VUCETICH AND PULLAR: SOILS

The Rotomahana shower was a most un-usual volcanic event, but nevertheless strik-ingly demonstrated the resilience of indigen-ous vegetation.

Ash showers may not always have becncatastrophic for vegetation, and even whereforest may have been obliterated by ashflows a return may not invariably haveinvolved a long time and protracted plantsuccessions. Long-lasting changes may beconsidered probable only where soils wercradically changed.

REFERENCESBIELESKI,R. L., 1959. Factors affecting growth and

distribution of kauri (Agathis australis

65

Salisb.); III, Effect of temperatuJ'e and soHconditions. Aust. J. Bot. 7: 279-294.

CRANWELL,L. M., and MOORE,L. 8., 1936.1 The occur.'rence of kauri in montane forest on Te Moe-hau. N.z. J. Sci. Tech. 18: 531-543.

I

KIRK. T., 1872. Notes on the flora of the Like districtof the"North' Island. Trans. N.Z. Il1st. 5: 322-345.

I

MASTERS,S. E., HOLLOWAY,J. T., and MsKELVEY"P.J., 1957. The national forest survey (1 New Zea-land, 1955, Vol. 1. Gavt. Printer, Wellington.

MILLENER,L. H., 1953. How old is the velgetation onRangitoto Island? Rept. 2nd Ann.1 Mtg., N.z.Ecot. Soc. 17-18.

NICHOLLS,J. L., 1959. The volcanic erupiions of Mt.Tarawera and Lake Rotomahana Imd effectson surrounding forests. N2. J. For. ,8: 133-142.

ASH BEDS AND SOILS IN THE ROTORUA DISTRICT

C. G. VUCETICH and W. A. PULLAR

Soil Bureau, Department of Scientific and Industrial Research, Christchurch: and

Whakatane

INTRODUCTION

During the Late Quaternary, volcaniceruptions of the explosive or paroxysmaltype (Taylor, 1953) occurred in the centralNorth Island about centres, which for con-venience, are designated Okataina, Waita-hanui, Maroa, and Tokaanu (Fig. 1). Theresulting ejectamenta formed thick layeredbeds largely of rhyolitic pumice. Most isknown about the distribution of beds fromthe Okataina and Waitahanui cruptivecentres where named beds have beenmapped in the Rotorua, Bay of Plenty, andGisborne districts (Vucetich and Pullar, inpress). Little is known about the Maroa andTokaanu centres except that they are con-sidered to have been important contributorsearlier in the Holocene and that they werealso active contemporaneously with theTaupo Pumice eruptions.

STRATIGRAPHY

The following ash beds have been recog-nised; the age where known is given inyears before 1950:

Named Beds ,

Tarawera scoria (and Rotomahana mud) erupted1886Kaharoa Ash 810+ 70'Taupo Pumice 1700+ 1501Taupo Subgroup, members 9-13Waimihia Ash 3420:t:70'Rotokawau AshWhakatane AshMamaku AshRotoma AshTaupo Subgroup, members 16--18 8850+ 1000'Waiohau AshRotorua AshRerewhakaaitu Ash

Unnamed Beds "'-

"'X' Beds"; (Ash and lapilli) three in number. inRerewhakaaitu locality and believed of restricteddistribution; not mapped because of I few expo-sures.

*"Pinkish-brown Beds"; multiple, of lash grade,weakly weathered; mainly northern p:trt of Kai~ngaroa Forest, Wairakei, Waiotapu and :~otorua.

*"Mauve Beds"; three in number' TIainly ".onMamaku Plateau. . ,

""'f'.. . ,., ,

"Yellow-block Bed"; mainly along Bay of Plenlyand. in ,Gisborne District; not traced

I

south andwest of Rotorua. .

* The "pinkish-brown" and "mauve" bids are notseen in stratigraphical order in one section andmay be coeval. Mapping of these together withlower.-beds is proceeding.. ,;.

I

.

66 VUJ.l"'A!\JTrTTV Af\JO

,"Whlte--block. Bed"; mainly along Bay uf Plt::Jlty;not traced west and south of Rotorua. (Both of theCiUUVC lUUY be a correlative of Ka:wcrnu Brcccit1.)

