floristic change during early primary succession on...

BRAUN-BLANQUETIA, vol. 46, 2010 225

FLORISTIC CHANGE DURING EARLY PRIMARY SUCCESSION ON LAVA, MOUNT ETNA, SICILY

Roger DEL MORAL*, Emilia POLI MARCHESE**

* Department of Biology, Box 351800, University of Washington, Seattle, Washington (USA)E-mail: [email protected]** Università di Catania, c/o Dipartimento di Botanica, via A. Longo 19, I-95125, Catania (Italia)E-mail: [email protected]

ABSTRACT

We investigated the degree to whi-ch vegetation becomes more similarduring primary succession and askedwhether the age of a lava site alonedeterminesspeciescompositiononothe-rwise similar sites or if site-specificfactors are more important. The studywasconfined to lava flows found betwe-en 1,000 and 1,180 m on the south sideof Mount Etna, Italy that formed from1892 to 1169 or earlier. Ground layercover was measured at 15 exposed sitesand 12 sites under shrubs, using ten 1-m2 quadrats in five plots at each site.Changes in species richness, cover, di-versity, dominance, and similarity wereonly loosely related to lava age regard-less of whether plots were exposed or inanunderstory.Thesemeasuresdidchan-ge predictably when compared to thedegree of site development measuredby vegetation cover. Analysis by non-metric multidimensional scaling orde-red vegetation by their degree of deve-lopment, not along an age gradient. Thepresence of leguminous shrubs alteredspecies composition and thus succes-sion trajectories. Variable initial surfa-ce morphology, landscape factors, hi-storical conditions, and random eventshave affected both species establish-ment rates and trajectories. Determini-stic processes (e.g. competition) havenot smothered the initial heterogeneity,nor have they after over 800 years for-ced understory development to a com-mon terminus. Evidence for conver-gence (e.g. increased similarity withinplots with increasing age) was obtainedonly if sites were arrayed by their deve-lopmental,notcalendar,age.Thus,usingdevelopmental agemay overcomesomeof the intrinsic pitfalls of the chronose-quence approach when assessing vege-tation dynamics.

KEYWORDS: chronosequence, conver-gence, primary succession, similarity,trajectory, volcanoes.

ABBREVIATIONS: ANOVA=analysis ofvariance; CV=coefficient of variation;DCA=detrendedcorrespondenceanaly-

sis; GF=growth-form; GPS=global po-sitioning system; HC=half-change;NMS=nonmetricmultidimensional sca-ling; PS=percent similarity.

NOMENCLATURE: Pignatti (1982).

INTRODUCTION

The mechanisms that guide theassembly of species are complex (KED-DY, 1992; WALKER & DEL MORAL , 2003).During primary succession, landscapecontext and chance produce mosaics(DEL MORAL, 1998), whose variationdeclinesover time(RYDIN&BORGEGÅRD,1991; DEL MORAL & JONES, 2002). Therelationship between composition andenvironmental factors usually stren-gthens to improve predictability (WIL-SON et al., 1995). However, neither re-duced heterogeneity nor stronger deter-ministic control of patterns ensures thattrajectories will converge.

The traditional view of succession(CLEMENTS, 1916; BRAUN-BLANQUET,1964; FACELLI & D’ANGELA, 1990) isthat all trajectories converge to a singleassociation. This view assumes thatbio-tic interactions are intense and can over-ride initial variations (MUELLER-DOM-BOIS, 2000) and was demonstrated onIcelandic lavas by BJÄRNASON (1991).The alternative view is that mosaicspersist due to priority effects and spe-cies traits (e.g. RAMENSKY, 1924; GLEA-SON, 1939; WHITTAKER, 1974; YOUNG etal., 2001). This “assembly” view asser-ts that species accumulate and emphasi-zes contingency and priority (EGLER,1954; DRAKE , 1990; DEL MORAL et al.,1995; HONNAY et al., 2001; DEL MORAL

et al., 2005). Multiple trajectories arecommononglacier forelands(MATHEWS,1992; FASTIE, 1995), slack dunes (ADE-MA et al ., 2002), sand dunes (LICHTER,2000), riparian sites (BAKER & WAL-FORD, 1995), disturbed forests (MCEUEN

& CURRAN, 2004; SVENING & WRIGHT,2005) and volcanoes (WHITTAKER &JONES, 1994; TAGAWA, 2005). There is adeveloping consensus, particularlyamong restoration ecologists, that com-munity assembly is often stochastic and

can lead to alternative stable vegetationtypes (FATTORINI & HALLE, 2004; TEM-PERTON &ZIRR, 2004;YOUNG etal. 2005).

