widespread throughout the British

Isles.

(C ) MIRE VEGETATION

Urtica dioica, Galium aparine and Cirsium arvense are

constants in Iris pseudacorus-Filipendula ulmaria mire (M28;

Filipendulion ulmariae), which is largely confined to the west

coast of Britain and is especially well developed in Scotland,

from Arran to north Sutherland and into Orkney and

Shetland, where it is virtually ubiquitous in suitable locations

(Rodwell 1991b).

U. dioica is also frequent in Filipendula ulmaria-Angelica

sylvestris mire (M27; Filipendulion ulmariae), Urtica dioica-

Vicia cracca sub-community which occurs in central, southern

and eastern lowland Britain, especially where the mire runs on

to drier ground.

(D ) MESOTROPHIC AND CALCICOLOUS GRASSLANDS

Urtica dioica is constant in Arrhenatherum elatius grassland

(MG1; Arrhenatherion elatioris), U. dioica sub-community

(Rodwell 1992), which is especially prominent in areas of inten-

sive arable agriculture throughout the lowlands of Britain. It is

also a constant in Arrhenatherum elatius-Filipendula ulmaria

tall-herb grassland (MG2; Arrhenatherion elatioris) together

with Angelica sylvestris, Dactylis glomerata, Epilobium monta-

num, Festuca rubra, Geum rivale, Heracleum sphondylium,Mer-

curialis perennis, Poa trivialis, Silene dioica and Valeriana

officinalis, a community which is confined to steep slopes, usu-

ally of northern aspect, on rendziniform soils overlying the

Carboniferous limestone of Craven and Derbyshire in north-

ern England.

U. dioica also occurs with lower frequencies in mesotrophic

Holcus lanatus-Deschampsia cespitosa grassland (MG9;Calthi-

on palustris), Holcus lanatus-Juncus effusus rush-pasture

(MG10; Calthion palustris), and Festuca rubra-Agrostis stolo-

nifera-Potentilla anserina grassland (MG11; Potentillion

anserinae). It is scarce in calcicolous Festuca ovina-Avenula

pratensis (Helictotrichon pratensis) grassland (CG2; Bromion

erecti), Brachypodium pinnatum grassland (CG4; Bromion

erecti), and in Avenula pratensis (Helictotrichon pratensis)

grassland (CG6; Bromion erecti).

(E ) MARIT IME VEGETATION

InHippophae rhamnoides dune-scrub (SD18; Salicion repentis

arenariae), Urtica dioica is a constant in the Urtica dioica-

Arrhenatherum elatius sub-community, which is well estab-

lished on the east coast between Kent and Fife, and especially

in northNorfolk and Lincolnshire (Rodwell 2000).

U. dioica is scarce in the Ammophila arenaria mobile dune

community (SD6; Ammophilion arenariae), Carex arenaria-

Festuca ovina-Agrostis capillaris dune grassland (SD12;

Corynephorian canescentis) and Elymus repens (Elytrigia

repens ssp. repens) salt-marsh community (SM28; Potentillion

anserinae).

(F ) VEGETATION OF OPEN HABITATS

Urtica dioica is a constant in several communities of open habi-

tats (Rodwell 2000). Together withGalium aparine, it is prefer-

ential in Stellaria media-Capsella bursa-pastoris community

(OV13; Fumario-Euphorbion), Urtica dioica-Galium aparine

sub-community, which occurs widely on fertile loamy soils,

among root vegetable, salad and cereal crops, throughout the

British lowlands. It is dominant in the Urtica dioica-Galium

aparine community (OV24; Galio-Alliarion), in nutrient-rich,

moist but well-aerated, disturbed soils throughout the low-

lands and a joint constant in Urtica dioica-Cirsium arvense

community (OV25; Galio-Alliarion), which is ubiquitous

through the British lowlands, and characteristic of disturbed

nutrient-rich loamy soils within badly managed pastures and

leys, on abandoned arable land, waysides, verges, waste

ground and woodland clearings. U. dioica is less constant in

the Epilobium hirsutum community (OV26; Convolvulion sepi-

um) and in Epilobium angustifolium community (OV27; Carici

piluliferae-Epilobion angustifolii).

Urtica dioica is less frequent in a number of other arable-

weed assemblages and communities of waste places. It occurs

in the Papaver rhoeas-Viola arvensis community (OV3; Arno-

seridion minimae), the Veronica persica-Alopecurus myosuro-

ides community (OV8), Matricaria perforata-Stellaria media

community (OV9), Poa annua-Senecio vulgaris community

(OV10) and Poa annua-Myosotius arvensis community (OV12)

(all referred to Polygono-Chenopodion polyspermum), in ara-

ble and gardens on fertile loams and clays, and in theAnagallis

arvenis-Veronica persica community (OV15) and Reseda luteo-

la-Polygonum aviculare community (OV17) (both referred to

Caucalidion platycarpi) on lighter calcareous soils. In trampled

habitats, it is found in the Poa annua-Matricaria perforata

community (OV19; Polygonion avicularis), Poa annua-Plan-

tago major community (OV21) and Lolium perenne-Dactylis

glomerata community (OV23) (both Lolio-Plantaginion). It

occurs in the seasonally flooded Agrostis stolonifera-Ranuncu-

lus repens community (OV28; Potentillion anserinae) and in

the Polygonum lapathifolium-Poa annua community (OV33;

Bidention tripartitae) of enriched depressions in damp

disturbed places.

The phytosociology of U. dioica in continental Europe has

been extensively described and reviewed by numerous

authors, including Dierssen (1996), Ellenberg (1988), Ober-

dorfer (1983), Srutek & Tecklemann (1998), Stortelder, Scha-

minee & Hermy (1999), Szafer (1966) and Westhoff & den

Held (1969).



IV. Response to biotic factors

(A ) COMPETIT ION

Urtica dioica is a ruderal described by Grime, Hodgson &

Hunt (2007) as having an established competitor strategy. In

summer, its dense, rapidly ascending closed canopy can pre-

clude the growth of other herbaceous species, which together

with the impact of relatively persistent stem litter and lateral

spread by means of rhizomes, often leads to U. dioica forming

monospecific stands (Al-Mufti et al. 1977; Srutek 1993). There

is a marked amplitude of seasonal variation in the abundance

of bryophtes attached to the litter, especially Brachythecium

rutabulum, expansion of which coincides with the moist, cool

conditions of spring and autumn.

Hara & Srutek (1995) found that as stands of U. dioica

developed there was a large decrease in shoot density resulting

in a survival rate at the end of the growing season of about

30%. Density-dependent shoot self-thinning with the death of

smaller shoots occurred from the beginning of the growing sea-

son. They hypothesized that the resources of smaller shoots

subjected to self-thinning are absorbed by larger living shoots,

resulting in the support of growth of the latter, and hence

reducing competition (see also VIA).

In fertile soils under a woodland understorey, shading

causes a premature decline in shoot dry mass and limited flow-

ering, coinciding with the full expansion of the tree canopy,

and reduces the competitive ability ofU. dioica (Al-Mufti et al.

1977). In a European beech forest, studies reviewed by Srutek

& Tecklemann (1998) compared the cover of U. dioica and

Mercurialis perennis and showed that the latter species was

dominant in areas where the relative irradiance was 3–5%,

whereas above 5%M. perennis was suppressed by the increas-

ing competition fromU. dioica.

The forest understorey herbs Anemone nemorosa, Lamia-

strum galeobdolon andVeronica montana are regarded as being

indicator species of ancient woodland sites where the soil fertil-

ity is often low. Hipps et al. (2005) carried out pot-culture

experiments in a shaded glasshouse, with vegetative propagules

of these three species plus U. dioica, one plant per pot of peat-

based compost (with water extractable P content of

0.2 lg ml)1), together with or without a competing U. dioica

plant. All the species responded to the addition to the soil of P

(0–10 mg L)1) in standard Long Ashton nutrient solution by

raising the concentrations of P in their shoot and root tissues

and increasing their biomass, resulting in an increased P

uptake. The growth of L. galeobdolon and V. montana was

restricted by competition with U. dioica, but growth of

A. nemorosawas unaffected by competition, probably because

of their different phenologies. In a further experiment with an

ex-arable soil (pH 7.4), two other treatments had the pH

adjusted to 5.8 and 4.3 respectively. Acidifying the soil

enhanced growth, but reduced the concentrations of N, P and

K in the leaves of all the species. The effects of competition

withU. dioicawere again evident across the pH range.

One of the few species capable of persisting in almost mono-

specific stands ofU. dioica, the scrambling winter annual Gali-

um aparine, is its most successful competitor. Because

G. aparine has less investment into supporting structures, it can

maintain growth and cause physical destruction of the canopy

of U. dioica by its weight resting on the host stems (Schulze &

Chapin 1987). When the shoots of U. dioica die back com-

pletely in winter, the seeds of G. aparine are able to germinate

at the lower temperatures and seedlings benefit from the

increased light flux (Taylor 1999).

