M. Luisa Martinez and Patricia Moreno-Casasola

Journal of Coastal Resear ch 162-174 Royal Palm Beach, Florida Winter 1998

The Biological Flora of CoastalDunes and Wetlands: Chamaecrista chamaecristoides(Colladon) I. & B.M. Luisa Martinez and Patricia Moreno-Casasola

Depto. de Ecologia VegetalInstituto de Ecologta, A.C.km 2.5 Antigua Carretera a CoatepecXalapa, Ver. 91000 Mexico

ABSTRACT _

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MART1NEZ, M.L. and MORENO-CASASOLA , P., 1998. Th e Biological Flora of Coas ta l Dun es and Wetl ands: Chamaecrista chamaecristoides (Colla don) 1. & B. Journ al of Coastal Research, 14(1), 162-174. Royal Palm Beach (Florida).ISSN 0749-0208.

Chamaecris ta chamaecristoides (Colladon) 1. & B. is a tropical shru bby legume end emi c to th e Mexican coas ta l sa nddu nes of the Gulf of Mexico and to a lesser exte nt , of the Pacific coas t . It grow s mainly on mobil e dunes, and it is th efirst colonizer. In this study we pre sen t inform ation regarding th e plant' s geographical distribution , range of habitats,physiological ecology, population biology, reproduction, its role in geomorphology and interactions with other species.Ch. chamaecristoides is very tolerant to drought and low nutrient availa bility in th e soil. Biomass accumula tion andrelative growth rates increase sign ificantly when seedlings are covered by sa nd. Th e legume may be considered as anurse plant that modifie s environme ntal conditions and facilitates the success ional process . Temper ature fluctuati onsin the sha de of adult plants decrea se noti ceab ly when compa re d with bare sa nd temper atures. Further, bacteri al andmycorrhizal associa tions are abunda nt. All these charact eri sti cs prob ably play an important role in dune dynamicssince late colonizers are only able to arrive afte r Ch. chamaecristoides has reached a rather high (2x2m) plant cover .Th e plan t' s hard coat ed see ds germina te gradua lly through out th e yea r, afte r see ds have been exposed to naturallyoccurring temperature fluctuations on th e sa nd surface . Seedling mort al ity is very high during th e first th ree months,and only 5% reach adult stage. Winter storms are a major cause of death, since dr ought and sa nd movement becomedrast ic during these months (November to March ). We found that seedling size plays an important role in surviva land reproduct ion. Larger seedlings are the ones with hi gher surviva l rates and repro duce ea rlier . Ch. chamaecristoidesplants decre ase sa nd movement and hold the subst ra te together, prom oting dune sta bilizat ion. Thi s is importa nt tohumans, since mobil e dunes continuously encroach nearby roads, cities and crop field s. Th er e is no pra ctical experiencein dune stabiliza t ion with this species .

ADDITIONAL INDEX WORDS: Dun e erosion, dune stabilization, coastal habitat , dun e vegetation.

INTRODUCTION

Chama.ecrista chamaecristoides (Colladon) I. & B. is a tropicallegume endemic to the Mexican coastal sand dunes of the Gulfof Mexico and , to a much lesser extent, of the Pacific coast aswell. It is the first colonizer of mobile sand dunes, and its shrubby growth form enables it to stabilize the substrate. Once itgrows and forms monospecific patches that cover approximatelya 4 X 4 m area, those species with a lower tolerance to sandmovement and drought arri ve. Thus, the legume may be considered as a nur se plant that modifies environmental condit ionsand facilitates th e successional process.

Th e plant's showy yellow flowers are produced abundantlyevery year, at the end of the rainy season in the Summer .Th e pods burst open and disperse the seeds. Once on thesand, they are disp er sed secondarily by wind , together withthe moving substrate.

This species is an important component of plant communitieson Mexican coastal sand dunes, and has received much attention over the last years (MORENO-CASASOLA, 1986; MARTINEZ,

96049 received 25 May 1996; accepted in revision 5 January 1997.

1994; MARTINEZ and MORENO-CASASOLA, 1993; 1996; MARTINEZ et al., 1994; MARTINEZ and RINCON, 1993; and unpublished data). However, we have never assembled all the information relative to th e plant's biology. In thi s review we presentinformation about th e physiology, population dynamics and biotic interactions of Chamaecrista chamaecristoides, although different aspects are still not well understood.

TAXONOMY AND VARIATION

Name

Chamaecrista chamaecristoides (Colladon) Greene, 1. & B.section Chamae crista (IRWIN and BARNEI3Y, 1982), family Leguminosae - Cae sa lpinioideae . Subtribe: Cassiinae. Synonym s: Cassia chamaecristoides Colladon; Cassia chamaecrista Mill. Gard. Diet . Cassia cinerea Chamisso & Schle chtendal, Linnaea .

Taxonom ic Descriptio n

Th e following account was based on th e descriptions madeby MCVAUGH, (1987 ) and IRWIN and BARNEBY, (1982 ).

Flora of Coastal Dunes 163

Seed Morphology

Seeds are compressed, light brown, ovate, acute at the attached end, finely shallow pitted, 3.5-5 mm long (Figure 1).The dull fuscous testa is sometimes paler at the hilum.

Seedling Morphology

Each of the pair of cotyledons is ovate, acute at the petiole.The leaves are alternate. The immature seedling does notresemble the mature plant until it has developed its firstleaves (Figure 1).

Shoot Morphology

Robust or slender shrub, commonly small-leaved but largeflowered. Always psammophilous plants, either monocarpicor perennant and suffruticose. The freely, often divaricatelybranching main stems are diffuse or decumbent, and formmats or low thickets (60 cm tall), up to more than 1 m across.The foliage is concolorous and the leaflets are as variablypubescent as the stems, often glabrous or glabrate above,rarely quite glabrous beneath. The stipules are erect, submembranous, pale-green, turning brownish-stramineous. Theleaves are usually widely arcuate-spreading in expanded outline broadly to narrowly ovate. Leaves fold readily, althoughcommonly, they are spreading in their natural position. Petiole with wrinkled pulvinus with a gland near the middle ofthe petiole. The profile of this gland is obconic to trumpetshaped. There are from 8 to 19 pairs of leaflets per leaf, inserted along the rachis, 6-7 mm apart. Leaflets are decrescent both, up- and downward from below middle of rachis.Leaflets are often falcately incurved, obtuse at the apex andcordate at the base on the proximal side.