Convergence can be recognized ifsample similarity increases with age,but chronosequence methods may con-found site and stochastic effects witheffects due to age. Though chronose-quence methods must be employed inlong trajectories (DEL MORAL& GRISHIN,1999), the underlying assumption thatall sites were initially identical has ra-rely been tested. Here we explore therelationship between time and develop-ment on a small part of Mount Etna,Sicily to explore the relationship betwe-en age and vegetation development.

We sampled a ~832 yr chronose-quence on lavas of Mount Etna to explo-re community assembly. Poli Marchesesuggested (POLI, 1965, 1970, 1971; POLI

& GRILLO, 1975; POLI et al., 1995; POLI

MARCHESE & GRILLO, 2000a) that con-vergence was likely at the plant-socio-logical class level after 1200 yr, but thatrates varied with surface morphology,microclimate, and dispersal (cf. MAKA-NA & THOMAS, 2004). Poli Marchese hasdescribed how many early stages beco-me a few shrub stages, and ultimatelywoodlandassociations in theclassQuer-cetea ilicis.

We explored ground layer compo-sition on substrates of different agesand contrasted variation in exposed si-tes with understory vegetation. We sou-ght evidence for convergence by exa-mining similarity changes along thechronosequence. Fundamental to theapplication of the chronosequence ap-proach is that these assumptions aretrue: a) differences on the site are duesolely to the age of the site; b) differen-ces during establishment (e.g. weatherpatterns) are unimportant; c) landscapeeffects are minimal; and d) dispersaleffects are similar at each stage. It isbecoming clear that if these assump-tions are not valid, the interpretations ofsuccession can be in error (JACKSON etal., 1988). Further, local substrate va-riation and constraints on plant growthcan cause succession trajectories to de-velop at significantly different rates(ELLIS, 2004), compromising the use of

226 BENSETTITI A., BIORET F., BOULLET V., PEDROTTI F., Centenaire de la Phytosociologie

space-for-time substitutions.

METHODS

STUDY AREA

Mount Etna dominates north ea-stern Sicily. This volcano reaches 3350m a.s.l. Chronic eruptions, from thesummit and from many fissures, haveoccurred since A.D. 500. We conductedthis study on its south slope between1,000 and 1,180 m elevation duringMay 2001, the height of the growingseason. Pastures, orchards, and quarries

affect this zone on very old substrates.Selected lavas date from 1169 (or ear-lier), 1536, 1537, 1634-1638, 1766,1780, 1886, and 1892, and were largelyfree from such disturbances. Youngerlavas (1910 and 1983) were common inthe study area, but they lacked signifi-cant vascular plant vegetation. All siteswere on a’a lava (POLI, 1970), whichfractures to facilitate succession. Thenearest weather station is in Nicolosi(698 m), where the mean annual preci-pitation of 111 cm, occurs primarilyduring autumn and winter. The meantemperature is 14.3 °C.

The vegetation samples were do-

minated either by tall shrubs, hencefor-th called shrub sites, or by ground layerspecies, henceforth called exposed si-tes (Tab. 1). Latitude and longitudewere determined by GPS, and plottedon a topographic map, from which ele-vations were determined (Fig. 1).

There is a vegetation mosaic for-med in response to lava age, surfacemorphology, microsites, and later di-sturbance (POLI MARCHESE & GRILLO,2000b). Stereocaulon vesuvianum andother lichens and mosses colonized bar-ren lavas that were formed since 1910.These species form soil that fills crevi-cesand facilitate invadingvascularplan-ts.Centranthus ruber and Rumex scuta-tus are early vascular plant pioneersbecause they can establish in crevices.

The 1892 flow had a mosaic ofcryptogams, with scattered annuals andperennial forbs confined to cracks. The1886 flow supported a denser mosaic.The nitrogen-fixing shrubsGenista aet-nensis and Spartium junceum had esta-blished sporadically. As these shrubsalter their surroundings, they may faci-litate herb establishment. Lavas formedin 1780 were floristically similar toyounger ones, with Micromeria graecalocally common. A flow deposited in1766 sustained scattered Genista andSpartium. A complex array of flowswas deposited from 1634 to 1638 (heretermed “1636”). Diverse herbaceousvegetation was in close proximity toextensive areas dominated by Genistaand Spartium . The 1537 flow extendsbeyond the 1886 flow, and shrubs do-minated the sample plots. The 1536flow, dominated by herbs, was in closeproximity to vegetation dominated byQuercus ilex that had established on aflow from 1169 or earlier. Thus, after~832 years, several distinct plant asso-ciations persist (POLI MARCHESE& GRIL-LO, 2000a), dominated by several deep-rooted species (cf. BORNKAMM, 1981)and annuals. Similar vegetation appea-red to occupy lavas of quite differentage.