(B ) IMPACT OF GRAZING

Kirby (2001) quotes the findings of studies which indicate that

the following deer eat U. dioica in quantity: fallow (Dama

dama L.), roe (Capreolus capreolus L.) and red (Cervus elephas

L). However, he reports that the frequency of the plant in

Quercus robur-Pteridium aquilinum-Rubus fruticosuswoodland

(W10) is unaffected by deer and sheep grazing. In the British

Isles, the plant is avoided by rabbits and cattle andmay be seen

standing up above closely cropped vegetation (Greig-Smith

1948). It has long been assumed that stinging hairs serve a pro-

tective function against mammalian grazing (Salisbury 1961)

because of their pain-producing properties against humans

and animals. There is no evidence, however, that U. dioica is

harmful to herbivores. The situation is not one of straightfor-

ward, complete avoidance. For example, some breeds of

domestic cattle avoid stinging nettles while other breeds eat

them readily (Uphof 1962). Selective grazing was studied in

Aland, south-western Finland, by Haeggstrom (1990). He

found that if grazing was intense, sheep droppings contained

numerous germinable nettle seeds, but cattle which graze selec-

tively did not eat the plant. However, Cosyns et al. (2005)

found that fresh dung samples from Shetland and Konik

breeds of horses and Scottish Highland cattle grazing two

coastal dune nature reserves contained seed and abundant

emerging seedlings of U. dioica. The dung of free-ranging

horses also contained large numbers of emerging seedlings of

U. dioica (Cosyns & Hoffmann 2005). Rabbits will eat small

quantities of nettles when food is scarce (Gillham 1955; Tho-

mas 1960), and remains of nettle epidermis, including stinging

hairs have been found in samples of rabbit dung.Urtica dioica

apparently possesses high nutrient quality (a high content of

protein and vitamins A and C), and is potentially a good

source of food for livestock, and hence mammalian grazing is

an obvious major threat and probably the main selection pres-

sure behind the stinging hairs.

Pollard & Briggs (1984b) compared the grazing preferences

of rabbits and sheep in captivity on nettle plants, obtained by

vegetative propagation from 11 individuals from four locali-

ties, with known stinging hair densities. Both herbivores pref-

erentially grazed plants with lower stinging hair densities. In a

field experiment in an area with intense rabbit grazing, greater

damage was observed on plants with lower stinging-hair densi-

ties. The patterns of grazing damage, and observations of the

herbivore behaviour, suggested that stinging hairs act to deter

consumption of significant amounts of plant matter, even by

animals which had not learned to avoid the plant. Pullin &

Gilbert (1989) compared stinging hair densities on plants of

1442 K. Taylor


U. dioica from livestock grazed and ungrazed areas, and before

and after cutting in the field. Plants from grazed fields had

significantly higher stinging hair densities than plants from

ungrazed fields, and the latter showed more variation in

density. Regrowth plants had higher stinging hair densities

than initial growth and shoots from the centre of patches had

lower densities than edge shootsmore exposed to grazing.

Urtica dioica is among the major sources of green plant

material consumed in the field by the snailsArianta arbustorum

L. and Cepaea nemoralis L. (Grime & Blythe 1969). Labora-

tory feeding experiments (Grime, MacPherson-Stewart &

Dearman 1968) have confirmed the palatability of stinging net-

tle leaves relative to those of other species in tests with Cepea

nemoralis.Mason (1974) reported that the snailsDiscus rotund-

atus (Mull.) and Hygromia striolata (Pfeiff.) feed on the fresh

leaves of U. dioica. Many species of insects are herbivores or

parasites of U. dioica (Davis 1991). In particular, the larvae of

several species of butterfly, especially the small tortoiseshell

Aglais urticae (L.) and the peacock Inachis io (L.), are com-

monly or exclusively found on stinging nettles in the British

Isles. Stinging hairs may not be a problem to caterpillars sim-

ply because of the large size of the hairs relative to the size of

the larva. The frequent association between insects and nettles

seems to imply an insignificant action of stinging hairs against

many insects. However, it is possible that more insects might

feed on U. dioica even more frequently, or consume plant bio-

mass at a faster rate, if stinging hairs were absent.

V. Response to environment

(A ) GREGARIOUSNESS

Urtica dioica is frequently found in very large patches, and

forms pure stands under favourable conditions. These patches

are not infrequently the product of a single individual that has

spread by means of horizontal rhizomes which can be up to

c. 50 cm ormore in loose soil (Greig-Smith 1948).

(B ) PERFORMANCE IN VARIOUS HABITATS

Urtica dioica tolerates partial shade, with >10% relative illu-

mination when the trees are in leaf (Ellenberg value for

light = 6; Hill, Preston & Roy 2004). It prefers moderately

shaded woodlands and hedgerows, but also occurs in open

habitats such as floodplains, pastures and meadows. Accord-

ing toOlsen (1921),U. dioica thrives in full daylight, but attains

its most luxuriant growth at 10–20% and minimum growth at

5–10% full daylight. In open woods, moderately high light

fluxes reach the floor during a large part of the spring and sum-

mer, and providing mineral nutrition is not limiting, rapid

growth rates are possible. Pigott & Taylor (1964) found that

seedlings of U. dioica grew best in woodland plots with added

phosphate where light fluxes were high, but in deep shade only

a few seedlings survived. Many open woods provide a suitable

environment for seedling establishment, similar to those found

in recently cleared, moist, productive sites with high illumina-

tion throughout the growing season (Grime 1966).

The seasonal pattern of the above-ground biomass of

U. dioica was determined in 1960 in the Carboniferous lime-

stone dales of Derbyshire, in Northcliffe Wood, Calver, and in

limestone grassland at the Winnats Pass, with a peak biomass

in August of 700 and 490 g m)2 respectively (Taylor 1963). In

1975, the aerial parts were measured at four sites by Al-Mufti

et al. (1977) and there was an exponential increase in biomass

of U. dioica peaking in August; two sites were on soils overly-

ingCarboniferous limestone in northDerbyshire in the vicinity

of Lathkilldale, one a bed of stinging nettle on the site of a dis-

used sheep-pen [peak biomass, 457±45 (SE) g m)2] and the

other in a stand ofChamerion angustifolium in a small roadside

hollow in a leached brown earth soil (67±39 g m)2); the third

and fourth sites were in mixed deciduous woodland and scrub

on a stream terrace on the Coal measures at Totley Wood,

Sheffield, one in a clearing (388±56 g m)2) and the other

immediately adjacent on the woodland floor (53±15 g m)2).

In the woodland-floor site, U. dioica was frequent but the

plants had low vegetative vigour and many shoots failed to

produce flowers.

Seedlings of Urtica dioica, growing in a standardised pro-

ductive growth-room environment, had a higher mean RGR

in total dry-mass per plant (2.0–2.4 week)1) after 5 weeks,

than species associated with infertile soils (Grime & Hunt

1975). Although U. dioica has a small seed mass (the initial

capital for growth), its ‘competitive’ strategy involves an excep-

tionally high relative growth rate (RGR). This coincides with

tall stature, extensive lateral spread and the tendency to accu-

mulate leaf litter, characteristics which facilitate the exclusive

occupation of productive sites. Although it is not possible to

extrapolate these laboratorymeasurements to the field in abso-

lute terms, the values still provide a clue to the productivity of

the species growing under natural conditions.

Wheeler (1981) studied the growth of woodland and pas-

tureland clones of U. dioica in the field in relation to the light

climate. Growth was analysed using the methods and notation

described in Evans (1972). Estimates of diffuse and direct irra-

diance, derived from hemispherical photographs (Anderson

1964), were made in deciduous woodland at the end of April

(pre-shade phase 60 cm from ground level) and in mid-June

(shade phase 120 cm from ground level), and at the same

position and time in pastureland. In the pre-shade phase

(November–April) in woodland, mean instantaneous diffuse

irradiance was 56.4% and direct irradiance was 37.3% (of inci-

dent), compared with the values of 91.5% and 82.5% respec-

tively in pastureland. In the shade phase (May–October) in

woodland, the mean diffuse irradiance was 26.6% and direct

irradiance 23.8%, compared with the values of 84.5% and

84.3% respectively in pastureland. By the end of April, the

woodland clones were significantly larger than the pastureland

clones in respect of length of internodes, drymass of shoot, dry

mass of leaves, length of petiole, leaf area and SLA

(mm2 mg)1). By the end of May these differences had

increased substantially, and the height of the woodland clones

became significantly greater than the pastureland clones. The

woodland clones in the shade made slightly more growth in

terms of both the dry mass of the shoots and of the



inflorescences than the clones in the denser stands in pasture-

land. However, the latter produced on average 82% more

seeds than the woodland clones. The plasticity ofUrtica dioica

allows it to maintain competitiveness in woodland areas of

moderate shading (20% irradiance), where it can grow as well

as, or even more successfully than it does in conditions of full

sunlight.

Wheeler (1981) also grew potted seedlings either in full glass-

house light or beneath shade screens. There was no significant

difference in total dry mass of plants growing at 25%, 35%,

67%and 100% irradiance. This result was due to the inevitable

reductions in unit leaf rate (ULR) being compensated for by a

large increase in the SLA, a smaller increase in leaf weight ratio

(LWR) and a reduction in root weight ratio. At very low light

(8% irradiance), extreme elongation of the hypocotyl took

place, leaf expansionwas limitedandgrowth impeded, followed

by heavy mortality due to fungal attack. There were no signifi-

cant differences between woodland and pastureland clones in

relation to the characteristics studied and therefore no evidence

of theexistenceofwoodlandandpasturelandecotypes.

In assessing the effect of shading on plant growth, Smith

(1981) andCorre (1983a) have emphasised the need to consider

the changes in the spectral distribution that occur in natural

shade and the effects of irradiance of different spectral quality.

Compared with sunlight, the spectral composition within and

beneath a leaf canopy is relatively poor in blue and red, rela-

tively rich in green and especially rich in far-red wavelengths.

Two wavelengths, 660 nm (red) and 730 nm (far red), are the

absorption maxima of the inter-convertible forms of the pho-

toreceptor pigment phytochrome and therefore shade light is

characterised by the red ⁄ far-red ratio (R ⁄Fr). At latitude

53�N, the R ⁄Fr varies between c. 1.15 in the open and c. 0.10

in dense shade where irradiance is<1%.