Root morphology

Chamaecrista chamaecristoides possesses a long, scarcelybranched root that extends more than 1 m into the soil. Whenengulfed by shifting sand sometimes produces adventitiousroots on main root or on branchlets (M.L. MARTINEZ, personalobservation). Bacterial root-nodules (CORBY, 1981) are characteristic of the genus, and they occur in no other Casiinae.In Ch. chamaecristoides root nodules are located in the sparseand very thin branches. Since roots reach great depths undernatural conditions, it is not known if nodules are distributedthroughout the length of the root or if they are concentratedeither in the surface or in deeper soils. Under natural conditions, the root is long and almost with no ramifications.However, when grown in optimal conditions (in the greenhouse, with unlimited availability of water and nutrients)roots branch profusely and the main root is not apparent.These ramified roots are very well nodulated.

Flowers

Peduncles are supra-axillary, with the raceme-axis up to 2em long. One to three flowers per peduncle. Buds are ovoid,acute, 1-1.3 em long. Sepals are thin, brownish or yellowish,lance-acuminate. Petals are yellow, and pinkish or brownishyellow when dry. Usually, they are very unequal, 3 of them

are shorter, oblong or oblong-obovate beyond the claw. Thelongest petal (abaxial) is flabellate, 13-19 mm long. Eachflower has 10 anthers, which are red or reddish. Ovaries areusually pale-pubescent; style is filiform, 4.5-9 mm long;ovules 19-14.

Fruits

Pods are 25-65 mm long and 3.5-6 mm wide. The purplishcastaneous and ultimately nigrescent in drying valves, arethinly pilosulous or glabrous.

Variability

Subspecies

According to IRWIN and BARNEBY (1982) Chamaecristachamaecristoides comprises three varieties, chamaecristoides,brandegeei and cruziana. The first two are perennial whilethe last one is monocarpic, sometimes overwintering. Theydiffer in the lengths of their leaflets, sepals, petals, anthersand pods. However, typical var. chamaecristoides and typicalvar. cruziana are hardly different in shape and venation ofthe leaflets and differ principally in proportion of flower toleaf. Var. chamaecristoides retains a large longistylousflowers alongside ofaxeromorphic reduction at once inlength and complexity of the leaves. Also, it is separableeasily enough by the syndrome of a perennial root, while theroots in var. cruziana are annual.

Ecotypes

Chamaecrista chamaecristoides var. chamaecristoides hasan erect growth-form along the coasts of the Gulf of Mexicowhile the populations from the Pacific are prostrate (P. MoRENO-CASASOLA, personal observation). On the other hand, aseries of populations along the Gulf coast northward fromTampico (mainly Texas) also show a prostrate growth formand have been considered as Ch. chamaecristoides var. cruziana. However, according to IRWIN and BARNEBY (1982), itis difficult to find two populations in this area in which theleaflets, the displacement of the midrib, the form and stipitation of the petiolar glands, the number of leaflets andgrowth-form are strictly identical. This mass of intergradientmaterial generates a taxonomical problem with different solutions. One of them has been to interpret the prostrate duneecotype as a northern extension of Ch. chamaecristoides. Another solution has been the suggestion of considering Ch. chamaecristoides as a multiracial megaspecies in which othertaxa should be incorporated. On grounds of overall morphological similarities and dissimilarities, the authors decided tomaintain the traditional species separation of this controversial group, considering it as a single species with three varieties and a close relationship with other species, such asCh. rufa, Ch. fasciculata and Ch. pedicelaris.

GEOGRAPHICAL DISTRIBUTION

The species is entirely Mexican and var. chamaecristoidesis the one that has been studied more thoroughly. In thisstudy we present information of this variety. Ch. chamae-

Journal of Coastal Research, Vol. 14, No.1, 1998

164 Martinez and Moreno-Cassasol a

Chamqurtdq. chama.cris/o/(IeSCo/. /1. Mg. r 13. r; /I,m SCn If'!f6

f3mga et'R. r:Q? [)C//JII S 1/(,

! Ills- E. Sb-ttverlra.

Figure 1. Chamaecrista chamaecristoides var, cham aecristo ides (a) mature plant with flowers and fruits; (b) two and six-days old seedlings; (c) seed.

J ournal of Coas tal Research, Vol. 14, No.1, 1998

;<:J~c·~c

°c• var. chamaecristoides (9~,.,• var. cruziana

A var. brandegeei

Flora of Coastal Dunes

USA

1

1

1

1

Mexico

1

1

1100° W

330~ 30I 270 26

l5 210 22

l! 150 18a.~ 90 14

II. 30

165

Gulf of Mexico

Figure 2. Distribution of Chamaecrista chamaecristoides and its three varieties along the Mexican coasts. Var. chamaecristoides is the one with thewider range of distribution. 1. Texas; 2. Tamaulipas; 3. Veracruz; 4. Tabasco; 5. Campeche; 6. Yucatan; 7. Sinaloa; 8. Jalisco; 9. Michoacan.

cristoides has a wide distribution on the Gulf coast from intertropical Tamaulipas to Yucatan and a more local one in thePacific (Sinaloa, Jalisco and Michoacan) (Figure 2), in isolated beaches. Ch. chamaecristoides var. chamaecristoides is locally abundant between the central zone of Tamaulipas andthe eastern tip of the Yucatan Peninsula; although on theextremes of its distribution range it is much less abundant.It is disjunct in the coasts of Jalisco and Michoacan,

The central zone of the Gulf coast (central and southernVeracruz) is the region with the more mobile dune systemsand higher sand movement rates (3 m a year- MORENO-CASASOLA 1982) and where populations are more numerous andindividuals more vigorous. Only Croton punctatus and Palafoxia lindenii are able to inhabit leeward and windward zonesof active dunes together with Ch. chamaecristoides. The restof its distribution area includes parallel dune ridges wheresand movement is less intense and vegetation cover is higher.Under these conditions Ch. chamaecristoides is associatedwith several other species and is more scarce.