SAMPLING METHOD

Nested sampling was used to par-tition variation and similarity at threescales (Tab. 2). The site was homoge-neous with a known age and little di-sturbance. There were three samplesper site, except that for understories,only one sample from 1766 and twosamples from 1537 were obtained. Sin-ce there were few areas in the study arealarge enough to satisfied the selectioncriteria, sites were selected subjecti-

Tab. 1 - Site age, location and general vegetation. Date is the year of the documented flow.Coordinates are for the most distant sites.

Fig. 1 - Locations of study sites. Key to symbols: =exposed-1892; =exposed-1780;=exposed-1636; =exposed-1536A; =exposed-1536B; =understory-1886;=understory-1766;=understory-1636; =understory-1537; =understory-1196.


vely, but starting sample locations weredetermined by tossing a stake (i.e. it washaphazard). Samples of a site were se-parated by at least 50 m. Each samplehad five plots, with four surroundingthe initial plot within 10 to 30 m (deter-mined randomly) at the four cardinaldirections. The plant cover was estima-ted using ten 1-m2 quadrats in eachplot. Quadrats were divided into 25squares to facilitate estimation. Forexposed sites, quadrats were located ina predetermined pattern. Due to irregu-lar and small shrub canopies, understo-ries were sampled as follows. A shrubanchored a plot. One or two quadrats(0.5 by 2 m to fit under the canopy) weresampled. Understories were sampled inan expanding circle of shrubs until tenquadrats had been described. A secondplot was established at least 30 m fromthe nearest sampled shrub. This proce-dure continued until five plots had beensampled. Sampling was based on shru-bs, not microsite differences.

STATISTICAL METHODS

Sites were described using rich-ness, the information theory [H’ = -Σpiln p

i], and Simpson’s Index comple-

ment [D = 1- Σpi2], each calculated at

several scales. The mean number ofspecies (richness) was determined forquadrats, plots, and samples.

We used nonmetric multiple di-mensional scaling (NMS; MCCUNE &MEFFORD, 1999) to visualize relation-ships among plots and samples. Preli-minary analysis showed both sets to betwo-dimensional (cf. MCCUNE & GRA-CE, 2002). The final analysis was startedfrom the best configuration, based onthe least stress. Varimax rotation maxi-mized the degree to which patterns werealigned with the axes.

Detrended correspondence analy-sis (DCA; MCCUNE & MEFFORD, 1999)was used to compare ground layer ve-getation and to determine heterogenei-ty. The eigenvalues from DCA at eachsite and scale estimated overall varia-tion.Species turnover (diversity)alongfloristic gradients was estimated usinghalf-changes in the floristic composi-tion.

The relationships among quadratsof a plot, plots of a sample, and samplesof a site were calculated by percentsimilarity (PS = 200Σmin (Xik, X jk)/Σ(Xik+ X

jk), where X = cover of species k in

samples i, j). Calculations (KOVACH,1999) used the percent cover (quadrats)or mean percent cover (plots and sam-ple).

Structural features of plot vegeta-tionwerecomparedwithone-wayanaly-sis of variance (ANOVA), followed byBonferroni pair-wise comparisons ofdifferences (ANALYTICAL SOFTWARE,2000). Graphs were produced withAxum 7 (INSIGHTFUL CORPORATION,2001).

RESULTS

Both exposed and understory ve-getation occupied a chronosequence se-veral centuries long. Exposed vegeta-tion was sampled on sites initiated du-ring episodes dated from 1536 to 1892.Understory vegetation was sampled onsites initiated from 1169 (or earlier) to1886. Shrubs alter microclimate condi-tions to favor growth and developmentof a somewhat different suite of speciesthan those that colonize bare lavas.