"Grey-banded Bed"; wide distribution from westof Tauranga to Gisborne 'but southernmost limitsnot kn01Nn.' (CoJ:'J:"elative of Rotoiti Bre<'c-i~.)'~[

,"Red Beds" or "ginger tuffs" '(Hamilton correla-tive); cOrI'Hated with known Hamilton beds westof Taurang;l and at Matamata; traced along Bay ofPlenty to Ciisbomei perhaps the most widespread

of aU a~h-fijll beds,,I

I

1. N.Z. lAC. 10. N.Z.J. ScL Tech., B36; 371-4(1954). I,l. N.£.. HC. 1. N.L.J. 5C1. Tech., 113.:5; 127-5 (19.:'13).3. N.Z. 14C.179

}N.Z.J. Geol. Geophys., 2: 208-41

N.Z. ItC. 165 (1'15'1)

ERUPTIVE EVENTS

The ash column records events separatedby considerable time intervals. Each eventc.i1u~cd c(n:nplctc or portia.! {' dC:5truction; of

the vegetation within an inner zone and,dm1ng the ensuing period I of quiescence,

veg,etatioll established itself and soils beganto lorm. ] n the preceding cold period (LastGlaciation) the ash column shows erosionbreaks mid it would appear that a numberof orupti~)n.c took plo.cQ ,"vhon tho. upland~

were bad, of vegetation and there was ex-tcn3ivc clo~ion.

I

Detailed studies of the Taupo Pumice and:soils dtai;/cd frOID it are uscd as a bash for

the pattei'n of forest destruction and for asuggested history of the forest during theLate Quaternary.

hnC<"J.OI of '\'ULO UIT nnd. '\':COC'ThTIOH

DESTRUCTION

In Ihe viclnlIy of the vents, blast amiglowing, avalanches' were the principalagents of destruction and shower-beddeddepo!i:itli:

I

beyond are cnnc,;:idprprl to h~vf"

f~1l9n l"l:\lA Qnrt tn h~"p Idl1Prtvpgpt~tinn hy

,burial. This principle is well illustrated inthe products of Jhe Kaharoa eruption wheret.:harrt::u lug5 al~ :st:::eu in locali5cd ash.flow

deposits dropped by glowing avalanches onthe' lower slopes of Mt. Tarawera" but theash-fall deposits lurther away do not con-tain charcoal. Charcoal occurs sparingly inash-fall deposits and is usually to be foundin contact with or within the buried soilrather thi~n in the overlying ash bed5.

,

Produt.:lS of ash-flow uroppcd by glowingclouds or nuees ardentes are often wide-spread and more difficult to plot because ofviolent eruption occurring simultaneouslytrom a number of vents:. The cloud~ tr~vPllf"CI

long distances, transporting vast quantitiesof pumice and ash at temperatures sufficient-ly high to char wood in their passage across

the landscape. Deposits from one eruptiveepisode vary according IO LOpography; Indepressions and on valley floors, particu-larly towards the foot of hill slopes. theyare characteristically thick pumice brecciasfroquontly containing charco~l, ~nd onclopoc, they arc ach brocciac Q good dQ~l

thinner but of irregular thickness.I I

Vegetatjon wa~ a]~o drowned or buriedby water and alluvium impounded in tem-porary lakes.I

From ash-fall plans prepared by Vucetichand Pullar (in pres:s:) the area covered byash more than 24 in. thick is 5,000 squaren'lilc.3 j 1TIOrc than 18 in., 7,000 .3quarc milc,:,;and nJ.on; than 12 in., 12,000 "quarc milc",

I in the combined districts of Rotorua, Taupo,Bay of Plenty, and Northern Hawke's Bay(FIg. I).The critical thickness of cold ash required

to destroy vegetation cannot be given withprecision. On Mt. Egmont A. P. Druce (pers.comm.) observes that the Burrell Ash

deposited cold some 300 years ago had thefollowing effect on cedar and Hall's totara;13 In. [lUck compleIe desIrucIlon12-15 in. almost complete destruction9-12 in. partial destruction

ThUUli:l:S (1000) levun:s ..hi:l.. ..ltt::: fUlt:::Sl Wi:lS

not destroyed on Mt. Edgecumbe whereTarawera scoria and ash' fell 14 in. thick.If a conservative estimate of ash thicknessfor complete destruction is 18 in., then des-truction on at least one occasion occurreduna iUI ct:u::::ct uf 7,000 ~y'UctH::: utik:s.