Var. brandegeei is known only from the shore line of Sinaloa in the Gulf of California, while var. cruziana is abundantfrom Aransas Bay, (coastal South Texas) to the Veracruz-Tamaulipas border.

RANGE OF HABITATS

Zone of Occurrence

The three varieties are found in dunes, beaches and sandyflats behind barrier beaches. Ch. chamaecristoides var. chamaecristoides's most typical habitats are foredunes, protectedzones behind the foredune and active dunes (Table 1). It isfrequently found in mono specific patches, in microenvironments with high sand movement rates. It has also been collected in the backshore, above the high tide mark, and alsoin humid slacks, but importance values are much lower. However, individuals along the beach and humid sites are notvery vigorous.

During three years sand movement was measured atmonthly intervals in a semimobile dune system in Veracruz.Sand accumulation and erosion was quantified by using fixedstakes (MORENO-CASASOLA, 1982). Vegetation was also analyzed in a 2 X 2 m quadrats placed at each stake. A total of53 plots were sampled. Ch. chamaecristoides was found in 38of the samples, and it showed high cover values in 75% ofthem (MORENO-CASASOLA, 1986). Cluster analysis indicatedthat the floristic group Palafoxia lindenii-Ch. chamaecristoides established mainly on both windward and leeward slopes


166 Martinez and Moren o-Cass asola

and also on the arms close to the dune cres t. Mean abs olutesa nd movement in a three year period was: 75.8 :t 67.9 (ranging from 8 to 156 em). A second floristic group was formedby 18 samples in which Ch. chamaecristoides was almostmonodominant. Thi s group also establishe d on the slopes,arms and crests where mean sa nd movement was 70.2 :t 54.3ranging from 8 to 181 em, Th e third group included Pectissaturejoides with high cover values, and es tablished mainlyin the arms and hollows wher e sa nd movement was considerably less (25.1 cm :t 27.2 and ranged from 0 to 81 cm ayear).

Substrate Characteristics

Substrate characteristics from the beaches and coastaldunes found along the Gulf varied with latitude. Along thestate of Tamaulipas, its northernmost limit of distribution,pH varied between 7.6 and 8.5, organic matter percentageranged from 0.59 to 2.02, 60-95% was fine sand, part quartzand part calcium carbonate (the latter varied very much betwe en sites: 3-43%). In Ver acruz and Tabasco the followingvalues were obtained: pH 7.4-8.0 , organic matter perc entage0.65-1.17; 57-94% was fine sa nd, quartz sa nd bein g predominant and calcium carbonate va ried betw een 17 and 23. InCampeche pH ran ged between 7.3- 8.5, organic matter percentage varied from 0.65- 2.08, medium and fine sand predominated and calcium carbona te conte nt fluctuated around90%. Organic matter was high er in Yucatan (0.8--6.3%) andpH va ried between 8.5- 9.2. Ca ribbea n beaches are composedprimarily of fine calcium carbona te sand (61-94% carbonates)originate d by the weathering of coastal lime stone, shells andnearby coral reefs (GARCIA, 1987; MORENO-CASASOLA, 1982;MORENO-CASASOLA a nd CASTILLO, 1992 ).

Climatic Requirements

Clima te changes along the ea ste rn coast from a sem iaridstepp e climate in th e north (BSx' ) and the eastern tip ofYucatan (BSw) to a tropical humid climate (Af and Am) in th erest of the Gulf and Caribbean (GARCIA, 1988) . Precipitationis seas ona l with the Summer as the main rainy season . Th emost imp ortant climatic element that varies drastically alon gthe coast is pr ecipitation (570 mm or less on the NorthernYucatan Peninsula to 624 in Ta maulipas and 2237 mm inTab asco). Th e rainy season takes place between June andOctober and represents between 69.3 and 83.7% of total yea rly rainfall. In most areas a doubl e summer maximum rainfalloccurs: an ea rly peak in June-July whi ch is followed by awetter peak in Septe mber or October. Low winter temp er atures wit h occass iona l freezing occur in the Northernmostdistribu tion ran ge of the species (Tamaulipas).

Nortes or winte r storms are atmospheric disturbances rest ric te d to the Gulf of Mexico which modify th e cool seas on.Temperatures can dr op as low as 10 °C and rainfall duringthis season represents 10-12% of the total annua l pr ecipitation. Nortes are frequent and 20 to 25 occur from October toMarch (THOM 1967 )- and bring strong wind s that producecons iderable sa nd movement (MORENO-CASASOLA 1982;POGGIE 1962).

Ta ble 1. Floristic associations in which Cha ma ecris ta cha maecristo idesappears as a dominant species (A) frequent and with high cover values andas an accompanying species, less frequent with high cover (B) or infreq uentwith low cover values (C). Floristic associations are described for theregions shown in Figure 2, in several habitats: beach and [oredune (BCH),mobile dun es (MOB), humid slacks (HUM), protected zones behind the [oredune (PRO) and stabilized areas (S 1'8) . Region I: includes Tama uliposand northern Veracruz; Region II: centra l and southern Veracruz; RegionIII : Tabasco; Region IV: Campeche and Yucatan. In order to make the tableeasier to read, accompanying and not very abunda nt plant species in someof the florist ic associations (num bered) are listed below.