FLORISTICS

Therophytes (59%)andHemicryp-tophytes (25.6%) dominated the sam-pledflora.TheremainingvascularplantswereChamaephytes (7.7%), Geophytes(2.6%) and Phanerophytes (5.1%). The-rophyte dominance results from theMediterranean climate, the immaturesoil, and nearby human disturbances.Of the Therophytes, 20.5% were nativexerophilous species common to expo-sed habitats (Helianthemetea guttataeclass) and an additional 23.1% wereruderal-nitrophilous species (Stellarie-tea mediae class) that spread on lavaafter human disturbances. The Hemi-criptophytes and the Chamaephytesestablished in cracks on a’a lavas be-cause they possess extensive root sy-stems. Such species as Rumex scutatus,Centranthus ruber, and some N-fixingPhanerophytes (Genista aetnensis andSpartiumjunceum)arecommon thepio-neers on Mount Etna, as are species ofthese plant functional types elsewhere(TSUYUZAKI, 1991). Once woody vege-tation has established, forest speciessuch as Geranium robertianum (Quer-co-Fagetea class) can invade and domi-

nate. POLI MARCHESE & GRILLO (2000a)provided a detailed description of thesuccessional dynamics of this vegeta-tion and listed the life forms and com-munity affiliations for each species.

Mean cover at ten sites is shownfor species with > 1% cover in at leastone plot (Tab. 3). Rumex multifidusdominated E1892 and E1536 sites, butwas scarce in better-developed exposedlavas and also in the understory. Bro-mus tectorum was best developed inlavas with good vegetation develop-ment (Tab. 4). Centranthus ruber wascommon across all sites and ages, butwas best developed on U1886, whichhas a rough surface to permit its esta-blishment. This species and other pe-rennials as Rumex scutatus form poc-kets of vegetation between blocks onthe flow. Its absence beneath Quercussuggested that it can persist in a chrono-sequence until dense shade, deep tan-nin-rich litter and deeper soils are for-med. Grasses, Asteraceae, annuals, andother small herbs dominated the lavasofE1780andE1636.The lavasofE1536were relatively barren, and were cha-racterized by Rumex multifidus, Isatistinctoria, and an assortment of annualsincludingAira cupaniana. Species typi-cal of exposed lavas were best deve-loped in sites of intermediate age(1780 and 1636), but were sparse orabsent from both younger and olderlavas.

Understory vegetation was relatedfloristically to adjacent exposed vege-tation. Shrubs establish in cracks whilethe ground layer can establish on surfa-ces with or without cracks. As a result,the establishment of shrubs does notdepend upon the establishment of her-bs, soweshouldexpectsimilaritybetwe-en the understories and the exposedground layers. However, the presenceof shrubs clearly alters the microclima-te, thus permitting species adapted toshade and more favorable conditions toexpand at the expense of more stress-tolerant species. Centranthus , Rumexscutatus, Isatis tinctoria, Geranium ro-bertianum, and grasses dominated siteU1886. Many of these species werecommon on young, exposed lavas. Bro-

Tab. 2 - Sampling design.

1 Except 1 sample for 1766 understory and 2 samples for 1537 understory.


mus madritensis, Geranium robertia-num, Briza maxima , Isatis, Cerastiumsemidecandrum , Crepis leontodontoi-

des, and Galium aparine dominatedU1766. U1636 was diverse, and contai-ned Centranthus, R. bucephalophorus,

Isatis,Poa bulbosa, Sedum tenuifolium,Geranium robertianum, and Microme-ria, a species common in all older un-derstories. U1537, which was locatedon a flow that was partially covered bythe lava of 1886, had a sparse canopy,with grasses, Centranthus, Isatis, seve-ral common forbs such as Crepis leon-todontoides, Sedum tenuifolium, Silenegallica, Galium aparine, andGeraniumrobertianum. This composition sugge-sted some disturbance and little sup-pression by shade. The vegetation un-derQuercus (U1169) was distinct fromtheother sites and included species fromthe class Querco-Fagetea such as He-dera helix, Geranium molle, G. rober-tianum, and scattered legumes. Com-mon native species included Microme-ria and Rubia peregrina, a species fromthe class Quercetea ilicis.

STRUCTURE

Plots were analyzed by linear re-gression of plot age with the structuralfeature in question: species richness,percent cover, information theory di-versity (H’), and the reciprocal of Sim-pson’s index (D). Patterns in richnesswere similar at each scale, so only plotvalues were analyzed by regression.Site richness (total number of species)was not linearly related to age in eitherexposed or understory plots (Tab. 4).Richness peaked in the intermediatesite (1636) and was lower in youngerand older sites. The pattern was similarin understory sites. Understories werericher in species in young sites, and inlavas from 1537, but low beneathQuer-cus (1169). There were significant dif-ferences between substrates at each sca-le, but the differences were not relatedto substrate age. Plot richness displayeda strong quadratic relationship to age (r2

= 0.64; P < 0.0001). There were twice asmany species per plot in samples thatwere 100 years younger. Richness be-neath the sparse crowns of Genista andSpartium increased significantly withage at each scale. If Quercus samplesare excluded, there is a significant line-ar increase in plot richness with age (r2

= 0.65; P < 0.0001). The relationshipincluding Quercus is quadratic (r2 =0.76; P < 0.0001).Quercus samples hadlow richness at each scale and werefloristically distinct from other com-munities (Tab. 3).