TAUPO PUMICE ERUPTIONS,

A :scdc:s of cl-uptions:: frol11 ccntl.cs aL

Waitahanui, Maroa, and' Tokaanu andoccurrtng at short mtervals are known asthe Taupo Pumice Eruptions. They termin-ated with particularly violent explosionsfrom vents which appearito have extendedbeyond the centres named and with ash-flQW material pend.5ting for long d15tal1ct:::t.

VUCETICH AND PULLAR: SOILS 67

FIGURE 1. Area in which forest was destroyed by vulcanicity on at least one occasionduring the Holocene. Destruction is postulated to be complete with deposits greater

than 18 in. thick and partial with those 12-18 in. thick.

The timetable, form of eruption, and pat-tern of vegetation destruction is given belowwith particular reference to the Waiotapu-Oranui locality.

Phase 1

Initial ash-fall deposits from centres nearWaitahanui; Hatepe Ash, "putty colouredlayer", and Rotongaio Ash caused localiseddestruction mainly by burial.

Phase 2

Widespread deposits of Taupo Lapillimainly from Waitahanui are shower beddedand caused localised complete de,;tructionby burning and blast, and by burial in anouter zone. Large trees remained standingthrough 18 in. thick deposits.

Phase 3

After a short interval a violent eruption

ERUPTIVE CENTRES

12" lSOAIrICH

18" ISOPACH...-

68 VULCANICITY AND VEGETATION

of the ":hyolitic block member" gave rise to,

a wide, pread thin bed of rhyoliticlap,illi. "

This eruption ~ppears to have its sequeni1'"

the Maroa centre where, on slopes nearOrakei{orako, rhyolite ash and lapilli andpumice lapilli form thick, mainly loose,deposit,! of irregular thickness. In valleybottom,' massive breccias up to 15 ft thickare cha:acterised by discontinuous verticalfissures with inclusions of loose ash and'

"..c'-

chalazo dites ( volcanic hailstones). Thiseruptioll caused localised forest destructionby bias c and burning as well as by burial.

Phase"

Uppe;' Taupo members include multipleflow bEds of indurated ash (and pumicelapilli) )n the uplands thinning off as slopeincreaSE s, and thick pumice breccias of the

"

valley f: oors. Some hill slopes in the lee' ofthe bel'eved direction of blast have little,or no a:;h deposit. '

e,

The pattern of breccias indicates thatnuees,Nhich were to some extent chan-nelled t y the Paeroa scarp into the Waikiteand Whirinaki valleys, were partiallyobstruc,ed by the orographic barriers ofrhyolite domes and Horohoro Bluff anddeflecte 1 to the west. The source of thenuees j,; not known but the occurrence ofthick heccias containing pumice blocksnear Ol'akei Korako suggests

ee that therewere eruptions from this placee'e The limitof breccia and ash-flow beds is also the limitof fore,t destruction. Forest would be des-troyed by blast, ,eby_bmning or charringwhere ,engulfed by nuees, and by the im-poundir g of valleys for a short period whendrainag:,e was obstructed 'by pumicebreccia,; for example, Reporoa valley,which ,':as a lake below the 1,070 ft contour.The area of destruction is roughly 4000 sq.miles, but it is believed that remnants offorest nay have survived on thinly coatedslopes v,'ithin this area and so.provided plantmateria, for eventual colonisation.':'..,,_; ..1.":'';1';,,, '

..-

e

Phase 5!

A final phase with source vents in theMaroa-1\[okai-Oranui area locally producedthick pumice breccias with restricted distil:butionJmt with more widespreadash-fall.""