Corre (1983a) grew seedlings of a number of sun and shade

plants, including U. dioica, experimentally in aerated nutrient

solution in a controlled environment room. In different experi-

ments, three different levels of irradiance (400–700 nm) were

applied; amoderately reduced level of irradiance with a normal

R ⁄Fr, and a very low level of irradiance with either a normal

R ⁄Fr or a low R ⁄Fr (see also VI E). With decreased irradiance

and a constant R ⁄Fr, there was a decrease in the RGR of

U. dioica. In the low R ⁄Fr treatment, there was an increase in

stem elongation and a concommitant increase in stem drymass

and shoot weight ratio (SWR). A small decrease in LWR

which occurred reduced the dry matter production. These

morphogenetic adaptations ofU. dioica to the low R ⁄Fr led to

differences in the extent to which RGR decreased, but changes

in ULR were of minor importance. It is suggested that the

R ⁄Fr ratio is more critical than reduced irradiance in the

plants’ response to shading.

In further experiments, Corre (1983b) assessed the growth

and morphogenetic reaction of U. dioica to a combination of

low irradiance and low nutrient supply, a combination typical

of woodland habitats. There was an interaction between the

effects of irradiance and NO3) supply: low supply of NO3

)

caused amuch greater decrease in RGRunder high irradiance,

because of large changes in dry matter distribution; LWR

decreased much more under high irradiance than under low

irradiance, while the effect on ULR was small and not depen-

dent on irradiance. No interaction was found between the

effects of irradiance and PO43) supply on RGR, because the

interactions between these effects on morphogenesis (leaf area

ratio, LAR) and on productivity (ULR) cancelled each other

out. With a low supply of PO43), the LAR only decreased

under a high level of irradiance, butULRonly decreased under

a low level; RGRdecreased to the same extent under both high

and low levels of irradiance. It is unlikely that the shade toler-

ance ofU. dioica is partly or wholly based on a lower sensitivity

to low nutrient supply under a low level of irradiance.

In plots sown with different strains and species of herbage

plants in a grazing trial on previously cropped agricultural

land, Ivins (1952) recorded the appearance of U. dioica as a

weed in some of the plots. It only succeeded in becoming estab-

lished in those plots originally sown with leguminous species

that increase the content of organic nitrogen in the soil. Gos-

ling (2005) investigated the apparent association of U. dioica

on acidic, nutrient-poor mining spoil with the N2-fixing, legu-

minous shrub Lupinus arboreus. There were significantly

higher concentrations of extractable soil P concentration and

higher values of soil pH beneath the L. arboreus compared

with the areas between bushes occupied by herbaceous vegeta-

tion. Litter mass inputs and P beneath the shrub were more

than two and a half times higher and three times higher in

terms of N than in areas between bushes. This did not lead to a

higher soil organic matter content beneath the bushes, how-

ever, probably because the high nutrient concentration of the

litter leads to rapid decay. A glasshouse pot-culture experiment

showed thatU. dioica grew poorly on soil collected from areas

between bushes of L. arboreus without the addition of supple-

mentary N and P fertilizer, indicating co-limitation by both N

and P. Growth of U. dioica on soil from beneath L. arboreus

wasmore than four times greater than in soil from between the

bushes. Colonization of the mining spoil by U. dioica can

therefore be attributed to increased soil N and P derived from

the litter of L. arboreus. However, in the field, U. dioica was

only found beneath dead and senescent bushes, suggesting a

period of inhibition caused by shading, before senescence of

L. arboreus allows light penetration to the nutrient-rich soil

below.

(C ) EFFECT OF FROST, DROUGHT, AND FLOODING

Frost

The shoot tips ofU. dioica are not affected by spring frosts, but

may die back following early frosts in the autumn and such

frosted shoots may produce lateral branches before their death

later in the autumn (Greig-Smith 1948).

Drought

Hill, Preston & Roy (2004) attributed Ellenberg indicator

values of 6 (on a scale of 1–12) for moisture (on moist or

damp but not on wet soils) to U. dioica. Ellenberg (1988)

1444 K. Taylor


measured the soil-water potential around the roots of a num-

ber of widely distributed herbaceous plants in a moist-soil

Oak-Hornbeam wood. For U. dioica, he found the values

normally about )1 bar ()0.1 MPa). In an unusually dry late

summer when the water potential reached )5 to )6 bar

()0.5 to )0.6 MPa) the plant started to go limp, and when it

reached )7 bar ()0.7 MPa), U. dioica was completely wilted.

Permanent wilting point is usually defined as )15 bar

()1.5 MPa) which suggests that the plant is not very drought

tolerant.

Boot, Raynal & Grime (1986) employed a technique

which involved daily transfers of the root system of seed-

lings (18–28 days old) of U. dioica between nutrient solution,

distilled water and air to provide a convenient and realistic

method of exposing plants to controlled intensities of

drought. This laboratory experiment allowed the duration

of drought to be controlled and simulated the diurnal pat-

terns of desiccation experienced in the field. Prolonged

drought (5 h and 7 h day)1) strongly inhibited vegetative

growth and flowering. The droughted plants only wilted

during the first week of treatment, thereafter, the species

maintained high-water potentials in the xylem ()0.2 MPa)

at moderate intensities of drought (1–3 h day)1). This

response suggested that a process of hardening, probably

involving stomatal closure, had occurred and coincided with

a pronounced decline both in yield and in the development

of nodes and inflorescences.

Waterlogging

Urtica dioica ssp. dioica cannot survive flooding of its roots

and rhizomes for long periods, as it does not have any special

adaptation to anoxic conditions. In central European flood-

plains, the plant is only found in shallow depressions where the

water-table does not stay above the soil surface for long

(Srutek 1993). The growth responses of terminal sections of

U. dioica have been studied at water-table depths ranging from

10 to 60 cm under the soil surface in a garden experiment by

Srutek (1997). He found that biomass (ranging from 14.5 to

1.9 g plant)1), height, branching of stems and rhizomes and

rhizome length decreased in containers with a higher water-

table. Transpiration rates of individual plants also decreased

remarkably with the water-table nearer to the surface, ranging

from 10.2 down to 1.0 mM m)2 s)1. In spite of the decreased

competitive ability of U. dioica in regularly flooded wetlands,

particularly in sites with a long-term presence of a shallow

water-table near or above the soil surface, the plant is able to

persist in un-mown wetlands for many years and form almost

pure stands.

VI. Structure and physiology

(A ) MORPHOLOGY

Young rhizomes are produced either from older rhizomes or

from stem bases. They are reddish in colour and bear stinging

hairs and scale leaves with a small rudimentary lamina and

large stipules. Roots usually arise immediately above the stip-

ules (four per node), although frequently they arise immedi-

ately below some or all of the stipules and occasionally

between the nodes. The roots branch profusely and form

numerous fine laterals. Older rhizomes and roots have a yellow

cork layer. In very loose soils rhizomesmay be found at depths

up to 30 cm or more (Greig-Smith 1948). According to Srutek

& Tecklemann (1998), rhizomes usually occur not deeper than

20 cm below the soil surface. They also provided evidence of

changes in the architecture of the rhizome system under condi-

tions of water stress; on a dry grassland, old rhizomes of

U. dioica form thick lignified stems which ensure water supply

and distribution during water stress. Under these dry condi-

tions, all rhizomes show a distinctive secondary growth, mani-

fested as an increase in the specific mass of rhizomes (dry mass

per unit rhizome length), which is not the case in other habitats

with abundant water supply.

The shoots are erect, 1.5–2 m rarely >3 m tall, bearing

opposite, decussate leaves which differ in shape according to

the position on the stem. Shoots are generally unbranched, but

occasionally show some branching at the top, especially in the

autumn (Greig-Smith 1948). On highly fertile sites,Urtica dio-

ica builds a dense canopy as early as the stage of seven

expanded leaves, but plants continue to produce new leaves

and abscission of lower leaves occurs due to self shading. This

leads to the total leaf canopy being replaced about three times

during the growing period. Despite a retranslocation of c. 60%

of nitrogen, the leaf nitrogen pool must be replaced twice dur-

ing each growing season (Schulze & Chapin 1987; Srutek &

Tecklemann 1998).

Samples of mature leaves of U. dioica collected from plants

growing in a woodland margin, in early September 2008, have

been examined to determine stomatal density (R. Sharp, pers.

comm.). There were no stomata on the upper (adaxial) leaf sur-

faces, making the plant hypostomatous. The mean stomatal

density on the lower (abaxial) leaf surfaces was 94. 8±3.3

(SE)mm)2.

(B ) MYCORRHIZA

Examination of more than 20 root-systems of U. dioica by

Abeyakoon & Pigott (1975) from natural habitats showed no

trace of arbuscular mycorrhizal (AM) colonization. The nettle

lectin, Urtica dioica Agglutinin is a protein found in the roots,

rhizomes and seeds. It exhibits carbohydrate binding specifi-

cally to chitin found in the cell walls of certain fungi, and pre-

vents the formation of mycorrhizal associations (Wheeler

2005).

Harley & Harley (1987) recorded slight colonization (29%)

by AM in British material, and both its presence and absence

in continental Europe.

(C ) PERENNATION: REPRODUCTION

Urtica dioica spreads mainly by means of far-creeping rhi-

zomes and stolons which continue to grow until the death of

the aerial shoots in the autumn. New rhizomes are produced



in autumn and they continue to grow at or just beneath the

soil surface until the death of the aerial shoots, when they

turn up to form new shoots (see Fig. 2 in Greig-Smith

1948). These small shoots survive in the winter and resume

growth in the following spring (chamaephyte or protohemi-

cryptophyte in severe winters). The age of the plant at first

flowering is not known, but it does not flower in the first

year (Greig-Smith 1948). Seedlings of U. dioica flowered 36–

67 days after germination under experimental conditions in

a growth room (Boot, Raynal & Grime 1986). Abundant

seed is produced every year, but the number of seedlings

varies greatly from year to year (Greig-Smith 1948). Germin-

able seed is dispersed in the dung of some breeds of cattle

and horses (see also IV). Seeds and rhizome fragments,

which readily reroot, are also transported with soil and, for

this reason the species is highly mobile (Grime, Hodgson &

Hunt 2007).