5 Eustachys petraea C6 Ambrosia artemisi ifolia C7 Panicum geminatum C. Solidago scabrida B, Rhynchospora

stellata C8 Eustachys petraea C, Chiococca alba C. Borrichia [rutescens

B, Ambrosia artemisiifolia C12 Cnidoscu lu s texan us C14 Iva asperifolia B, Amaranthus greggii C, Alysicarpus vagina l-

is C17 1'amonea curassavica C18 Porophyllu m pu nctatu m B19 Bouteloua repens B, ln digofera suffruticosa C20 Digitaria run ifolia B22 Cnidoscu lus texanus C, Crotalaria sagittalis C, Citharexylum

berlandieri B, Cissus sicyoides C, Sp orobolus jacq uemontiiE, Malvaviscus arboreus C, Tridax purpusii C, Verbesinapersicifolia C

25 Panicum geminatum C28 Cuscuta am ericana C33 Agave angu st ifolia C, Metopium browneii C, Vigna vexillata C,

Gomphrena decumb ens C, Psychotria erythrocarpa C34 Cuscuta americana C, Borrichia [rutescens C, Agave angusti-

folia E, Metopium broumeii C, Gomphrena decumbens C,Psychotria erythrocarpa C, Lan tana involucrata C

COMMUNITIES

Ch. chamaecristoides is associate d with different speciesalong its range of distribution. Veget ation analysis alon g 44beach a nd dune sys te ms throughout the Gulf and Ca ribbea ncoasts have provided us with a fairly good view of it s geogr aphical a nd local distribution and also its association withother species (MORENO-CASASOLA and ESPEJEL 1986, CASTILLO et al. 1991, and unpublish ed data). Table 1 shows asynthesis of th ese res ults. It always app ears as a companionspecies along th e coast of Ta ma ulipas and northern Veracru z,except in mobile zones where it is ass ociate d with oth er species th at tolerate sa nd burial : Croton pun ctatus , Palafoxia lindeni i. On the beach it is found with Uniola pan iculata whichreaches it s southe rn distribution limit in this region, as wellas with othe r species such as Croton pun ctatus , Ipomoea stolonifera, Sporobolus virgin icus, Ipomoea pes-caprae. Cent ra la nd southe rn Veracru z is its main area of distribution, wheremobile and sta bilized dune sys te ms 25m high are found. It isone of the dominant floristic associations in active dunes andappears as a compa nion species in severa l associa tions ma inly in protected hab itats behind the foredune, and with lowerva lues in humid habitats and stabilized zones. In general as socia tio ns are rich in species. In Tabasco (reg ion II I) it is partof differ ent flor istic groups found in protected hab itats. It isfound on the beach with other pantropical species such asSporobolus uirginicus, Ipomoea pes-caprae and th e rud er alPassiflora foetida. Dun es in this region have been alte red by

J ournal of Coastal Resea rch , Vol. 14, No.1 , 1998

Tab

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2021

122

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124

2526

2728

2930

3113

233

34

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168 Martinez and Moren o-Cassasola

Ta ble 2. Mean dry biomass (g) and root / shoot ratio measured for Chamaecr ista chamaecris toides growing in a green house du ring 140 daysunder contrasting nutrient conditions. S mall letters indicate significantdi fferences between treatm ents at the end of the experiment (one-way AND VA, p < 0.005, F = 35.65).

a)1.0

0.8

0.6

1!

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aa

b

c.... control.... moderately dry--A- spray.-y . very dry

Nutr ient Conditions

Con tinuously poorContinuously richPred ictab le pulsesUnpredict ab le pu lsesSand from th e study site

Dry Biomass(g )

0.12 (0.02) a13.22 (2.4) c3.56 (0.72) b2.13 (0.23) b2.8 (0.2) b

Root/Shoot Ratio

2.28 (0.52) a2.42 (0.62) a1.72 (0.41) a1.92 (0.26) a1.23 (0.44 ) a

0.0 +---,---,---,---,-- -,-- -,---.----

o 20 40 60 80 100 120 140

Time (days)b)

12 -,-- - - - - ------- ---,

10

§ 8Zl~ 6.2m

a 4

2

ocontrol moderate spray extreme

Watering conditions

Figure 3. (a) Seed ling surviva l under contrasti ng waterin g condit ions .Treatm ents as expla ined in text. Differ en t letters indicate significant differences in survivorship curves (p < 0.05). (b) Dry biomass accumula t ionafter 140 days of exposure to differ ent wateri ng treatments . Biomass accumulation in the cont rol lot was signi ficantl y la rger (p < 0.05).

cattle gr azing and coconut plantations. Man y of th e charac teristic species in these associations are secondary speciessuch as Crotalaria incana, Panicum maxim um , Bidens pilosa.Finally, in Campeche and Yucatan it is mostly found as acompani on species in protected and sta bilized zones. It s distribution frequ ency alon g the coast becomes highly irregul artowards Yucatan . Several Caribbean elements appear inth ese asso ciations: Coccoloba uuifera, Ernodea littoralis, Scaevola plumieri, Ambrosia hispida.

PHYSIOLOGICAL ECOLOGY

Physiology

The species is very tolerant to dr ough t. In 1994, MARTINEZ,et al., studied Ch. chamaecristoides's res ponse in surviva l andgrowth under different levels of water availability. In a greenhouse experiment , they applied four watering regim es to 10days-old seedlings growing in sa nd sa turated with 500%Long-Ashton nutrient solu t ion. The following treatmentswere applied: (a) Cont rol. Watering was done every oth er daywith 100 ml of tap water . (b) Moderate drought. Wateringtook pla ce once a month with 120 ml of tap water. The waterapplied in th is treatment was equivalent to the amount ofmonthly rainfall th at usu ally occurs during the dry season

(GARciA, 1988). (c) Spray- wate ring. Every othe r day 10 ml oftap water were sprayed on the seedlings . d) Severe drought.The pots were never watered afte r th e initial water ing . Waterconte nts in th e soil were measured at th e end of the experiment with a psychrometer and zero values indicated thatth ere was no remaining water in th e sand. Seedling survivors hi p was followed during 140 days, and at the end of theexperiment plants were oven-dried at 70 °C du ring 48 hrs.Dry weight was obtained for each plant. Survivors hip curv eswere compa red usin g the log-rank Pet o and Peto test (PYKEand THOMPSON, 1986) and dry biomass was compared witha one way ANOVA (ZAR, 1984)

Su rvivorship of seedlings of Ch. chamaecristoides was notsignificantly different among th e four treatments during th efirst 80 days (Figu re 3a ). After th e third month of exposureto the dry conditions, seedlings subject to spray- watering andsevere drought started wilting and dyin g. Survival was thesa me between th e control lot and moderate dr ought . Biomasswas ten times gre ate r in th e cont rol seedlings and did notdiffer signi ficantly betw een th e three dr ought regimes (Figure 3b). The ability of these seedlings to withstand total lackof watering for more th an 80 days probably allows th em tosurvive during the dry months of the year.