Vegetation cover in exposed siteswas not linearly related to site age (Tab.4). Percent cover demonstrated a signi-ficant quadratic relationship (r2 = 0.61;P < 0.0001). Cover of the understories

Tab. 3 - Mean cover of species with a mean cover > 1% in at least one plot. Genistaaetnensis, Spartium junceum and Quercus ilex were excluded because canopies wereuniformly high (80 to 99%). Species listed by in order of DCA Axis 1 (all plots).T=Therophytes;H=Hemicryptophytes,Ch=Chamaephytes,G=Geophytes,P=Phanerophyte.GF=growth-form.

Tab. 4 - Richness and cover in ground layer communities. All quadrats are included incalculations, even those without vascular plants. Date is the year of the lava flow; N is thenumber of quadrats with vascular plants of 150 sampled; parentheses give the size of thesample unit. There were five plots per sample of a site.

Note: exposed and understory values were analyzed separately; superscripts indicatecommonmembership ingroupsdeterminedafter significantANOVA(P<0.01) by Bonferronitests (P<0.05).


was similar to each other, except forthosebeneath Quercus, which date from1169 or earlier. The quadratic relation-ship among all understory cover wasstrong (r2 = 0.72; P < 0.0001), but exclu-ding 1169 lavas eliminated any signifi-cant relationship.

H’ changed significantly on bothexposed and understory plots at eachscale (Fig. 2). Quadrats of E1892 hadsignificantly lower H’ than most oldersites because many quadrats had onlyone species. The highest H’ values onexposed lavas at each scale were depo-sited in 1780 and in 1636. The lowest H’was on E1536, perhaps due to adversesurface conditions. Only the quadraticregression was significant (r2 = 0.59; P<0.0001). UnderstoryH’ increasedwithage (r2 = 0.29; P < 0.0001), but the densecanopy of the 1169 plots reduced H’dramatically.

Dominance changed significantlyin both exposed and understory vegeta-tion (Fig. 3). At the quadrat level, expo-sed sites increased in D between 1892and 1636, but the 1536 samples wereless diverse. At the plot level, exposedlavas differed significantly, but the li-near regression was poor (r2 = 0.08; P <0.02). The quadratic relationship pre-dicted D based on age to a much greaterdegree (r2 = 0.34; P < 0.0001) demon-strating that structural changes are notrelated primarily to lava age. The un-derstories were relatively similar, butthe quadratic regression was significant(r2 = 0.57; P < 0.0001). When Quercusplots were excluded, there was a linearrelationship of D with age (r2 = 0.45; P< 0.0001).

COMMUNITY PATTERNS-NMS

The 24 samples (Quercus samplesexcluded) were analyzed by NMS toclarify thefloristicpattern.Sampleswerecomposed of the mean of 50 quadrats(10 in 5 plots). The observed stress of9.7 (instability < 0.06) provided a use-able analysis.

The floristic pattern on exposedsamples was poorly related to site age(NMS-2; Fig. 4). The pattern was signi-ficantly different from random and anyinferences would be robust (stress < 9.5instability < 0.07). Exposed plots weredistinct from understory plots. The flo-ristic gradient, which describes patternsof accumulating species richness andcover, is not correlated to substrate agefor either exposed or understory plots.Within each group, there was a correla-tion between the ordination and the de-gree of plot development.

TURNOVER

Turnover is the floristic variationbetween samples and is termed ß-diver-sity. We examined turnover at the plotscale with DCA because this methodartfully measures floristic variation.Turnover was estimated by half-chan-ges (HC) in DCA scores of the ordina-tion (Tab. 5) when 120 plots were analy-zed (excluding U1169). If sites weremore homogeneous with age, then tur-nover should be reduced with time, and

the variation among plots of a sample,measured by standard deviations of theDCA scores, should decline.

The first DCA axis revealed notrends of HC with age or between expo-sed and understory plots. Understoryplots also showed little pattern. Here,turnoverwas influenced byfactors otherthan age, in particular the surface mor-phology. If sites were arranged by thedegree of vegetation development, thenexposed sites demonstrated a decline invariation with time.

Fig. 2 - Vegetation diversity (H’) on nine young lava sites. “Quadrats” is the mean valuefor each quadrat (from 75 to 150) at a site; “plots” are the mean value for plots at a site;“sample” the mean value for each sample at a site. Values were analyzed by one-wayANOVA, with exposed and understory vegetation analyzed separately. At each level,letters over the bars indicate group membership as determined by Bonferroni comparisons(P<0.05).