,

,-r"'c,

TEMPERATURE OF ASH-FLOW.,., ., .

,Ash-flows must have had a substantialburning effect on the, vegetation, but whilethe minimum temperature required to charwood is 250"C, there is no sign of bakingof a buried soil and this would suggestlowest temperatures at the base of depositsand a temperature gradi,ent within the over-lying ash. If the buried soil was not appreci-ably heated seeds, rhi>:omes''.' and fungalhyphae may be expected' to' survive andbecome important plant:;material to colonisethose,hill slopes which, although overrun bynuees, are thinly ash c'oated. These viewsmay have substance from the, observationsof Miller et al ( 1955) who recorded tempera-tures as high as 600"C during scrub fires atTaita Experimental Station.

SOILSe

Profiles of the buried soils are comparedwith present day soils, and matched fordevelopment of pedologic horizons anddegree of weathering as ,indicated by colourand content of amorphous clay minerals.With few excepMons the evidence suggeststhat'forest reclothed. each successive ashdeposit but there is little indication as tothe rate of recovery.

Soils derived from Kaharoa Ash

After 800 years, subsoil characters arevery weakly expressed in the loose, coarseKaharoa Ash. Under podocarp-dominantforest along Hongi's Track ne"";m Lake

eRotoiti, 'subsoilsee are '"

hiimus stained andlenses of fine ash are weakly cemented' withhumus and iron. In the :>Kaingaroa :Forestand Galatea basin (Vucetich et aI, 1960) pro-files are more strongly developed on hillslopes under a manuka scrub community(Lepto.spermu111,:scoparium) and least so onflattish sites in both basin and plateau undera manoao (lJracophyllum;suljulatum) andtussock (Poa caespitosa) cover. Differenoomay be explained by a relatively, unfavour-able microclimate. .' ,

Unfortunately, .firing of thevegetatio(l,bythe,early ,Maori makes :interpretation difli'ctilt:.: This point is well illustrated nearKafngaroa Forest Headquarters, where cadre"~Rere subsoil from "Kaharoa Ash shows dis-

VUCETICH AND PULLAR: SOIl.S

rupted shower bedding about a site onceoccupied by a tree with a basal diameter ofabout 10 in. A thin weakly-stained lens offine ash in the subsoil with further ironstaining in the buried Taupo Pumice soilbelow indicates that a podzolising tree sur-vived a 12 in. thi2k deposit of Kaharoa Ashand, furthermore, continued to podzolisethe new material. But this forest was des-troyed by a fire prior to European settle-ment.

Soils from Taupo Pumice

Present day podocarp-dominant forestshave left a marked imprint in Taupopumice soils particularly on the hills. Soilhorizons are best exhibited in a 'saucer-shaped soil, podzolised under rimu (Dacryd-ium cupressinum) which has, under,itsmor-litter, it pale brown, bleached A2 and adark brown, humus illuvial, B2 horizon.These features show up well in sOme 24 in.of ' indurated silty sand, representative ofthe ash-flow type of deposit, and the podzolB 'horizons are well preserved where theforesfhas been destroyed and where a blacktopsoil, enriched with bracken fern or scrubhumus, has subsequently formed. By com-parison, broadleaved trees produce mulllitters arid a humus-rICh' A horizon over athin B except where grown on sites pre-viously occupied by rimu or similar pod-zolising trees, e.g., where rata supplants ahost rimu, a mull litter and a humus-richA overlie the podzol B horizon.,

- ,"

,'

" Soils derived from thick ash-flow beds on";'

flattish basin sites are commonly shallowwith a pumice pan restricting plant roots.' Acommon profile under manoao or manukascrub shows a black topsoil resting on adark brown subsoil stained reddish-br.9wnand weakly cemented, and passing abruptlyto a massive silty sand at 10--12 in.

On the uplands, near Waimihia, scrubsoils have similar characteristics also attri-butable to podzolisation. Taylor (1953) didnot consider these shallow rooting soils tobe normal "pumice podzols". But the weightof indirect evidence strongly suggests a pre-vious shallow-rooting forest. Such a forestwould be vulnerable to fire and may explaincomplete destruction before Europeansettlement.