In anothe r gre en-h ouse experiment see dlings of Ch. chamaecristoides were exposed to different nutrient supplies inorder to st udy their growth responses (MARTINEZ and RINCON, 1993 ). The plants wer e grown for 140 days und er consta nt and pul sed nutrien t conditions , using pure silica sa nd:continuously nutrient poor, consta nt nutrient rich, predictable and unpredictabl e nutrient pul ses. A fifth condition consis te d in using sand from th e tropical dunes wher e th e plan tnaturally occurs. Thi s lot was watered with distilled water .Final dry biomass and root/sh oot ratio were compared usin gone-way ANOVA (ZAR, 1984). The authors found almost ate n-fold difference in biomass between th e constant nutrientrich treatment and th e rest (Table 2). There were no significant differ ences between pul ses and plants growing in origina l sa nd. Nutrient-poor plants showed th e lowest biomassva lues . Bioma ss partitionin g was simila r in th e five treatments (Table 2) and the re was always a higher allocati on toroot s than to shoots . In this study evidence was provided toprove that Ch. chamaecristoides is able to tolerate ext remelynu trient limited conditions and th at seedling growth can besus tained in dune sa nd. Nutrient pul ses wer e not associa tedwith differences in plan t growth.

J ournal of Coastal Research, Vol. 14, No. I , 1998


100 a)~ Emergence date 100

0) 80ex> c=J April 88 ~0)

i 10'r" rzza July 88.s 60

~ April 89 sf/) ~CI) u. 1>ts 40::3

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a:: Plant cover (cm2)

0

Size Categories (cm3)

Perennation

POPULATION BIOLOGY

b)1.0

0.8 Size categories (em")a ___ 1001-10000

0.6 ___ 101-1000~ c -A- 11-100

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c: (::s:::sJ 101-1000Q) ~ >1000::J 40CT~u,

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Buried Exposed DryCauses of death

Population Dynamics

In 1980, MORENO-CASASOLA measured plant cover of 250Ch. chamaecristoides individuals growing naturally in a sanddune system (unpublished data). Her data show a leptokurticdistribution of sizes (Figure 5a) in which smaller individualswere more frequent than larger ones. MART1NEZ and MoRENO-CASASOLA (1993) observed that plant size was not related to age. A range of sizes may be measured for a givenage, which probably reflects differences in genetic potentialand in micro site conditions. Patterns of seedling survival under natural conditions were followed during a three yearstudy in the state of Veracruz, central part of the coast of theGulf of Mexico (MART1NEZ and MORENO-CASASOLA, 1993).The authors mapped and quantified growth and survival of2031 seedlings distributed throughout four 50 X 60 m mobiledunes. They found that the largest mortality rates occurredduring the nortes and also that size rather than age playedan important role in survivorship (Figure 5b). The largerseedlings suffered significantly less mortality during the nortes than smaller ones and the same trends were observed for

Figure 5. (a) Plant cover distribution showing the larger frequency ofsmaller individuals. (b) Survivorship curves for seedlings during the nortes season. Smaller ones showed a higher mortality than larger ones.Significant differences between survivorship curves are indicated by different letters. (c) Causes of death shown for different size categories(height X width X length). Smaller individuals were the most susceptibleones.

>100000>10000> 1000

Chamaecrista chamaecristoides is a short-lived perennialshrub (phanerophyte) that may live up to 6 years (MARTtNEZ,unpublished data). Larger individuals are able to withstandsand movement during the nortes season and occasionally,produce adventitious roots on buried shoots. Some times aftersevere erosion exposed roots may also produce small leaves.However, vegetative propagation has never been observed.

Flowering takes place once a year at the end of the rainyseason (September-October). It is synchronic in most individuals covering the dunes with a yellow tinge. Fruiting takesplace after the rainy season, before and during the beginningof the winter storms (October-November) (CASTILLO andCARABIAS, 1982). After the reproductive event, adult plantsshed most of their leaves and remain quite leafless duringthe dry season. Leaves are produced again with the first rainshowers, in April.

MARTINEZ and MORENO-CASASOLA (1993) observed thatflowering is related to individual size rather than to its age(Figure 4). Their information indicates that it appears plantsmust to achieve a minimum size before they are able to reproduce. In this case, the microenvironment where the plantsare growing plays an important role since it affects theplant's chances of growth and survival and, as a consequence,of reproduction as well.

Twenty five percent of the seeds are able to germinate assoon as they are dispersed, while the remaining 75% are hardcoated and need to experience naturally occurring temperature fluctuations in order to germinate (MORENO-CASASOLAet al., 1994). This set of seeds germinates continuouslythroughout the year, although two peaks have been observed:in April, at the end of the nortes season, and in July, duringthe rainy season. Both cohorts play an important role in population dynamics (MARTINEZ and MORENO-CASASOLA, 1993).

Phenology

Figure 4. Flower production in terms of individual size. The larger individuals represent a higher percentage of reproductives, independentlyof their emergence date.


- - --------------- -

170 Marti nez and Moren o-Cassasola

1988, 1989 and 1990 . See dlings suffer ed high mortality owin gto desiccation (51%). On the oth er hand, sa nd movemen t(both burial and erosion) accounted for 17% of see dlingdeaths. Seedlings were not abl e to withstand prolonged a ndcontinuous total burial. Simila rly, they could not maintainthemselves in a cons tantly eroding surface. Th ese causes ofseedling mortality also varied with see dling size: the sma llerones wer e the most sus ceptible to bu rial and desiccation,whil e sand erosion affected a larger size ran ge (Figu re fie).

Germination date and local growing condit ions greatly affected the size that see dlings achi eved by th e time the nortesseason started. Early cohorts suffered larger initi al mortali tybu t wer e better able to survive the nortes since th ey hadgrown for a longer peri od of time. Contrary to this, late cohorts suffered less in it ial mortality but their timing for gro wing was much shorter. Thus, it see ms that in an unpredictably changing environment as th e dunes, one cohort mightbe consi dere d as "ins urance" against total loss of the earlieror later ones. Similar to othe r dune species (LAING, 1958;SYMONIDES, 1977; MAUN, 1981 ), it is of selective adva ntagefor Ch. chamaecristoides to maintain cohorts germinatingduring differ ent seasons of the yea r . Such a continuous germination pattern is achieved through a hard coat dormanc ywhich is gradually broken through naturally occurring temperature fluctuations, as will be shown in the following secti on.