Fig. 3 - The complement of Simpson’s index (D) on nine young lava sites. Analysisdescribed in Fig. 2.


Turnover also was estimated bysimilarity among plots. The samples ofa site were compared with each of thesamples of the comparison sites andanalyzed by ANOVA. There was signi-ficant turnover in response to age andcanopy (Tab. 6). PS in exposed sitesranged from 3.6% to 51.5%, indicatinglarge turnover. There were no trends ofsimilaritywithageamongexposedplots.Similarity declined regularly if siteswere arrayed by structure. For example,the youngest site (E1892) was mostsimilar to the oldest sites (E1536A, B)because the development of these oldsites was least.

Understory similarities werealways lowest when compared toU1169. There were no clear patternsbetween understory similarity and theirage differential. The similarity of otherunderstories to U1169 increased withsite age, suggesting that understory tur-nover tends towards composition foundunder denser canopies. Species such asMicromeria, Geranium robertianum,Galium aparine, Poa and Sedum tenui-folium were common beneath Quercusand denser Genista and Spartium cano-pies.

WITHIN-SITE HETEROGENEITY

As a site matures, species invade,expand, and interact. An initially hete-rogeneous site should become less va-riable because as biomass increases,microsite variation is subdued. We me-asured heterogeneity using DCA of theindividual sites and floristic similaritywithin sites at three scales.

The ground layer vegetation wasexplored to examine floristic variation(Tab. 7). The eigenvalues shown forE1892 and E1536-A are arbitrary be-cause so many quadrats had no speciesin common. The eigenvalues of expo-sed sites were least at intermediate agesand highest in youngest and oldest sites.This variation is correlated to vegeta-tion development, not substrate age.HC (turnover) was reduced slightly atthe quadrat level, but at plot and samplelevels HC was minimal at intermediateages. Understory vegetation variationincreased with plot age, as did HC, ateach scale. Rather than becoming morehomogeneous, each there is evidencefor greater differentiation within a site.The low values in U1766 are due in partto the lower sample size. Exposed vege-tation generally expressed greater va-riation at a given scale than did theunderstories of a comparable age. Theexception was quadrats in 1636, where

Fig. 4 - Nonmetric multidimensional scaling (NMS) ordination of 120 plots from 8 sites.U 1169 was excluded because it was too distinctive.

Tab. 5 - Turnover (HC) in ground layer vegetation and the variation in DCA scores whenplots were examined in a single data set (U1169 excluded). SD is the variation of the sampleDCA scores of the site.

Tab. 6 - Turnover estimated by floristic similarity among understory and among exposedsamples.

Note: Comparisons were made among all understory sites and among exposed sites(ANOVA, P<0.05) followed by Bonferroni comparisons; values with the same superscriptare within the same group.


the understory vegetation was slightlymore variable than the exposed sites.

We determined percent similarity(PS) among all quadrats of a site, among10 quadrats in each of the plots in a site,and among the five plots in each sampleat a site (Fig. 5). The PS of exposed sitesdiffered significantly with site age ateach scale (ANOVA, P << 0.001).However, in no case was there a simplerelationship between substrate age andcommunity similarity, due primarily tosurface effects. PS at exposed sites waslower than that of the understory atcomparably aged sites, except in 1636,where theunderstoryvalueswere lower.ShrubspermittedhigherPSthan inexpo-sed sites at each scale, presumably bycreating a more homogeneous environ-ment. Though PS among understoryplots differed significantly, there wasnoparticularpattern.Quercus plotswerethe least similar, but at smaller scales,these old sites were comparable to othersites. Over this chronosequence, varia-tion within a site remains more impor-tant than effects that could create ho-mogeneity.

CANOPY EFFECTS

Shrubs strongly affected speciescomposition and thus altered thetrajectory of succession. We comparedthe five exposed sites to the five under-story sites (Tab. 8). The youngest un-derstory was most similar to older expo-sed sites, while the youngest exposedsites had very little in common with anyunderstory. Exposed and understory ve-getation had limited similarity. The pre-sence of a dense evergreen canopymarkedly altered the species understorycomposition, such that its highest simi-larity was to thatof theadjacentE1536Asite. U1169 does not appear to be the“target” to which all trajectories arecurrently aimed. Intermediate stages dooccur in the general area, but not withinthe immediate area sampled (POLI MAR-CHESE & GRILLO, 2000a).