69

Profiles suggest a good recoveryI

of theforest after the Taupo eruptions, beijIg mostrapid on the hills where pockets 'jf plantmaterial survived and, then e

~

pandedslowly across lowlands and plateaux It maybe inferred that recovery wouldha(e beenquicker after the ash-fall type of eJuption,which destroyed smaller areas of for\,st, andwhich provided a more suitable loose parentmaterial for forest growth. "

,

Buried Soils and Past Vegetation

Information gained from prof les ofpresent-day surface soils and from ~inds ofvegetation growing on them is use\l for a~rovisional interpretation of

.the

Jvegeta-

tlpnal and climatIc history dunng tjIe LateQuaternary.,

"

Apart from the black scrub soilsI

(Kaha-roa Ash) buried beneath Tarawera orRotomahana ashes, almost all buri~d soilsare brown or yellowish brown in Jcolour.Distinctive A horizons are not gcnerallyfound and the buried soils comprl se onemain horizon over yellowish brown! parentmaterial. Most of the buried soi~s haveirregular thickening of the old B horizonswith tongues of similar material extending10--12 in. below. The best example of'tonguing' occurs in soils bearing fine-tex-tured, indurated lenses, and having I a paleyellowish-brown colour penetrattd bystrongly coloured dark brown tlmgues.These features together with the infl equentoccurrence of charcoal fragments in ],ocketsare indicative of forest. The followipg is abrief, inventory of characters for

I buriedsoils:

,--1,;01-, .

Rotokawau Ash; basic, but containing rhyoliticash; one of the most weathered of I-~olocenea~n~s (sandy loam texture), and tHe moststrongly and uniformly coloured; comm9.nly red-dish yellow; very limited area; stratigraphicrelationship to Waimihia Ash not known I

Waimihia Ash; more uniformly humus Jnd ironstained than overlying present dayl TaupoPumice soils and considered to br moreweathered but not yet confirmed by clay

I

mineralanalyses (note that comparison IS ma;:le withsoils derived from ash-fall' deposits o~ TaupoPumice east of the limit of ash-flow deposits).The climate may have been slightly warn1ler thanafter the Taupo Pumice eruptions.

I

Whakatane Ash; yellow brown silt~, sandand sandy loam; texture distinct from greyshower beds at base.

- - t

Mamaku Ash: sirong brown or brighil yell()w.

70 VULCANtCtTY AND VEGETATiON

brown coloured soil, more weathered than eitherWhakatane above or Rotoma below; soil colourdistinctive for separating these beds.Rolome Ash; blackish brown soil contrastingsharply with marked shower bedding at base>Taupo subgroup members 16--18; str<?ng brownsandy 'oam for fine ash and yellowIsh brownsilty sand for coarse ash; comprises several ash-fall acrretions some probably from the Maroaeruptive centre; fine flecks of charcoal from soilon member 18 give an age of 8,850+ 1000 years,but the sample contained charcoal and organicmatter existent in the soil before burial and sothe result gives a maximum age and not neces-sarily the time of eruption.Waiohl/U Ash; silty sand or sandy loam and thecolour is similar to Taupo subgroup 16-18; bearsforest imprint.Rotorua Ash; soil is the most weathered of theHolocctlc ash beds; sandy loam throughout withstrong-brown or yellowish brown colours andcharac !:erised by having fine pumice lapilli in thesurfac{' horizon; colour and texture remarkablyconsta;1t over a range of sites and rainfall condI-tions and indicate soil formation under anappreciably warmer climate than today; littleclear Jorest imprint but occasional pockets ofcharcoal discovered; no features suggesting pod-zolisat'on attributable to kauri trees (Agathisaustrai'is ).