(D ) CHROMOSOMES

The cytotypes of U. dioica have a basic number of either

x = 12 or x = 13, with two tetraploid forms ofU. dioica ssp.

dioica 2n = 48 and 52 (reports of 2n = 26 require confirma-

tion) and the diploidU. dioica ssp. galeopsifolia 2n = 26 (Stace

2004). Cytological intermediates between the two taxa occur

(see VIII B).

Grime, Hodgson & Hunt (2007) gave a 2C value of 3.1 pg

for the nuclear DNA content, where the C (constant) value is

the amount of nuclearDNA in the haploid nucleus.

(E ) PHYSIOLOGICAL DATA

Photosynthesis

Corre (1983a) grew seedlings of a number of species of differ-

ent shade tolerance, including U. dioica, in an aerated nutri-

ent solution in a climatic room (daylength 16 h, day

temperature of 20 �C and a night temperature of 15 �C).Two light treatments were used: one moderate energy fluence

rate (400–700 nm) of 8 W m)2 with either a normal (1.00) or

low (0.11) R ⁄Fr. Measurements of photosynthesis and respi-

ration were made in an assimilation chamber by infra-red gas

analysis. Four plants of each species were measured from

both light treatments every 5 days, then leaf areas and leaf

and stem dry masses recorded. At moderate light fluxes

LAR, SLA, SWR and LWR for U. dioica appeared to be

unaffected by the reduction in the R ⁄Fr. Not unexpectedly

photosynthetic capacity (light-saturated photosynthetic rate

at 330 ppm CO2) and photosynthetic efficiency at non-satu-

rating levels of irradiance did not seem to be influenced by

the R ⁄Fr. However, dark respiration at low R ⁄Fr was

decreased probably because of the increased energy demand

of the elongating stem.

Ogren & Sundin (1996) compared the photosynthetic

responses for species fromhabitats differing in light availability

and dynamics grown under the same controlled conditions.

An open gas-exchange system was used to analyse the two

principal components of photosynthetic induction: opening of

stomata and the activation of ribulose-1, 5-bisphosphate

carboxylase ⁄oxygenase (Rubisco). Leaves of U. dioica when

transferred from low (25 lmol m)2 s)1) to high light

(800 lmol m)2 s)1), took 21.4±2.1 (SE) min to reach 90%of

the final photosynthetic rate and 5.0±0.5 min to reach 90%

of the final CO2 use efficiency (a measure of Rubisco activa-

tion). In contrast Paris quadrifolia, a typical shade plant, took

7.1±0.7 and 5.2±0.4 min respectively to respond. The

rapidity of the response in the latter species was explained by

stomata only partially closing in the low-light period, so that

full stomatal aperture was reached very quickly when light

availability was increased. The slow response of U. dioica was

probably the result of lower rates of both Rubisco activation

and stomatal opening.

Nutrient supply and growth

The amount of nitrogen and phosphorus (g m)2), and indeed

probably all nutrients, held in the vegetation and used in the

production of aerial shoots is highly significantly greater in

dense stands ofU. dioica than in extensive colonies ofMercuri-

alis perennis or Deschampsia cespitosa (Taylor 1963; Pigott &

Taylor 1964). Most of the nutrients in the aerial shoots of

U. dioica are apparently redistributed between the shoots and

roots or rhizomes, with some uptake from the soil. Supporting

evidence is also provided by the results of plant tissue analyses

of mature, non-senescent leaves of U. dioica collected from a

wide range of sites (Table 1). The concentrations of nitrogen,

phosphorus, calcium and magnesium are at the upper end of

the ranges reported for herbs in many surveys (Thompson

et al. 1997b).

Taylor (1963) investigated the nitrogen requirements of

seedlings ofU. dioica and a number of other woodland species,

on mull soil from under a luxuriant growth of the plant in

NorthcliffeWood, Calver, Derbyshire, in a glasshouse pot-cul-

ture experiment. Nitrogen mineralization was reduced by add-

ing sucrose solution to the soil at regular intervals, to lower its

C ⁄N ratio, but an adequate supply of other essential mineral

nutrients was added to each pot. The immobilization of soil

nitrogen resulted in a greater reduction in the total dry-matter

production of U. dioica (50–60% of control yield) than Des-

champsia cespitosa (36–53%). When NH4NO3 was added with

Table 1. Mean concentrations of nutrient elements in mature, non-

senescent leaves ofUrtica dioica sampled from a number of sites

Source

Number

of sites

Concentration of element (% dry mass)

N P K Ca Mg

A 6 3.51 0.44 2.53 6.82 0.56

B 28 4.26 0.51 2.61 3.87 0.67

A, analytical chemistry, Centre for Ecology and Hydrology, Lan-

caster; B, Thompson et al. (1997b).

Chemical analyses were carried out using the methods described

by Allen et al. (1989).

1446 K. Taylor


sucrose (1 part N to 4 parts C), the dry mass of the seedlings

was similar to, or greater than that of the control plants. The

reduction of soil nitrogen mineralization resulting in decreased

growth was accompanied by a change in the proportions of

total dry mass in different plant parts a decrease in leaf dry

mass with a corresponding increase in root drymass and a sim-

ilar reduction in total leaf area althoughULRwas unaffected.

Falkengren-Grerup (1995) carried out three flowing-solu-

tion experiments, using field-relevant soil solution concentra-

tions at pH 4.5, in a glasshouse to test the importance of

NH4+ vs. NO3

) + NH4+ to the growth of a number of spe-

cies, incudingU. dioica.The results showed that the plant grew

better withN supplied asNO3) + NH4

+ rather thanNH4+.

Havill, Lee & Stewart (1974) assayed in vivo NRA in the

leaves of various species in a number of contrasting habitats, a

good indicator of the availability of NO3) in these habitats.

They found that U. dioica growing on wasteland had a mean

activity of 7.93 lmol NO2) h)1 g)1 fresh mass (range 3.60–

15.60) which indicated a nutritionally significant utilization of

nitrate. The addition of saturating concentrations of sodium

nitrate produced, after 72 h induction, an enzyme activity of

16.10 lmolNO2) h)1 g)1 freshmass, the highest rate of any of

the species measured. Al Gharbi & Hipkin (1984) found the

highest levels of NRA in the leaves of 27 ruderal species, grow-

ing in cultivated plots receiving an annual spring dressing of

rotted compost and fertilizer, with a mean activity of

4.39±0.84 lmol NO2) h)1 g)1 fresh mass. Urtica dioica had

the highestmeanNRAof 8.75 (range 7.0–10.0).

Gebauer, Rehder & Wollenweber (1988) listed the contents

of nitrate, NRA and organic nitrogen in individual organs of

many Central European plants, in a range of habitats. Urtica

dioica had high concentrations of NO3) with the highest con-

tents in the shoot axis plus petioles fraction, having mean val-

ues 6.7±4.7 (SE) lmolNO3) g)1 dry mass in occasionally cut

grassland, and 350.3±163.7 on the slopes of river banks.

NRA values were highest in the laminae in the same two sites,

16.6±9.1 and 31.7±6.6 lmol NO2) h)1 g)1 dry mass

respectively, activities which are especially high. The organic

nitrogen content of U. dioica was characteristically high in the

river bank community, especially in the laminae and reproduc-

tive fractions, with values of 41.0±4.2 and 38.4±6.4 mg

N g)1 drymass respectively.

The growth of Urtica dioica is affected by the extent of the

supply, source, utilization and storage of nitrogen. Rosnit-

schek-Schimmel (1982) studied the influence of low (3 mM)

and high (22 mM) NO3) or NH4

+ supply on biomass and N

distribution in seedlings grown for 8 months on vermiculite

with continuously applied flowing nutrient solutions. At high

NO3) supply, maximal shoot length of U. dioica was doubled

and leaf area increased threefold. Dry matter production of

plants supplied with NO3) exceeded that of seedlings supplied

with NH4+ irrespective of the concentration of the N source.

LowN supply favoured the partitioning of biomass to the root

plus rhizome fraction, whereas high N supply resulted in pref-

erential growth of the aerial parts. Growth on high NO3) or

NH4+ caused a higher content of total N, NO3

) and free

amino acids (particularily asparagine, arginine and glutamine)

in all plant parts as compared with that found in plants grown

on the low N sources. The distribution of total N between the

various plant parts was similarly affected by the concentration

of N supply as was that of the biomass. The main N storage

compounds wereNO3) whenNO3

) was the N source, and free

amino acids when NH4+ was the N source. NO3

) was prefer-

entially accumulated in the leaves and stems, whereas amino

acids were more equally distributed between leaves, stems and

roots plus rhizomes. When NH4+ was the N source, the con-

centrations of amino acids were significantly highest in the

stems.

In a further experiment, Rosnitschek-Schimmel (1983) grew

plants of U. dioica from seed in garden mould in a glasshouse.

A large proportion of the biomass was found in the root and

rhizome fraction, and the distribution of total N followed in

general the trends of biomass partitioning. NO3) reduction

occurred almost exclusively in the leaves where some 98% of

total NRA was found. The proportion of nitrate in the stems

was eight times higher than in the leaves. A high accumulation

of nitrate and nitrogen-rich amino acids, asparagine and argi-

nine, and a number of other free amino acids and related com-

pounds occur in roots and rhizomes of U. dioica. There is a

possible role of these free amino acids in the detoxification of

abundant NH4+ and in storage and transport of nitrogen

(Rosnitschek-Schimmel 1983).