REPRODUCTION

Sexual Reproduction

Polination and fertilization

Flower s of Ch. chamaecristoides are visi te d by a widenumber of insects , many of them Hymenopter a (GARCIAFRANCO, personal communication ). However, it is not kn ownwhether they act as pollinators or not. It is also not kn ownif flower s are self-compa tible. Evid en ce of nectar thieves hasbeen observed in field conditions (GARCIA-FRANCO, personalcommunication ).

Seed Production and Composition

Adul t individuals of Ch. chamaecristoides flower inten sively at the end of th e rai ny seas on. In 1991 , MARTINEZ (unpublish ed data ) tagged 5 branches in 15 adult individua ls andfollowed bud and flower dynamics du ring two mon ths, at biweekly intervals. On average, she found that a branch mayform up to 14.4 ± 4.05 buds. Th e maximum number of budsper br an ch was 24, and the minimum 7. From a total of 1082flower bud s only 224 (21%) produced pods which were maturetwo months after bud form ation. Th e mean number of podsper branch was 3.03 ± 1.89, and the calcula ted poten ti almaximum see d production per pod was 10.22 ± 1.79. Ofthese, 19.4% are lost du e to seed attack by insect s and 8.8%are aborte d. As a result of these losses, mean number ofheal thy seeds per pod was 7.3 ± 2.65.

Seeds are covered by a mucil aginous layer which form s ahumid coat after bein g in contact with water.Che mical ana lysis of see ds gave th e followin g composition: protein > 900

(mg) g- I, starch 36 (rng) g 1, no lipid s. Anthocyanins werepresent although not in high quantities.

Dispersal

Pods bu rst open when they ripen and dry out . Dispersaldistan ces from the parent plant vary from a few centimetersto almost two met er s. Th e seeds are then secondarily dispersed by stro ng wind s together with the shifti ng sandgrains . Very few seeds are buried by the sa nd, due to th eirflat shape (Figu re 1), size (0.48 em long) and weight (0.018g) (MORENO-CASASOLA et al., 1994 ). Thi s set of cha racteris tics ena ble see ds to slide on the sand surface when moved bythe wind . MARTINEZ and MORENO-CASASOLA (unpublisheddata ) found that seeds are deposited along wind -corridors inclumps from 5 to 250 see ds distributed in a 21 X 15 ern area.

In semimobile dunes (grassla nd and active dunes), withfew Ch. chamaecristoides adult individu als, see d trap s placedvertically but touching the sa nd surface captured a total of632 seeds from March 1990 to February 1991 (ACOSTA, 1993).The largest see d rain den siti es (seeds/rn- ) were registeredfrom Septe mber to J anuary (Sep. 953, Oct . 377 , Nov. 103,Dec. 260 a nd J an . 283) whi ch coincided with the peak produc tion of seeds. Less than 53 seeds/m" per month were captured during th e rem aining months of th e yea r.

Seed Bank and Seed Size

Seed bank in mobile dunes was qu antified by ALTAMIRANOand GUEVARA (1982 ). Th ey qualtified th e see d bank in thea rms , cres ts , windward and leeward slopes of two mobiledunes. At each site th ey took ten sa mples distributed in 1 m",by using a cylinder 8 cm long with a diam eter of 8 cm. Thus,in ea ch site they sa mpled 500 em- and a volum e equivalentto 4020 em", Th ese authors only register ed seven species,Chamaecrista chamaecristoides bein g the most abunda nt one .A total of 709 seeds were found in the four sa mpling datesfrom March 1979 to J anuary, 1980. Yearly seed den sity(seeds/rn") in the sand varie d grea tly among microsites. Thelargest den sit ies ranged form 820 to 3760 seeds/m" and wererecord ed in sa mples from th e a rms and crests wher e th er e issand accumulation. Seeds were record ed at much lower densities (from 160 to 340 see ds/m") in leeward a nd windwardslopes .

Germination Ecology and Establishment of Seedlings

Only 25% of the see d popul ation are able to germina te immediately. Th e rest rem ain ungerm inated until dorm ancy isbroken by na turally occurring te mperat ure fluctu ations. Th isdorman cy may also be br oken by manual scarification with aknife, or through te mperature fluctu ations which frequentlyoccur on mobile dunes wher e seeds are found (MORENO-CASASOLA et al., 1994).

ALTAMI RANO and GUEVARA (1982) obse rved that germination speed was different between seeds collected directlyfrom th e paren t plant and seeds picked from th e sa nd surface . Th e first lot reach ed 50% germination a fter 30 days,while the latter reach ed th e sa me germination percentage ata much fas ter rate, 23 days sooner. Thi s may be explai ned

Jo urnal of Coas ta l Resea rch, Vol. 14, No. 1, 1998


Figure 6. (a) Biomass accumulation measured every six weeks, afterexposure to different burial conditions: control, 50% and 100% of thegrowth meristems. (b) Root/shoot ratio of buried plants. Different lettersindicate significant differences between treatments (p < 0.05)

by the scarifying effects that fluctuating temperatures haveon these hard-coated seeds (MORENO-CASASOLA et al., 1994).

Germination experiments in controlled laboratory conditionsshowed that 40% of seeds of Ch. chamaecristoides were ableto germinate after being exposed to daily temperature fluctuations of 10°C. Germination percentage increased evenmore (80-900/0) with wider daily temperature fluctuations (20to 35 °C). Wider fluctuations were effective after 15 days,while narrower fluctuations only increased germination afterseeds had been exposed to these conditions during threemonths. The same results were observed in a similar experiment under field conditions. These experiments demonstratehow important daily temperature fluctuations on the sandsurface are in the process of breaking hard coat dormancy ofCh. chamaecristoides seeds.

Hard-coated dormancy and the gradual effects of temperature fluctuations give as a result a continuous seed germination under natural conditions. There are two major flushesof newly emerged seedlings, which coincide with the end ofthe nortes season and the onset of the rainy season. All seedlings show large mortality values during the first threemonths (from 50 to 70%), giving a Deevey curve type III. Establishment of seedlings is affected by the same factors thatinfluence adult plants, namely: sand movement (burial anderosion) and drought. Smaller-sized individuals are more susceptible to these factors than larger ones (MARTINEZ and MoRENO-CASASOLA, 1993).