The fine ash beds which overlie and seal theRotonr.a Ash provide a strong contrast inweathering WhICh supports the premise thatweathering is relatively ineffectual after burial.Nor h there evidence that the Rotorua Ash ismore basic in composition or weathers morereadil~' than other pumiceous materials. RotoruaAsh is thus an important marked bed in Holo-cene [\sh.fall deposits but no time span can beallotted until 14C dates have been determinedon sanples already collected.*

Rerewhakaaitu Ash; the basal named bed of theHoloc,~ne ash; brown in colour but less weath-ered than Rotorua; bears forest imprint."X Beds"; three in number and each of limitedextent; silty sand texture; weak forest imprint."Pinklsh.brown beds"; multiple beds, sand orsilty ~;and texture; no distinct pedologic hori.zons; no evidence of forest."Maut'e beds"; multiple beds with weak pedo-logical features; fine ash weathered to sandysilts, ~;ilty loams, and silty clays; firm, compact,massi-;/e; no evidence of forest."White block bed"; fossil soil not conspicuous."Grey banded bed"; pedological features weakand no definite forest Imprint; at Te Puke, fossilsoil a sandy loam while at Opotiki more a"slipp~ry" loam and silt loam.

* Age oj the Rotorua Ash is considered to be lessthan 15,000 years for the following reason: atPiarere, and along the Waihou River from Te Poito near Te Aroha, the high level terraces of Hinu.era F( rmation (pumice alluvium) are capped bytwo 2sh-fall beds, the upper one of which isthought to include Rotorua Ash. Two samples ofwood have been dated from the Hinuera alluviumand tbe younger age is 15,000 years.

"Red beds" (correlative 'of Hamilton beds):pedolo~ic features strong induding colour ofyellowIsh red and dark red, clayey textures, andstrongly developed blocky structures with clayskins on faces of aggregates: slippery and non-

-sticky; much amorphous clay mineral- allo-phane; most distinctive soils of all pyroclasticdeposits in !he Rotorua, Taupo, Bay of Plentyand Gisborne districts. No ObVIOUSforest imprintnoted but charcoal occurs sparingly in the lower"grey banded beds" and Ihis implies forestdestruction.The evidence from all named beds is that

forest was the main vegetation on pyroclasticserupted during the Holocene.

NATURAL HISTORY AND THE PATTERN OF

VULCANICITY

On uplands, Holocene ash deposits downto and including the" Waiohau Ash arewholly mantling beds and without uncon-formities; but the older beds are not alwayspresent and their absence is an indication ofthe incidence, degree and time of erosion.All older beds are present on the westernmargin of the Mamaku Plateau towardsTirau but they are absent on the crest of theMamaku Plateau, where Rotorua Ash restsdirectly on ignimbrite,' and largely so onKaingaroa Plateau where Rerewhakaaituash mantles a thin weakly-weathered pink-ish brown soil containing fragments ofignimbrite; on Taupuke Saddle (HuiarauRange) the same ash rests on "pinkish-brownbeds" which in turn rest on fragmented grey-wacke. In the Gisborne district all olderbeds are present on a high level terracealong the coast at Uawa, on Hamurian shaleat Whangara, and on, a terrace remnantamidst spectacular accelerated erosion, inthe headwaters of the Waipaoa River. ,

From this rather fragmentary evidence,a major erosion break is established some30,000 years ago when the older Hamiltonbeds were stripped from the uplands anderoded from greywacke ranges to the east.The" grey banded beds", probably eruptedabout this time, were also stripped fromthese surfaces as were later deposits duringthe last glaciation - "pinkish brown beds"from some uplands and "mauve-beds" fromthe crest of the Mamaku Plateau.

In Figure 2 trends' in climate are ex-pressed relative to weatheredness of LateQuaternary ash beds. A weathering indexhas been determined using 14C dates of

Rotoma----------.-

hg 16-18

Waiohau---------..

Rotorua----------R~rewhokaai-----------'x'be~'x' be-----------'x' bed-----.---_.-inkisn-brown'.-rriou.Ye-------ello'W.block'~-fi.C-Dl--1i''w I eo OC'

-feYMDon(le:r

,

VUCETICH, AND PULLAR: SOILS

eruptive events, and a date previously dis-cussed establishes a tentative chronology forthe older beds. A zero rating (0) for "pink-ish-brown", "mauve", "yellow-block" and"white block" beds implies slight weather-ing but not sufficient to rate as (1).