Hofstra, Laanting & de Visser (1985) investigated the nitro-

gen metabolism of U. dioica and its dependence on the supply

of mineral nutrients. Twenty-six days after seed germination,

seedlings were transferred to a nutrient solution containing

either 100% or 2% of quarter strength Hoagland’s solution.

After a further 17 days, half of the seedlings were transferred

either from 2% to 100% or vice versa. Each solution of pH 6.0

was renewed twice a week. During these intervals, depletion of

nutrients occurred in both solutions and the pH could also

change. Plants were harvested 11 days after the switch and

plant mass, organic nitrogen (soluble and insoluble) and

nitrate content were measured. The activities of two nitrogen

assimilating enzymes, NRA and glutamine synthetase (GS)

were determined. Fresh masses of leaves, stems and roots

reflected the nutrient status of the culture solution. Shoot and

root growth was reduced at constant low nutrient supply. The

content ofNO3 (lmol g)1 freshmass) of all the plant parts cor-

responded with the NO3) level of the nutrient supply. After a

100 ⁄2% switch the shoot ⁄ root ratio hardly changed, but

plants were showing symptoms of senescence. Net redistribu-

tion of N compounds from shoot to root took place, and also

leakage from the root into the nutrient solution. The disap-

pearance of nitrate from the shoot could be accounted for by

nitrate reduction (as calculated from growth and NRA data).

It is suggested that in these conditionsGS reacted to this down-

ward flux. A correlation occurred between nitrate reduction

andGS activity in the shoot ofU. dioica. In the root, there was

a combined influence of reduced nitrate and redistributed N

onGS activity.

Pigott & Taylor (1964) determined the dry-mass response

(mg plant)1) of seedlings of U. dioica and a number of other

common woodland species to the addition of nitrogen and



phosphorus when grown under glasshouse conditions in

plastic pots containing top soil (calcareous gleyed brown

earth) collected from under a dense stand of Mercurialis

perennis (Leaf Area Index 3.0–4.0) in Buff Wood, Cam-

bridgeshire. Re-analysing these results using the approach

suggested by Clymo (1995) provides mean estimates of rate-

limiting, yield-limiting and nutrient factors for U. dioica.

Addition of nitrogen when no extra phosphate was added

gave a slight reduction in yield ()1 mg) compared with the

control treatment, while phosphate behaved as a nutrient

and a limiting factor when no nitrogen was added

(+86 mg). Together they produced a response (+125 mg), a

positive interaction (+40 mg), greater than a simple addition

of the individual responses (+85 mg).

Pigott (1971) carried out a more detailed analysis of the

growth of seedlings of U. dioica in the top soil removed from

Buff Wood, Cambridgeshire, in a well-illuminated glasshouse

pot-culture experiment. Addition of NH4NO3 led to a slight

reduction in the growth of the seedlings ofU. dioica, and addi-

tion of all essential mineral nutrients except PO43) produced

only a small growth response. However, there was a large

response to addition of soluble phosphate, as Ca(H2PO4)2 and

a further increased response to the addition of both

Ca(H2PO4)2 and NH4NO3. This was caused by a high rate of

absorption which allowed a short period of high RGR related

to a rapid, but transient increase of LAR, and an increase in

growth of the stem relative to that of the roots. These responses

depended upon the roots coming into contact with a soil solu-

tion which contains as well as other nutrients, at least 0.01 mg

ions PO43) L)1 (10)5 M), the concentration found in soils

whereU. dioicawas growing vigorously. A high rate of uptake

of nutrients per unit mass of root system was then possible.

The relative increase of LAR caused by the addition of phos-

phate was greatly reduced in shade. Neither in the rate of

absorption of PO43) from dilute solutions, nor in the produc-

tion of dry matter relative to the quantities of PO43) absorbed,

does U. dioica differ significantly from species which show a

small response to PO43) on this soil type. Abeyakoon & Pigott

(1975) also found that seedlings ofU. dioica responded to addi-

tion of soluble phosphate when grown on freshly collected top

soil from two other major soil types, an uncultivated rendzina

and a brown forest-soil over Carboniferous limestone. How-

ever, these two soil types differ in the mineral content of their

inorganic phosphate fractions. In the rendzina apatite predom-

inates, while in the brown forest-soil iron phosphate predomi-

nates. There was no response to the addition of apatite on the

former soil, but there was a large growth response to the min-

eral on the latter. Clearly although the growth of U. dioica is

limited by the shortage of phosphate on the rendzina, it is

unable to make good this deficiency from apatite. In contrast,

on the more acid brown forest-soil the species showed a signifi-

cant growth response to the addition of apatite. The simplest

interpretation of this result is that the solubility of apatite is

controlled by soil pH. In the rendzina (pH 7.6), the additional

apatite is unavailable to plant roots, whereas in the brown for-

est-soil (pH 5.5) solubility is adequate to allow a high rate of

absorption.

Rorison (1968) described an investigation into the growth

of seedlings of U. dioica, Rumex acetosa, Scabiosa colum-

baria and Deschampsia flexuosa in water culture. Phosphate

was supplied as K2HPO4 at concentrations varying from

10)7 to 10)3 M. Over the range 10)7 to 10)4 M K2HPO4,

the RGR over the first 6 weeks after germination increased

with phosphate concentration for all species and the natural

logarithm of dry mass of the seedlings at 6 weeks was almost

directly proportional to the logarithm of phosphate concen-

tration. Urtica dioica was the only species which showed a

continuation of this linear response beyond 10)4 M K2HPO4

to 10)3 M. However, Pigott (1971) pointed out that these

concentrations are almost ten and a hundred times those

normally found in water expressed from soils where U. dio-

ica grows vigorously. Nassery & Harley (1969) investigated

the uptake of 32P-labelled KH2PO4 by excised root apices of

seedlings of U. dioica and Deschampsia flexuosa grown in

standard culture solution containing 10)5 M phosphate for

4–6 weeks. The roots of the two species showed no differ-

ences in the rate of uptake over a wide range of concentra-

tions of 10)6 to10)3 M phosphate. After transference to

phosphate-free solutions, the higher RGR of U. dioica fell to

that of a rate maintained throughout by Deschampsia flexu-

osa, indicating that the efficiency of utilization of phosphate

for producing dry mass was the same for both species.

However, Nassery (1970) found that the rates of uptake by

cuttings of the two species do differ, though not significantly,

over the range of concentrations which are found in soils.

When the cuttings were transferred from solutions with con-

centrations above 10)3 M KH2PO4 to phosphate-free solu-

tions, the greater removal of phosphate to the shoots

because of the high RGR of U. dioica also fell to that of a

rate maintained throughout by Deschampsia flexuosa. The

concentration of phosphate in the tissues fell to much the

same value in both species (2 lg P mg)1 dry mass).

Srutek (1995) studied the effects of different levels of

nutrient supply on the production and patterns of biomass

allocation in plants of U. dioica during the first growing sea-

son after germination. Seeds from unmanaged floodplain

meadows were used in a pot-culture experiment set up in a

plastic unheated glasshouse. Three treatments with a com-

bined N-P-K fertilizer (12.5% N, 8.5% P, 16.0% K) added

as an aqueous solution at a rate of 75, 225, and 375 kg ha)1

with harvests at monthly intervals. Biomass allocation was

significantly affected by the nutrient supply; higher nutrient

doses resulted in less biomass allocation to roots and

rhizomes, whereas the period of intensive production of

the above-ground biomass was prolonged. Shoot height

increased with nutrient supply. Within each treatment and

each harvest, inflorescence biomass was positively correlated

with shoot height. Branching of the main shoots to produce

laterals was positively correlated with plant height and chan-

ged with time. The number of new rhizomes was affected

by both treatment and harvest, especially in older plants.

The results indicate that high nutrient supply increased the

allocation of biomass both to reproductive and vegetative

organs.

1448 K. Taylor


(F ) B IOCHEMISTRY

The stinging hair ofU. dioica consists of a large cell embedded

in a multi-cellular pedestal base. The extremely large stinging

cell (1.8 mm long) and the main part of the cell (1.6 mm long)

is pulled out into a tapered needle or shaft that is closed at the

end by an asymmetrically placed hollow terminal knob. The

shaft has a thick cellulose cell wall that is glass-like and brittle

near its tip. The base is swollen into an elongated sphere with a

flexible cellulose wall and forms the basal bulb (Wheeler 2005).

When the stinging hair comes into contact with the skin, its ter-

minal knob breaks off leaving an asymmetrical hypodermic

tip. The downward contact force compresses the flexible walls

of the basal bulb and fluid is forced upwards and expelled into

the wound.

Chemical factors which primarily cause the stinging sensa-

tion when brushing contact is made with a stinging hair of

U. dioica have been extracted, and a variety of putative pain-

producing compounds has been proposed during the study of

these hairs (Thurston & Lersten 1969): histamine, acetylcho-

line, 5-hydroxtryptamine, serotonin and formic acid. These

substances are now cited in most texts, but the references

ignore a large body of negative findings. Recently it has been

demonstrated that in Urtica thunbergiana in Taiwan, formic

acid, histamine and serotonin were not present in significant

amounts in the stinging hairs. However, oxalic acid and tar-

taric acid were identified as major long-lasting pain-inducing

toxins (Fu et al. 2006).

