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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 sis; GF=growth-form; GPS=global po- can lead to alternative stable vegetation sitioning system; HC=half-change; types (FATTORINI &HALLE, 2004; TEM- Weinvestigatedthedegreetowhi- NMS=nonmetricmultidimensionalsca- PERTON &ZIRR,2004;YOUNG etal.2005). ch vegetation becomes more similar ling; PS=percent similarity. Convergence can be recognized if during primary succession and asked sample similarity increases with age, whether the age of a lava site alone NOMENCLATURE: Pignatti (1982). but chronosequence methods may con- determinesspeciescompositiononothe- found site and stochastic effects with rwise similar sites or if site-specific effects due to age. Though chronose- factors are more important. The study INTRODUCTION quence methods must be employed in wasconfinedto lavaflowsfoundbetwe- long trajectories (DEL MORAL&GRISHIN, en 1,000 and 1,180 m on the south side The mechanisms that guide the 1999), the underlying assumption that of Mount Etna, Italy that formed from assembly of species are complex (KED- all sites were initially identical has ra- 1892 to 1169 or earlier. Ground layer DY, 1992; WALKER & DEL MORAL, 2003). rely been tested. Here we explore the cover wasmeasured at15exposed sites During primary succession, landscape relationship between time and develop- and 12 sites under shrubs, using ten 1- context and chance produce mosaics ment on a small part of Mount Etna, m2 quadrats in five plots at each site. (DEL MORAL, 1998), whose variation Sicilytoexploretherelationshipbetwe- Changes in species richness, cover, di- declinesovertime(RYDIN&BORGEGÅRD, en age and vegetation development. versity,dominance,andsimilaritywere 1991; DEL MORAL &JONES, 2002). The We sampled a ~832 yr chronose- only loosely related to lava age regard- relationship between composition and quence on lavas of MountEtna to explo- lessofwhetherplotswereexposedorin environmental factors usually stren- recommunityassembly.PoliMarchese anunderstory.Thesemeasuresdidchan- gthens to improve predictability (WIL- suggested (POLI,1965,1970,1971; POLI ge predictably when compared to the SON et al., 1995). However, neither re- &GRILLO, 1975; POLI et al., 1995; POLI degree of site development measured duced heterogeneity nor stronger deter- MARCHESE &GRILLO, 2000a) that con- by vegetation cover. Analysis by non- ministic control of patterns ensures that vergence was likely at the plant-socio- metric multidimensional scaling orde- trajectories will converge. logicalclasslevelafter1200yr,butthat red vegetation by their degree of deve- The traditional view of succession rates varied with surface morphology, lopment, not along an age gradient. The (CLEMENTS, 1916; BRAUN-BLANQUET, microclimate, and dispersal (cf. MAKA- presence of leguminous shrubs altered 1964; FACELLI &D’ANGELA, 1990) is NA &THOMAS, 2004). Poli Marchese has species composition and thus succes- that all trajectories converge to a single described how many early stages beco- sion trajectories. Variable initial surfa- association.This view assumes thatbio- me a few shrub stages, and ultimately ce morphology, landscape factors, hi- tic interactions are intense and can over- woodlandassociationsintheclassQuer- storical conditions, and random events ride initial variations (MUELLER-DOM- cetea ilicis. have affected both species establish- BOIS, 2000) and was demonstrated on Weexploredground layer compo- ment rates and trajectories. Determini- Icelandic lavas by BJÄRNASON (1991). sition on substrates of different ages stic processes (e.g. competition) have The alternative view is that mosaics and contrasted variation in exposed si- not smothered the initial heterogeneity, persist due to priority effects and spe- teswithunderstoryvegetation.Wesou- nor have they after over 800 years for- cies traits (e.g. RAMENSKY, 1924; GLEA- ght evidence for convergence by exa- ced understory development to a com- SON, 1939; WHITTAKER, 1974; YOUNG et mining similarity changes along the mon terminus. Evidence for conver- al.,2001).This “assembly” view asser- chronosequence. Fundamental to the gence (e.g. increased similarity within tsthatspeciesaccumulateandemphasi- application of the chronosequence ap- plots with increasing age) was obtained zes contingency and priority (EGLER, proach is that these assumptions are only if sites were arrayed by their deve- 1954; DRAKE, 1990; DEL MORAL et al., true: a) differences on the site are due lopmental,notcalendar,age.Thus,using 1995; HONNAY et al., 2001; DEL MORAL solely to the age of the site; b) differen- developmentalagemayovercomesome et al., 2005). Multiple trajectories are ces during establishment (e.g. weather of the intrinsic pitfalls of the chronose- commononglacierforelands(MATHEWS, patterns) are unimportant; c) landscape quence approach when assessing vege- 1992; FASTIE, 1995), slack dunes (ADE- effects are minimal; and d) dispersal tation dynamics. MA et al., 2002), sand dunes (LICHTER, effects are similar at each stage. It is 2000), riparian sites (BAKER &WAL- becoming clear that if these assump- KEYWORDS: chronosequence, conver- FORD,1995),disturbedforests(MCEUEN tionsarenotvalid,theinterpretationsof gence, primary succession, similarity, &CURRAN, 2004; SVENING &WRIGHT, succession can be in error (JACKSON et trajectory, volcanoes. 2005) and volcanoes (WHITTAKER & al., 1988). Further, local substrate va- JONES, 1994; TAGAWA,2005).Thereisa riation and constraints on plant growth ABBREVIATIONS: ANOVA=analysis of developing consensus, particularly can cause succession trajectories to de- variance; CV=coefficient of variation; amongrestorationecologists,thatcom- velop at significantly different rates DCA=detrendedcorrespondenceanaly- munity assembly is often stochastic and (ELLIS, 2004), compromising the use of 226 BENSETTITI A., BIORET F., BOULLET V., PEDROTTI F., Centenaire de la Phytosociologie