Burial ConditionsTime

(weeks) Control 50% 100% F

11 0.054 (0.01) 0.072 (0.01) 0.06 (0.01) 1.2817 0.009 (0.007) a 0.012 (0.01) a 0.033 (0.01) b 7.1923 0.006 (0.02) a -0.027 (0.006) b 0.009 (0.02) a 7.39

Response to Burial

During the nortes season adult plants of Ch. chamaecristoides may be partially or totally covered by shifting sand. Ifsome green tissue is left on the sand surface, then it is possible that shoots will elongate and emerge from the sand(MARTINEZ, M.L. field observations).

MARTINEZ and MORENO-CASASOLA (1996) studied experimentally the effects of burial by sand on seedling growth ofsix tropical sand dune species. In a green-house experiment,they covered two week-old seedlings with dry sand from thestudy site. Treatments consisted of the following depths: 0%(control, no sand was added); 50% (one half of the growthmeristems were covered) and 100% (all the growth meristemswere covered, which left one half of the upper leaves abovethe sand surface). Three harvests were obtained at 6 weeksintervals. Plants were divided into roots, stems and leavesand then were oven-dried at 70°C during 48 hrs. Dry weightswere used to calculate relative growth rates according toHUNT (1982) and treatments were compared by using ananalysis of variance followed by Tukey test when ANOVAsindicated significant differences (ZAR 1984).

When all meristems were covered with sand, biomass increased significantly in Ch. chamaecristoides (Figure 6a).Root/shoot ratio decreased and the more sand accumulatedaround the plants the greater biomass was allocated toshoots, thus, decreasing the root/shoot ratio (Figure 6b). Relative growth rates increased significantly as seedlings of Ch.chamaecristoides were covered by sand (Table 3).

This study showed that Ch. chamaecristoides is not onlytolerant of burial, but that its vigor may be enhanced afterplants are covered by sand. Although the authors argue thatit is possible that the tolerance limits are higher than theburial treatments used, field experiments performed later on(unpublished data) showed that total burial leads to death ofthe adult plants, even within a week after burial. Therefore,it appears that some green tissue on the sand surface is necessary in order to recover after a burial event.

The distribution of Chamaecrista chamaecristoides may beinterpreted in terms of the differences in tolerance limits tosand movement. Hence, based on these results, Ch. chamaecristoides should be more abundant on microhabitats expe-

GEOMORPHOLOGICAL INTERACTIONS

Vegetative Reproduction

There is no evidence that Ch. chamaecristoides is able toreproduce vegetatively.

Table 3. Mean relative growth rate (g g 1 day 1) of seedlings ofChamaecrista chamaecristoides under various burial conditions. Different lettersin each row indicate significant differences between treatments (p < 0.05)(SD).

c=J controlrzn 50%&S:SJ 100%

b

b

23

a

b

17

ab a

115

Time after germination (weeks)

a)1.4

1.2C;';;; 1.0t/)t'OE 0.80.s~ 0.6

"'0

S 0.40....

0.2

0.0

b) 5

40~

~ 3<50s:t/) 2".;::)0e

0


172 Martinez and Moren o-Cassasola

riencing substantial sand movement. In th e st udy by MoRENO-CASASOLA (1986) this wa s in fact, th e cas e.

Role in Geomorphology

Usu all y, it is said that perennial grasses and trailing vinesplaya direct and dynamic part in coastal geomorphology (EHRENFELD, 1990; DEVALL, 1992 ). It is a lso argued that twogrowth forms are important in affect ing accre tion and dunestabilization: rhizomatous, in whi ch upward growth allowsgras ses to emerge from an accre ti ng surface; and stoloniferous in which lateral growth by long stolons enables plants tosurvive burial (DEVALL, 1992). Rhizomatous growth is supposed to be mainly temperate in distribution whil e th e stoloniferous form is as socia ted with the tropics. Ch. chamaecristoides does not fa ll into either of th ese categori es and nevertheless, is a tropical mobile sand dune stabilize r . Along theGulf coast of Mexico, mobile dunes are stabilized by two endemic shrubs (MORENO-CASASOLA, 1990 ): Ch. chamaecristoides and Palafoxia lindenii (Cornpositae). We have obse rvedthat these two species, es pecia lly the first one, a re the firstcolonizers. We do not kn ow the mechanism s that enable th emto emerge from an accreting surface, a lthough production ofadv entitious roots on buried ste ms has been observed (unpublish ed ). Also, occasi onally leaves are produced on exposedroots of Ch. chamaecristoides (MARTINEZ, M.L. field observations ).

MARTINEZ (1994 ) mapped a mobil e dune and monitoredvegetat ion dyn amics in 140 4 X 4 m quadrats during threeyears . She found that Ch. chamaecristoides was the first baresand colonizer. Seedlings were observed along wind corridorswh ere sand began to accumulate as seedlings grew andreached a mature state. Ch. chamaecristoides plants ca usethe accretion of sand. That is, sand accumulation has beenobserved in areas with this species , whil e it does not occurwhere these plants are absent, even within the sa me wind ward slop e. Eventually, a sm all arm was formed within thedune, and after two years, 1m of sand had accumulated atthese microsites. Also , other species such as Schizachyriumscoparium, Trachypogon gouini , Triplasis purpurea (grasses)and Pectis saturejoides (Cornpositae) had arrived and theirplant cover increased in time.

INTERACTIONS WITH OTHER SPECIES

Positive Interactions

Adult plants of Chamaecrista chamaecristoides see m to facilitate seedling germination and establishment of other species . MORENO-CASASOLA (1982 and unpublished data) monitored temperature and soil moisture underneath and outsidethe shade of the sh rub. Th e growth form -a low, not verydense shrub-of this species has an important effect in microclimate. Maximum temperatures and range of daily fluctuations decreased under ind ividual s of Ch. chamaecristoides.This effect could be see n on the sa nd surface bu t also at different depths on a sunny day in June. From 8:00 AM to 16:00 PM temperature on the sand surface in the shade of Ch.chamaecristoides fluctuated between 32 and 48 °C. Contraryto this, temperature fluctuations on bare sand ranged from

23 °C to 62 °C. Variations in shrub den sity produc ed differences in this response, but the maximum value reached wasalways lower under the shru b than on bare sand (MORENOCASASOLA, 1982 ). Sure ly, these differen ces in temperatureregimes are important in the seed ecology of the species andcould be ben eficial for the establis hment of late colonizers.