The seasonal dynamics of nitrogenous compounds in the

different plant parts of U. dioica have been investigated

under natural conditions during the course of a year by

Rosnitschek-Schimmel (1985a,b). In the below-ground

parts, a nitrogen store was built up during summer and

autumn, consisting mainly of free amino acids. This over-

wintering nitrogen store consisted mainly (up to 80%) of

asparagine and arginine. Although asparagine was domi-

nant in rhizomes, up to 1 year old, arginine was specifically

accumulated in the older rhizomes and roots. In early

spring, first the nitrogen stored in asparagine and, with a

delay of about 3 weeks, that in arginine was mobilized and

translocated to the rapidly growing shoots and used for

protein synthesis. In seeds, proteins were the major nitrogen

reserve.

VII. Phenology

The new small shoots produced at the soil surface from new

rhizomes in the autumn survive the winter and resume growth

in the following April (see also VI C) reaching a peak in shoot

biomass between July and September, depending on the habi-

tat (see V B). In the autumn, the leaf canopy deteriorates rap-

idly, and byNovember only the stems remain.

Urtica dioica is a long-day plant and may need up to 16 h

daylength for flowering (Grime, Hodgson & Hunt 2007).

Flowering takes place from late May to August, and viable

seed is shed or may remain on the dead stems up to December

or January. Occasional seedlings may be found in the autumn,

but the main production of seedlings occurs in the spring

(Greig-Smith 1948; Roberts &Boddrell 1984).

VIII. Floral and seed characters

(A ) FLORAL BIOLOGY

Urtica dioica is normally dioecious in parts of its range, and

with monoecious or hermaphrodite populations in other

parts of the range (Wheeler 2005). Reproduction is amphi-

mictic. It is usually wind-pollinated; the immature stamens

are incurved and under tension in the flower bud, and spring

back, scattering the pollen explosively. The pollen grains are

extremely small measuring 13.4 · 14.5 lm (Hyde & Adams

1958). An average pollen catch of 88.3 grains cm)3 year)1

(range 21.6–156.4) in the air from late May to late Septem-

ber, with a peak in June–August has been recorded. These

values are very high but less than grasses with a mean of

2106 grains cm)3 year)1 (range 725–4459), and the wind-pol-

linated trees Ash and Oak with means of 271 (range 89–505)

and 178 (range 10–504) respectively (Proctor, Yeo & Lack

1996). In open habitats, female clones of U. dioica flower in

advance of male clones and are slightly in advance of wood-

land clones (Wheeler 2005).

Although the individual flowers are highly reduced, green

and inconspicuous, occasional pollination by insect visitors

may occur. The flowers are visited by four species of thrip

(Thysanoptera), Taeniothrips atratus (Hal.), T. vulgatissimus

(Hal.), T. picipes (Zett.), and Thrips tabaci Lind. (Greig-Smith

1948). The small pollen-eating beetle Meligithes flavimanus

Sturm (Nitidulidae), which is very common, is also found on

the flowers. These small beetles and thrips, which fly from

flower to flower with their bodies covered in pollen, may be sig-

nificant pollinators.

(B ) HYBRIDS

Urtica dioica ssp. dioica and ssp. galeopsifolia differ cytologi-

cally and are usually separable using the morphological char-

acters described by Rich & Jermy (1998). Hybrids with

intermediate morphology and chromosome number are found

in places where populations of the two taxa meet; so far all

such plants encountered have been female (M. F. Godfrey,

pers. comm.).

(C ) SEED PRODUCTION AND DISPERSAL

Wheeler (1981) found a considerable variation in seed mass

within both woodland and pastureland clones in north Devon.

The mean seed masses were 0.199±0.024 (SD) mg and

0.191±0.016 mg respectively (n = 3000). The low seed mass

enables the production of vast numbers of seeds; a pastureland

clone produced c. 30 000 seeds per shoot, but a woodland

clone considerably fewer.

Dispersal is effected when the persistent, hispid perianth seg-

ments of the fruits adhere to animal fur, feathers and clothing,

and the perianth probably also assists in wind dispersal. Viable



seeds have also been found in the excrement of sheep, cattle,

horses and fallow deer (see IV).

(D ) V IABIL ITY OF SEEDS: GERMINATION

The ‘seed bank’ of U. dioica is of the persistent type (Thomp-

son & Grime 1979), in which few of the seeds germinate in the

period immediately following dispersal, and the species main-

tains a seed bank that changes little in size with season and is

large in relation to the annual production of seeds. Wheeler

(1981) found a seed bank which varied from 1754 to 9090 via-

ble seeds m)2 in floodplain pasture, and 88–1664 in woodland.

In Northwest Europe, Thompson, Bakker & Bekker (1997a)

have calculated that the average seed density at a soil depth of

0–3 cm is 4924 m)2 and at 0–10 cm is 23819 m)2. This type of

seed bank confers the potential for regeneration in circum-

stances where disturbance, temporally and ⁄or spatially unpre-dictable, allows the seedlings of U. dioica to evade the

potentially dominating effects of established plants. Roberts &

Boddrell (1984) sowed seeds of U. dioica, freshly collected in

autumn, on to sterilized soil that was confined in cylinders

sunk in the ground outdoors and periodically cultivated. Seed-

ling emergence was recorded for 5 years and the seasonal pat-

tern of emergence was determined. Peak emergence occurred

in April and in greatest numbers in the first year (68% of the

seeds sown), and an average of only 3% of the viable seed ini-

tially sown still remained after 5 years.

The ability ofU. dioica seeds to germinate and develop on a

range of soils under controlled laboratory conditions has been

assessed by Rorison (1967). Germination was most rapid and

complete, seedling survival rate high and growth strong on cal-

careous soils (pH 6.1–7.4), but slower and less complete on

increasingly acid soils (pH 3.5–4.1).

Pigott (1971) conducted a series of experiments using 50 ach-

enes of U. dioica sown on to the surface of soil in plant pots,

with five replicates in each treatment. The soil was a calcareous

mull, phosphate-deficient, woodland soil (pH 6–7 in the top

10 cm). Addition of phosphate to all treatments did not signifi-

cantly increase the production of seedlings after 3 weeks.How-

ever, germination was controlled by both the amounts of

irradiance and its spectral distribution. In pots placed in a

woodland glade where there was 60% transmission of daily

irradiance (400–750 nm) and the R ⁄Fr ratio was of c. 1, 68–

88% of the achenes produced seedlings. In the same woodland

under a closed canopy of Acer pseudoplatanus where there was

1% transmission of daily irradiance and a significantly

decreased R ⁄Fr ratio, no seedlings were produced. However,

in controlled environment cabinets with pots placed beneath a

spectrally neutral shade screen, with only a 1% transmission of

daily irradiance and a normal R ⁄Fr ratio, 8–24% of the ach-

enes still produced seedlings. Mediated through the phyto-

chrome system, the higher proportion of far-red inhibits

germination of the light-promoted seeds of U. dioica. Jan-

kowska-Blaszczuk & Daws (2007) have also demonstrated

experimentally that the very small seeds of U. dioica require

light with R ⁄Fr ratio of 0.9 or greater for 50% germination.

These responses to light indicate that seeds are only likely to

germinate in micro-sites near the soil surface, and in the

absence of over-topping vegetation or leaf litter. Such micro-

sites are comparatively rare which may necessitate persistence

in the soil seed bank (see before).

Grime et al. (1981) using a standardised laboratory proce-

dure have studied the germination characteristics of seeds col-

lected from a wide range of habitats. When freshly collected

seeds of U. dioica were placed on moistened filter paper in

Petri-dishes and transferred to a growth roomproviding visible

radiation at a flux of 40 W m)2 (‘warm-white’ fluorescent

tubes + tungsten bulbs) over a 15-h day at 20 �C and a night

temperature of 18 �C, 60% of the seeds germinated, with time

to 50% germination (t50) of 7 days. After dry storage in dark-

ness at 5 �C for up to 6 months, batches of 100 seeds were

either subjected in darkness to stratification at 5 �C for periods

of 3, 6 or 12 months, or placed at 20 �C for <1 month, then

tested for germination under the above conditions. In all treat-

ments with one exception (3 months’ chilling), germination

rates were not significantly different from that in freshly col-

lected seed. Germination of imbibed seeds in the growth room

was 99% in shade (2.5% of the light treatment c. 1 W m)2)

and 80% in the light regime (40 W m)2), but completely inhib-

ited by darkness. They found that when seeds were exposed to

a range of temperatures of 5–40 �C on a thermogradient bar at

constant temperatures under fluorescent light, germination

occurred over a range from 22 to 35 �C (t50 6 days). Germina-

tion of the wetted, minute seeds of U. dioica is stimulated by

even low fluxes of light and fluctuating temperatures but inhib-

ited by darkness; thus it flourishes in moist open and disturbed

sites and dense vegetation cover is unfavourable for regenera-

tion by seed.

(E ) SEEDLING MORPHOLOGY

Germination is epigeal. The achene splits at the apex and a

radicle emerges. The hypocotyl elongates and straightens,

withdrawing the cotyledons from the remains of the achene.

The cotyledons are oblong, emarginate and rather succulent

(Fig. 3). The germination and early stages of seedling devel-

(a) (b)

(c)

(e)(d)

10 mm

(a-e)

Fig. 3. Stages in seed germination ofUrtica dioica (a, b) and develop-

ment of seedlings (c, d) up to the first leaf stage (e).

1450 K. Taylor


opment have also been illustrated by Greig-Smith (1948). A

later stage in seedling development is illustrated by Muller

(1978).

IX. Herbivory and disease

(A ) ANIMAL FEEDERS

Insecta

A large number of phytophagous insect species has been iden-

tified onU. dioica, andDavis (1991) provides an illustrated key

and a taxonomic summary (see also Greig-Smith 1948;

Wheeler 2005). The fauna are listed in Table 2.