DISCUSSION

The focus of this study was todetermine if, during succession onyoung lavas, vegetation converges. In arelatively confined geographic area, canthe chronosequence approach be usedto assess this question? Alternatively,do even minor differences in site quali-ties and location lead to persistent vege-tation differences?

Measures of vegetation structure

changed dramatically among the expo-sed sites, but changes were not linearlyrelated toage.Speciesaccumulatedovertime at different rates, leading to a com-plex mosaic of similar vegetation onsubstrates of different ages. Two exam-ples are the similarity between exposedvegetation on lavas of 1892 and 1536,and the similarities among understoryvegetation on lavas of 1766 and 1537.In contrast, vegetation of nearly thesame age may differ strongly, even asi-de from the low similarities among thesamples of a site (Fig. 5). For example,the lava of 1636 sustains both well-developed exposed vegetation and

shrub-dominated vegetation. The lavaof 1892 lacked shrubs, while that of1886 had a significant shrub popula-tion. The effect of a canopy is to accele-rate the development of the understoryand to permit a more rapid accumula-tion of vegetation cover.

Observations of young lavas sug-gest that species in this chronosequenceinvade sporadically and individualisti-cally. They establish episodically, in-fluenced strongly by stochastic and con-tingent factors and by variations in soildepth, surface texture and morphology.

Priority effects (BELYEA & LANCA-STER, 1999) may alter trajectories. Whi-

Tab. 7 - Summary of DCA of individual sites on the first axis at three scales. Eigenvalueestimates variance, HC is the number of half-changes on the gradient. Shrub plots wereanalyzed without shrubs.

Note: A=values could not be calculated due to low species richness in many quadrats andthe number of times quadrats had no species in common.

Fig. 5 - Percent similarity at three scales. Quadrats at site = mean pair-wise comparisonsamong all quadrats (75 to 150) in one site; Quadrats in site = the mean of pair-wisecomparisons of quadrats within each sample; Plots = the mean similarity among the plotsat a site. Statistics are described in caption for Fig. 2.


le the Quercus plots sampled in thisstudy showed no evidence of havingonce supported Genista or Spartium ,other nearby sites at lower elevationshave Q. ilex that has invaded these shru-bs, and which sustains a more diverseunderstory. It is quite possible that theQuercus studied here did invade shrubsthat long ago disappeared. Therefore,while species accumulate and structuredevelops through time, the process pro-ceeds at different rates on different la-vas and is affected by many factors. Asnoted by FASTIE (1995) in relationshipto succession at Glacier Bay, the pre-sence or absence of a legume can sub-stantially alter the subsequent succes-sion (cf. DEL MORAL & ROZZELL, 2005).

Species turnover clearly occurredon exposed sites. Many species thatwere initially sparse in E1892 sites werecommon on older substrates. The pre-sence of shrubs, whether establishedprior to, simultaneous with, or afterground layer, has altered the herb layer,but different dominants occurred in dif-ferent plots of a site. The compositionof shrub understory appears less varia-ble when assessed by NMS (Fig. 4).Shrubs reduce options available to un-derstory species and enhance local ho-mogeneity. However, variation at thesample scale remains similar to that ofexposed sites. Variation patterns provi-de no evidence for convergence on anybut the smallest scale. One small areamightdevelophomogeneity under shru-bs, but substantial between-sample va-riation persisted even in the oldest sites.Shrubs increased the homogeneity ofunderstory vegetation and changed spe-ciescomposition. Shrubs increased sha-de and soil organic matter and alteredthe competitive balance among under-storyspecies.Overstorydominancemaybe the principal mechanism by whichconvergence to a single association oc-

Tab. 8 - Floristic similarity (percent) of ground layer vegetation samples: exposed sites vs.understory sites. Values are the mean of the nine pair-wise comparisons among the threesamples of one site and the three samples of the comparison site.

Note: Comparisons were made between all understory sites and all exposed sites (ANOVA,P<0.05) followed by Bonferroni comparisons; values with the same superscript are notdifferent.

curs (GLENN-LEWIN & VAN DER MAAREL,1992). However, for the process to becompleted, initial effects due to land-scape (cf. DEL MORAL & LACHER, 2005),persistentof initial colonizers, andchan-ce (cf. DEL MORAL & ECKERT, 2005)must be extinguished. To the extent thatsuch confounding factors remain, con-vergence cannot occur, even if homo-geneity is increasing (WALKER & DEL

MORAL, 2003).Turnover (HC) in exposed plots

declined with vegetation developmenton DCA-2, but not with lava age. Thepresence of shrubs tended to increase,not decrease understory variation. Theunderstory beneath Quercus was evenmore variable at these scales, and wasso distinct that we did not analyze thesesamples along with other plots. The rateof succession appears to vary on eachlava flow, leading to nonlinear changesin turnover with age.