Warmer temperatures are marked by diS'tinct soil formation on the unnamed "X"beds and a further increase in temperatureby stronger weathering of the Rerewhaka-"aitu soil. An interval for a "thermal maxi-mum" is acknowledged during the Rotoruaepisode extending with perhaps decreasingtemperature to the time of eruption of TaupoSubgroup members 16-18. From the Rotomaepisode to the present day the record showsrelatively little change although a compari-son of Taupo and Waimihia soils each with

the same time of soil formation would indi-cate a slight decrease in the intens'ty of soilweathering after,the Waimihia eruption.

CONCLUSIONS

The impact of Holocene and Late Quater-nary vulcanicity on the vegetation i 1 the cen-tral region is summed up as follow, :

1. Has speeded the establishment "f vegeta-tive cover on the, eroded uplands afterthe Last Glaciation by providir g a root-ing medium over fretted ignimhrite, andlikewise, speeded colonisatior of thegreywacke axial ranges.

2. Caused subsequent short-lived destruc-tion of forests around eruptiv" centres.An area of 5000 square miles has beenaffected at one or more times during theHolocene.

3. Each eruptive event has mantled a largezone beyond the central zone OJ'destruc-tion with an accretion of fresh mineralsand so offset the progressive degradingof soils by weathering, lead ing, andpodzolisation. Benefit is optimL m wherethe accretion is 12 in. or less oi' fine ashand where the vegetation is m,t greatlydisturbed.

"

4 . Each accretion increased the thicknessof ash mantle on the steep slop es of thegreywacke ranges and has assured pro-tection from erosion and so providedwatershed control.

5. Taupo Pumice eruptions were the mostdevastating to vegetation.

6. Thick deposits of Taupo Pumice on theKaiweka Range, for example, are highlysusceptible to actual or potentid erosion(A. P. Druce pers. comm.).

7. Indirectly, the effect of vulcanici ty is alsodemonstrated in the widespread destruc-tion of forest following the intlOductionof fire by man. Yellow-brown pumicesoils, unlike the more weath"red andmore moisture-retentive yellcw-brownloams and yellow-brown 'earth" dry outperiodically to a depth of abc ut 12 in.(Will, 1962) and at these times theirvegetation is susceptible to lire. Thescrub vegetation at the time of Europeansettlement was largely inducer. by fire

FIGURE Z. Trends in climate expressedrelative to weatheredness of Late Quarten-ary ash beds. A weathering index is basedon 14 C dating of four volcanic eruptions.

72 VULCANICrTY AND VEGETATION

andnct by vulcanicity. Succession,fromfire-induced. scrub to forest" ,hashas been prevented or slowed down bysoil d,'gradation including both strongleachirg and unfavourable physical pro-

.pertIes.

REFERENCES "

MILLER,R. B., STOUT,J. D., and LEE"K. E.; 1955:Biologica and chemical changes following scrubburning (In a New Zealand hill soil. N.Z.J.Sci.Tech., B37; 295. ,

"'

TAYLOR,N. H., 1953: The ecological significance ofthe central North Island ash showers - The soil

pattern. Rep. 2nd Ann. Mtg. N.l. Ecol. Soc. 11-22.THOMAS,A. P. W., 1888: Report on the eruption of'Tarawera and Rotomahana, New Zealand. Govt..'Printer, WeHington.VUCETICH,C. G., et aI, 1960: Soils, forestry, andagriculture of the northern part, Kaingaroa StateForesl and the Galatea Basin. NZ. Soil Bur. Bull.18. '

VUCETICH, C. G., and PULLAR, W. A. (in press):The stratigraphy of Holocene ash in the Rotoruaand Gisborne district. Part II. N.l. Geol. Surv.Bull. n.s. 73.

WILL, G. M., t962: Soil moisture and tempera lurestudies under radiata pine, Kaingaroa StateForest, 1956-8.N.l. J. Agric. Res. 5; 111-20.