The colonization by the insects of four isolated patches

(9 m2) of U. dioica established in East Anglia in 1970 was

studied by Davis (1975). Of the numerous species that were

recorded Liocoris tripustulatus (Heteroptera, Miridae), Da-

sinura urticae (Diptera, Cecidomyiidae) and two Homoptera,

Eupteryx urticae (Cicadellidae) and Trioza urticae (Psillidae)

were particularily good colonizers. Davis (1991) also

provided an introduction to the insect fauna of U. dioica in

the British Isles, and published the results of a field survey

of the European distribution of insects on U. dioica (Davis

1989).

Mollusca: Gastropoda

Both Arianta arbustorum and Cepaea nemoralis feed on fresh

and senescing leaves ofU. dioica (Grime,MacPherson-Stewart

& Dearman 1968; Grime & Blythe 1969). Discus rotundatus

(Mull.) andHygromia striolata (Pfeiff.) feed on the fresh leaves

ofU. dioica (Mason 1974) and the slugsArion fasciatus andA.

subfuscus also find the leaves ofU. dioica highly palatable (Jen-

nings &Barkham 1995).

Mammalia

The following deer eat U. dioica in quantity: fallow (Dama

dama), roe (Capreolus capreolus) and red (Cervus elephas) (see

IV).

(B ) PLANT PARASITES

Dicotyledons

The holoparasiteCuscuta europaea L. (Greater dodder) occurs

primarily on U. dioica, often near water, but is now scattered

and rare in England, north to Northamptonshire (Preston,

Pearman&Dines 2002;Wheeler 2005).

Fungi

At least 20 different microfungi found on U. dioica are

listed in Table 3 (see also Greig-Smith 1948; Wheeler

2005). They include parasites and saprophytes, the former

found on living stinging nettle plants and the latter being

involved in the decomposition of plant material both living

and dead. In a dense stand of U. dioica Al-Mufti et al.

(1977) found that litter, composed almost exclusively of the

remains of stems of the plant, was present in abundance

and showed little variation throughout the year

(c. 500 g m)2 dry mass). Bell (1974) reviewed experimental

studies on microfungi isolated from incubated stem sections

of U. dioica. The primary microflora was present on grow-

ing stems and colonization proceeded basipetally. In the

summer the predominant microflora included the following

Mitosporic fungi: Alternaria tenuis, Botrytis cinerea, Clados-

porium herbarum and Epicoccum nigrum on senescing leaves

and adjoining stalk segments. These fungi persisted

through the winter in lesser numbers, but became active

and reached maximum numbers the following summer.

The secondary microflora developed in the first winter after

litterfall and included A. tenuis, B. cinerea, Cephalosporium

sp., Cladosporium herbarum, Stachybotrys atra and Torula

herbarum.

X. History

Fossil records indicate that Urtica dioica was present in the

Middle Pleiocene andEarly Palaeolithic periods of the Tertiary

(Greig-Smith 1948). Godwin (1975) provides plant records for

the Quaternary. Fruits of U. dioica have been found through-

out the Pleistocene record.Asmight be expected for such a pre-

valent weed, the later Flandrian records include many from

archaeological sites (Neolithic, Bronze Age, Iron Age, Roman

and Medieval). Pollen and fruit records establish the abun-

dance of the plant in all LateWeichselian zones and zone IV of

the Flandrian. The records indicate persistence through the

interglacial stages, with greater abundance in the warmer mid-

dle sub-stages. It persisted through the Flandrian and

expanded on zones VIIb andVIII as a consequence of disfores-

tation and the spread of agriculture. The high latitudinal and

altitudinal ranges of U. dioica are consistent with the evidence

that it survived the last glaciation and possibly earlier ones in

the British Isles.

The Romano-British villages on the Grovely Ridge near

Salisbury, Wiltshire, where overgrown by woodland, are

marked by dense nettles living on the phosphate-rich debris of

an occupation which ended 1600 years ago. Also in woodland

on the deserted medieval manors of Overhall in Boxworth,

and Wratforth in Wimpole, Cambridgeshire, and the deserted

village of Little Gidding, Huntingdonshire, all sites having an

accumulation of phosphate (Rackham 1980).

In historical times U. dioica was first recorded in 1562 (At-

kins &Atkins 2004). It is difficult to investigate trends in a spe-

cies which was too common to have been recorded in detail

until the advent ofmodern systematic surveys, but EdwinLees,

one of the most perceptive 19th century observers, noticed that

in Worcestershire it tended to spring up in the footsteps of

workmen in sites from which it was previously absent, and in

this area it was so closely associated with human activity that

he thought that it was probably alien (Preston 2003). A num-

ber of studies suggest that it has increased in Southern England



Table 2. Phytophagous insect species recorded fromUrtica dioica. Information supplied by the Biological Records Centre – Data base of Insects

and their Food Plants (http://www.brc.ac.uk/dbif/)

Taxonomic group Ecological notes Source

Thysanoptera

Thripidae

Thrips urticae F. Both larvae and adults feed 12, 18

Hemiptera

Miridae

Calocoris alpestris (Meyer-Dur) Larvae polyphagous 22

C. fulvomaculatus (Degeer) Larvae and adults polyphagous 22

C. norvegicus (Gmelin) Larvae and adults polyphagous 22

C. sexguttatus (F.) ssp. insularis Reuter Larvae and adults feeding 22

Dicyphus errans (J. F. Wolff) Larvae and adults polyphagous 22

Heterotoma planicornis (Pallas) Polyphagous larvae and adults 22

Liocoris tripustulatus (F.) Larvae and adults polyphagous 1, 22

Ligocoris contaminatus (Fallen) 22

L. lucorum (Meyer-Dur) 22

L. pabulinus (L.) Larvae and adults polyphagous 22

L. spinolai (Meyer-Dur) Polyphagous 22

Lygus punctatus (Zetterstedt) Polyphagous 22

L. rugulipennis Poppius Polyphagous 22

L. wagneri Remane Polyphagous 22

Macrosteles variatus(Fallen) 20

Orthonotus rufifrons (Fallen) Larvae and adults feed on leaves 22

Orthotylus ochrotrichus Fieber Polyphagous 22

Plagiognathus arbustorum (F.) Larvae polyphagous 22

P. chrysanthemi (J. F. Wolff) Larvae polyphagous 22

Lygaeidae

Drymus sylvaticus (F.) Larvae feed on seeds, polyphagous 8

Heterogaster urticae (F.) Both larvae and adults feed 22

Scolopostethus affinis (Schilling) Larvae feed on seeds 8

S. decoratus (Hahn) Larvae feed on seeds 8

S. thomsoni Reuter Larvae feed on seeds 8

Homoptera

Cicadellidae

Agallia consobrina Curtis 20

Empoasca decipiens Paoli 17

Eupteryx aurata (L.) Larvae feed on leaves 16, 27

E. cyclops Matsumara Larvae feed on leaves 20

E. florida Ribaut 20

E. stachydearum (Hardy) 20

E. urticae (F.) Larvae feed on leaves 20, 27

Macrosteles variatus (Fallen) 20

Aleyrodidae

Pealius quercus (Signoret) Both larvae and adults feed 15

Aphididae

Aphis urticata Gmelin Adults and larvae feed on leaves

and shoots, rolling, gregarious

4, 24

Microlophium carnosum (Buckton) Leaves and stems are attacked 4

Phorodon humuli (Schrank) 4

Triozidae

Trioza urticae (L.) Larvae feed 12

Coleoptera

Apionidae

Apion urticarium (Herbst) Larvae and adults feeding on stems 14, 26

A. vorax Herbst Adults feed 5

Cerambycidae

Agapanthia villosoviridescens (Degeer) Polyphagous larvae feed on leaves, forming mines 6