There is little evidence for conver-gence in this chronosequence if oneuses chronological age. However, if weapply theconceptof developmental age,there are some indications of partialconvergence.Similarityamongquadratsat a site, within a sample, and, to a lesserdegree within plots, increased with de-velopmental age (Fig. 5), but under-story vegetation demonstrated no trend.We found no evidence for floristic con-vergence at the level of a sample.

Similarity between exposed sitesdeclines not with their age differential,but with differences in development.For example, E1892 is most similar toE1536A, which is the sample most si-milar to it in cover and measures ofdominance (Tab. 6). The PS of U1886to other understories declines not withage but with structure. These resultssuggest that within this small area, spe-cies composition will tend to developalong somewhat similar trajectories.

The rate of vegetation develop-ment on young lavas of Mount Etna hasbeen fitful. Sites of the same age candiffer subsequently due to surface cha-racteristics, nature and proximity topools of potential colonists, and contin-gent factors. The chance establishmentof large herbs (e.g. Centranthus or Isa-tis) or tall shrubs (e.g. Spartium) altersthe chronosequence. As a result, eachsite experiences a unique vegetationtrajectory. Given the importance of le-gumes and of landscape effects, webelieve that while convergence amongdominants (e.g. Quercus) may be ex-pected, understory differences will per-sist. Early in the chronosequence, mi-crosites diverge under the influence ofdifferent pioneers, and it is only laterthat there is the potential for even par-tial convergence. Because Quercus ilexand Q. pubescens (s.l.) are both long-lived species capable of exerting stronginfluences in the understory, it is likelythat heterogeneity within a site will de-cline. Increasing similarity and reducedoverall variation reflected by NMS sup-port this assumption. However, becau-se Quercus individuals, even within asmall area, may establish at differenttimes under varied circumstances, va-riation between samples may remainhigh (as it did in this study).

The situation on the lower southslopeofMountEtnasuggests ahypothe-sis that can be tested over a long periodusing a modified chronosequence con-cept. Proper comparisons arenotbetwe-en samples based on their calendar age,but on their relative age, determined bymeasures of vegetation development.Completely deterministic succession isunlikely to occur because the effects ofearly, often stochastic or contingenteventspersist indefinitely.Duringa longchronosequence new factors are intro-duced that affect young sites that neverinfluenced older ones. For example,disturbances due to roads, quarries, andpastures alter the landscape; new spe-cies that potentially can alter growingconditions are transported to the site;and climate patterns shift. However,there are some deterministic processesthat do mute heterogeneity, limit thespecies capable of persisting, and canpromote convergence. Legumes (e.g.Lupinus, Vicia) alter soil nitrogen, whi-le tall vegetation modifies the site tofavor mesophytic species. The diversi-ty and abundance of annuals are redu-ced in favor of persistent herbaceousperennials.

What we observed on Mount Etnais that several communities assembleduring the first few centuries on similar


substrates. Each is the result of uniquecombinations of variable substrate mor-phology, landscape factors, competiti-ve interactions, and stochastic proces-ses. Collectively the vegetation forms amosaic. At a given site, the vegetationbecomes more homogeneous over time,but the rate of change is variable, con-trolled primarily by substrate factorsthat do not strongly correlate with age.The oldest exposed lava supports vege-tation similar to that of the youngestbecause it hasdecomposedmoreslowly.Once established, species can fill openspace (hence the importance of priorityeffects.) Increasing homogeneity canproduce convergence. Even initially si-milar sites can develop distinct patterns(e.g. the two 1636 samples). Eventual-ly, canopy shrubs and small trees mayexert strong dominance on the under-story, but within the span of this study,their understories remained variable.The understories of Quercus vegeta-tion, dominated by persistent species,remain captive to their histories. Whilethe development of a canopy spurs spe-cies turnover, it does not necessarilyproduce convergence. Vegetation de-velopment on the young lavas of MountEtna combines assembly with succes-sion, but succession processes have yetto manifest strongly. This Mediterra-nean climate prolongs colonization, sothat a longer chronosequence is requi-red to provide a better assessment of thedegree of convergence on this volcano.

ACKNOWLEDGEMENTS

We thank Maria Grillo for plantidentification and translations. RdM isparticularly grateful to the students ofthe Ecology Section, University of Ca-tania for their valuable assistance andhelp in the field.

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