Further evidence of this positive interaction was providedby MARTINEZ (1994 ). In her study on vege tation dyn am ics ina mobil e dune, sh e found that see dlings from late colonizersare only able to arr ive and grow in th e mobil e dunes onceindividuals of Ch. chamaecristoides have reached a minimumplant cover of 2 X 2 m.

We have also obse rved th at see ds from grasses such as Trachypogon. gouini and Sch izachyrium scoparium var. littoralis(forme rly Andropogon scoparium var. littoralis ) become entangled on the branches of Ch. chamaecristoides. Eventually,these ca pture d seeds germinate under the shru b during thefollowing rain-sho wer. Vegetation ana lys is have shown thatthese grasses are frequ ent companions of Ch. chamaecristoides (MORENO-CASASOLA and ESPEJ EL, 1986 ).

Bacteri al nodul es are abunda nt in th e young root s of Ch.chamaecristoides, and it has been shown that mycorrhizal associations are very important for th e plant's performance.CORKIDI a nd RINCON (personal communication) found thatthe species is highly dep endent on this association . When Ch.chamaecristoides was inoculated with arbuscular mycorrhizalfun gi, total dry biomass increased four-fold . Relative growthrate, leaf area and leaf number were also sign ificant ly largerthan in non-mycorrhizal plants.

CORKIDI (1996) sugges ts that, considering the low prob ability of infect ion in the mobil e dunes, Ch. chamaecristoidescould be act ing as a source of mycorrhizal inocu lum. Latecoloni zers are al so highly dependent on mycorrhizae and th eshrub ma y also be beneficial in this sense, besides the differences in microclimate produ ced by its shade.

Predation

Herbivores do not affect Ch. chamaecristoides seedlingsgreatly. MARTINEZ and MORENO-CASASOLA (1993) observe dsurvivorship of protected and unprotected seedl ings. Of the212 see dlings protected by a pla stic mesh from predation inJuly 1989 , only 56 (26%) had survived by December 1989.Similarly, those see dlings from th e control lots sh owed lowsurvivorship values (53 out of a total of 165; 32%) wh ich didnot differ sign ificantly from the exposed ones. All seedlingsin both pr otect ed and unprotected sites died before reachingmaturity, owin g to sand movement and desiccation , ratherthan to herbivory.

Although pr edation does not affect seedl ing growth andsurviva l, a large number of insect s are found feeding on Ch.cha maecristoides. During the plant' s reprodu ctive seasonRICO-GRAY (199 3) observed six ant species feeding on extrafloral nectaries: Conomyrma sp ., Crematogaster brevispinosa, Forelius sp., Monomorium sp., Pseudomyrmex filiformisand Pseudomyrmex gracilis. He also obse rve d four a nt species(Azteca sp., Conomyrma sp., Forelius sp., Monomorium sp.)feeding on coccids standing on br anches and on homoptera

Jo urnal of Coastal Resear ch, Vol. 14, No. I , 1998


(scales and aphids) iCremaiogaster breuispinosa). These latterobservations occurred during the dry months of the year.

RESPONSE TO WATER LEVELS

Although Chamaecrista chamaecristoides is typically a mobile dune shrub, it might be found growing on the beach, behind the high tide mark. In central the Gulf of Veracruz itwas observed that it can tolerate fresh water inundationwhen the water table rises during heavy rainstorms. On thisoccasion, shoots developed adventitious roots and the adultindividuals seemed rather healthy. Contrary to the above,adult plants of Ch. chamaecristoides do not tolerate salt waterinundation and the beach population went extinct after extreme high tides during hurricane Roxanne in October 1995(PEREZ-MAQUEO, unpublished data).

ECONONUCIMPORTANCE

Dune stabilization

Ch. chamaecristoides is an important mobile dune colonizeralong the coasts of Mexico. It is the first species to colonizeextremely mobile bare sand, and facilitates seedling germination and growth of other late colonizer species. Within twoyears of continuous growth, the shrub reaches a 2 X 2 mplant cover and different grass species and composites willbegin to increase their plant cover as well. The plants decrease sand movement and hold the substrate together, promoting dune stabilization. This is important to humans, sincemobile dunes may be an important problem along the coastsof Veracruz, where roads and fields are continuously encroached by the surrounding large mobile dune systems.There is no practical experience in dune stabilization withthis species.

Regeneration of Dune Vegetation

Artificial removal of plant cover in 1 m 2 quadrats locatedin humid slacks, dry slacks and the beach showed differentrecovery rates and different species acting in the regeneration process. Ch. chamaecristoides played an important rolein the beach and dry slacks or hollows, but not in humid andwet slacks. Colonization by this species took place throughseed dispersal and germination. From a total of 20 speciescolonizing the beach, it reached the highest values of relativeimportance value. Canavalia rosea and Palafoxia lindeniiwere also important. Bare sand was soon covered by vegetative growth of creepers. In dry slacks Pectis saturejoides andCommelina erecta had the highest values followed by Ch. chamaecristoides (GONZALEZ-LoREA and MORENO-CASASOLA,1982). Regeneration was a slow process.

ACKNOWLEDGEMENTS

The results of the studies described in this manuscriptwere supported by several grants: CONACYT 0064-N9106,225260-5-3465N and 1841P-N and Instituto de Ecologia, A.C.902-16. We are very grateful to T. Valverde and O. PerezMaqueo for their assistance during our field work. Thanksare also due to E. Saavedra and R. Landgrave for drawing

Figures 1 and 2 respectively, and to M. Devall and J. Ehrenfeld for their helpful comments on an earlier version of themanuscript.

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Journal of Coastal Research, Vol. 14, No. I, 1998

M. Luisa Martinez and Patricia Moreno-Casasola

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