Curculionidae

Ceutorhynchus pollinarius (Forster) Larvae feed on roots 26

C. urticae Bohemen 26

Cidnorhinus quadrimaculatus (L.) Larvae feed on stems 26

Otiorhynchus clavipes (Bonsdorff) Larvae polyphagous 14

Phyllobius pomaceus Gyllenhall Larvae feed 14

1452 K. Taylor


Table 2. Continued


Chrysomelidae

Crepidodera ferruginea (Scopoli) Polyphagous larvae form mines and webs 26

Nitidulidae

Brachypterus glaber (Stephens) 26

B. urticae (L.) 26

Meligithes flavimanus Stephens Adults feed on pollen 26

Diptera

Cecidomyiidae

Dasineura dioicae (Rubsaamen) Larvae produce galls, form webs 3

D. urticae (Perris) Larvae produce galls 3

Chloropidae

Tropidoscinis albipalpis (Meigen) Larvae polyphagous, gregarious 25

Agromyzidae

Agromyza anthracina Meigen Larvae mine the leaves 23

A. pseudoreptans Nowakowski Larvae mine the leaves 23

A. reptans Fallen Larvae mine the leaves 23

Melanagromyza aenea (Meigen) Larvae feed on stems 23

Phytomyza flavicornis Fallen Larvae feed on stems 23

Lepidoptera

Glyphipterigidae

Anthophila fabriciana (L.) Larvae form webs 26

Tortricidae

Cnephesia asseclana (Denis & Schiffermuller) Polyphagous, form mines and webs 2

C. incertana (Treitschke) Polyphagous larvae form mines and webs 2

C. stephensiana (Doubleday) Larvae form mines and webs 2

Olethreutes lacunana (Denis & Schiffermuller) Polyphagous larvae form webs 21

Arctiidae

Arctia caja (L.) Larvae polyphagous 19

A.villica (L.) Larvae polyphagous 19

Callimorpha dominula (L.) Larvae polyphagous, gregarious 1, 19

Euplagia quadripunctaria (Poda) Larvae polyphagous, gregarious 19

Phragmatobia fuliginosa (L.) Larvae feed on leaves 19

Spilosoma luteum (Hufnagel) Polyphagous larvae 19

Pyralidae

Pleuroptya ruralis (Scopoli) Larvae roll the leaves 7, 9

Udea olivalis (Denis & Schiffermuller) Polyphagous larvae form webs 9

U. prunalis (Denis & Schiffermuller) Larvae feed on leaves 9

Nymphalidae

Aglais urticae (L.) Larvae feed on leaves, form webs, gregarious 1, 13,19

Araschnia levana (L.) Larvae feed 13

Cynthia cardui (L.) Larvae form webs 1, 19

Inachis io (L.) Larvae form webs, gregarious 1, 13,19

Nymphalis polychloros (L.) Larvae form webs, gregarious 10

Polygonia c-album (L.) Larvae feed 1, 13,19

Vanessa atalanta (L.) Larvae form webs 1, 13,19

Noctuidae

Abrostola trigemina (Werneburg) Larvae feed on leaves 1, 11,19

A. triplasia (L.) Larvae feed on leaves 1, 11,19

Autographa bractea (Denis & Schiffermuller) Larvae feed on leaves 1, 19

A. gamma (L.) Larvae polyphagous, form webs 19

A. jota (L.) Larvae feed 1, 11,19

A. pulchrina (Haworth) Larvae feed 1, 11,19

Axilia putris (L.) Larvae polyphagous 19

Diachrysia chrysitis (L.) Larvae feed 1, 11,19

Eugnorisma depuncta (L.) Larvae gregarious 19

Eumichitis lichenea (Hubner) Larvae polyphagous, gregarious 11

Gortyna flavago (Denis & Schiffermuller) Larvae mine the leaves 11

Hypena obesalis (Treitschke) Larvae feed on leaves 1, 11

H. obsitalis (Hubner) Larvae feed on leaves 1

H. proboscidalis (L.) Larvae feed on leaves 1, 11,19

Laconobia oleracea (L.) Larvae polyphagous 19

Noctua comes (Hubner) Larvae polyphagous 1, 19



Table 2. Continued


Orthosia gothica (L.) Larvae polyphagous 10

Standfussiana lucernea (L.) Larvae feed 19

Lymantriidae

Euproctis similis (Fuessly) Larvae polyphagous, gregarious 1

1, Allan (1949); 2, Balmer (1982); 3, Barnes (1949); 4, Borner (1952); 5, Cockbain, Bowen & Bartlett (1982); 6, Duffy (1953); 7, Emmet

(1988); 8, Eyles (1964); 9, Goater (1986); 10, Heath & Emmet (1979); 11, Heath & Emmet (1983); 12, Hodkinson & White (1979); 13,

Howarth (1973); 14, Hoffmann (1958); 15, Hulden (1986); 16, Le Quesne (1973); 17, Le Quesne & Payne (1981); 18, Mound et al.

(1976); 19, K. Noble, unpublished; 20, Ribaut 1952; 21, J. Robbins, unpublished; 22, Southwood & Leston (1959); 23, Spencer (1972);

24, Stroyan (1984); 25, Uffen & Chandler (1978); 26, Walsh & Dibb (1954); 27, Whittaker (1964).

Table 3. Microfungi mainly recorded fromUrtica dioica and part of plant affected

Oomycota

Peronosporales

Peronosporaceae

Pseudoperonospora urticae (Lib.) Salmon & Ware A downy mildew which forms greyish-brown or greyish-lilac patches on the

yellowed leaves, September and May

Ascomycota

Dothideales

Leptosphaeriaceae

Leptosphaeria acuta (Hoffm) P. Karsten Very common especially near the base of dead stems, February–May

Mycosphaerellaceae

Mycosphaerella superflua (Auersw.) Petrak Common on dead stems and usually in good condition in March

Erisyphales

Erysiphaceae

Erisyphe urticae (Wallr.) Klotzsch A powdery mildew on living leaves, September–October, oligophagous

Leotiales

Dermataceae

Calloria neglecta (Lib.) Hein Very common on dead stems, March–May

Laetinaevia carneoflavida (Rehm) Nannf. Ex Hein On damp dead stems, June–July

Pyrenopeziza urticicola (Phill.) Boud. Common on dead stems, April–August

Hyaloscypheae

Erinella discolour Mouton On dead stems, November

Rhytismatales

Rhytismataceae

Naemacyclus caulium Hohnel Near the base of dead stems, April, rare

Sordariales

Lasiosphaeraceae

Plagiosphaera immerse (Trail) Petrak On dead stems, May–August

Familia incertise sedis

Acrospermaceae

Acrospermum compressum Tode Common especially near the base of dead stems in damp situations,

February–June

Basidiomycota

Uredinales

Pucciniaceae

Puccinia caricina (DC) A rust commonly found on living leaves of Urtica dioica growing in damp

places in the aecidial stage, other stages on Carex spp., very common

Mitosporic Fungi

Aporhytisma urticae, Apolmelasmia state Can encircle the dead stems, February–March, not common

Arthrinium urticae M. B. Ellis On dead stems, November, uncommon

Botryosporium pulchrum Corda Forms extensive, white, cobwebby or fluffy colonies on living plants,

and may be seen any time from May to November

Endophragmia atra (Berk. & Br.) M. B. Ellis On dead stems, October–January, fairly common

Gyrothrix verticillata Pirozynski On dead stems, September, uncommon

Polyscytalum berkleyi M. B. Ellis On dead stems just above soil level, March–May, common

Pyrenochaeta fallax Bres. Pycnidia immersed in grey areas on dead stems

Septoria urticae Rob. & Desm. Causes greyish-brown spots on the leaves which eventually fall out

leaving shot-holes on the leaves

Source: Ellis & Ellis (1997). Classification based on Hawksworth et al. (1995).

1454 K. Taylor


in recent decades. A resurvey in 2003–2004 of 107 plots (each

with an area of 66 m2) originally recorded in 1949–1951 in a

range of habitats in Bedfordshire showed that U. dioica had

increased significantly both in the number of plots in which it

grew (from 18 to 54) and in its abundance within plots

(Walker, Preston & Boon 2009). There is also evidence for a

significant increase in cover (from 9% to 19%) in 139 British

woodland plots initially surveyed in 1971 and resurveyed in

2001 (Kirby et al. 2005) and for an increase in arable fields in

Oxfordshire and Berkshire between 1977 and 1997 (Sutcliffe &

Kay 2000).

XI. Conservation

Since 1998,Urtica dioica has thrivedmore than ever in the Brit-

ish countryside, where management of the land has been

reduced through schemes such as set-aside. By 2007, the spe-

cies had become the most abundant dicotyledonous plant

recorded in the Countryside Survey (Carey et al. 2008). Thus,

no specific measures are necessary to conserve the species. In

many semi-natural habitats, U. dioica is an important compo-

nent of the ecosystem. It is the food plant of the larvae of a

number of very attractive butterflies and other phytophagous

insects (Davis 1991; Wheeler 2005; see also IV). Pullin (1986,

1987) tested the hypothesis that the interruption of normal

plant phenology inU. dioicamay provide opportunities for the

insect herbivores Aglais urticae and Inachis io to escape the

nutritional constraints imposed by their food plant. Leaf water

contents and nitrogen levels were high during the spring fol-

lowed by a decline during flowering in June. Subsequently,

plants subjected to summer cutting showed increased water

content and nitrogen levels in the regrowth or complete

renewal of the foliage compared with the control. The caterpil-

lars exhibited a correlated improved feeding efficiency, faster

development, greater mass at pupation and possibly increased

fecundity.Aglais urticae normally completes two generations a

year, but a third generation occurred due to the increased

availability of nutrients.

The young shoots ofU. dioica have many uses including the

following: as a green vegetable; a pot herb; Cornish Yarg, a

modern speciality cheese, is wrapped in its leaves; as a medici-

nal plant with numerous beneficial properties; as an herbal

infusion; when dried as cattle cake fodder; and as a beverage,

including the recent production of ‘Stinger’, an organic ale.

The strong fibres of the plant can be used as a substitute for

linen, cotton and wool to produce woven garments (Wheeler

2005).

However, in many situations, Urtica dioica is a persistent

weed. It will not withstand repeated cutting and this is a recom-

mended means of eradicating it from arable and managed

grassland, whenever new shoots reach a height of 15–30 cm

(Greig-Smith 1948; Srutek &Tecklemann 1998). The plant can

also be controlled by the use of any of the herbicides simazine,

atrazine, bromacil, monuron or diuron, glyphosate, sodium

chlorate, dichlobenil and picloram (Fryer &Makepeace 1978).

Hipps et al. (2005) emphasized the difficulty of establishing

semi-natural woodland vegetation in the presence of the

shade-tolerant competitor U. dioica, especially in situations

where new woodlands are planted on fertile, ex-agricultural

soils containing large residual concentrations of P. Manipulat-

ing soil pH as a means of facilitating the establishment of

woodland indicator species in new farm woods is unlikely, in

the short term, to be effective where U. dioica is present (see

IV).

However, U. dioica ssp. galeopsifolia apparently has a more

restricted distribution and further survey work is needed to

determine howwidespread it is in the British Isles. Once the full

geographical distribution and abundance is known, then its

conservation status can be assessed.

Acknowledgements

I am indebted to Dr Russell Sharp for determining stomatal densities. I thank

Stephanie Ames, Biological Records Centre, for providing the map for Fig. 1,

Dr David Roy who supplied information from the Phytophagous Insects Data

Bank and Martin Godfrey for unpublished information on ssp. galeopsifolia. I

also thank Chris Preston, Michael Proctor, David Streeter andMike Usher for

their helpful comments on the text and additional information supplied. I grate-

fully acknowledge the useful comments of Tony Davy on the text and for his

detailed editing of the final manuscript.

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