Tab. 1 - Site age, location and general vegetation. Date is the year of the documented flow. minated either by tall shrubs, hencefor- Coordinates are for the most distant sites. thcalled shrubsites, orby ground layer species, henceforth called exposed sites (Tab. 1). Latitude and longitude were determined by GPS, and plotted on a topographic map, from which elevations were determined (Fig. 1). There is a vegetation mosaic formed in response to lava age, surface morphology, microsites, and later disturbance (POLI MARCHESE &GRILLO, 2000b). Stereocaulon vesuvianum and other lichens and mosses colonized barren lavas that were formed since 1910. These species form soil that fills crevi- cesandfacilitateinvadingvascularplan- ts.Centranthusruber and Rumex scutatus are early vascular plant pioneers because they can establish in crevices. The 1892 flow had a mosaic of cryptogams, withscattered annualsand perennial forbs confined to cracks. The 1886 flow supported a denser mosaic. Thenitrogen-fixingshrubsGenista aetnensis and Spartium junceum had established sporadically. As these shrubs space-for-time substitutions. affect this zone on very old substrates. alter their surroundings, they may faci- Selected lavas date from 1169 (or ear- litate herb establishment. Lavas formed lier), 1536, 1537, 1634-1638, 1766, in 1780 were floristically similar to METHODS 1780,1886,and1892,andwerelargely younger ones, withMicromeria graeca free from such disturbances. Younger locally common. A flow deposited in STUDY AREA lavas(1910 and1983)werecommonin 1766 sustained scattered Genista and the study area, but they lacked signifi- Spartium. A complex array of flows Mount Etna dominates north ea- cant vascular plant vegetation. All sites was deposited from 1634 to 1638 (here stern Sicily. This volcano reaches 3350 were on a’a lava (POLI, 1970), which termed “1636”). Diverse herbaceous m a.s.l. Chronic eruptions, from the fractures to facilitate succession. The vegetation was in close proximity to summit and from many fissures, have nearest weather station is in Nicolosi extensive areas dominated by Genista occurredsinceA.D.500.Weconducted (698 m), where the mean annual preci- and Spartium. The 1537 flow extends this study on its south slope between pitation of 111 cm, occurs primarily beyond the 1886 flow, and shrubs do- 1,000 and 1,180 m elevation during during autumn and winter. The mean minated the sample plots. The 1536 May 2001, the height of the growing temperature is 14.3 °C. flow, dominated by herbs, was in close season. Pastures, orchards, and quarries The vegetation samples were do- proximity to vegetation dominated by Quercus ilex that had established on a flow 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 different age.

SAMPLING METHOD

Nested sampling was used to par- tition variation and similarity at three scales (Tab. 2). The site was homogeneous with a known age and little disturbance. There were three samples per site, except that for understories, only one sample from 1766 and two samples from 1537 were obtained. Sin- Fig. 1 - Locations of study sites. Key to symbols: =exposed-1892; =exposed-1780; cetherewerefewareasinthestudyarea =exposed-1636; =exposed-1536A; =exposed-1536B; =understory-1886; large enough to satisfied the selection =understory-1766;=understory-1636; =understory-1537; =understory-1196. criteria, sites were selected subjecti- BRAUN-BLANQUETIA, vol. 46, 2010 227 vely,butstartingsamplelocationswere Tab. 2 - Sampling design. determinedbytossingastake(i.e.itwas haphazard). Samples of a site were se- parated by at least 50 m. Each sample had five plots, with four surrounding the initial plot within 10 to 30 m (determined randomly) at the four cardinal directions. The plant cover was estimated using ten 1-m2 quadrats in each plot. Quadrats were divided into 25 1 Except 1 sample for 1766 understory and 2 samples for 1537 understory. squares to facilitate estimation. For exposed sites, quadrats were located in Structural features of plot vegeta- nate. POLI MARCHESE &GRILLO (2000a) a predetermined pattern. Due to irregu- tionwerecomparedwithone-wayanaly- provided a detailed description of the lar and small shrub canopies, understo- sis of variance (ANOVA), followed by successional dynamics of this vegeta- ries were sampled as follows. A shrub Bonferroni pair-wise comparisons of tion and listed the life forms and com- anchored a plot. One or two quadrats differences (ANALYTICAL SOFTWARE, munity affiliations for each species. (0.5by2mtofitunderthecanopy)were 2000). Graphs were produced with Mean cover at ten sites is shown sampled. Understories were sampled in Axum 7 (INSIGHTFUL CORPORATION, for species with > 1% cover in at least an expanding circle of shrubs until ten 2001). one plot (Tab. 3). Rumex multifidus quadrats had been described. A second dominated E1892 and E1536 sites, but plot was established at least 30 m from wasscarceinbetter-developedexposed the nearest sampled shrub. This proce- RESULTS lavas and also in the understory. Bro- durecontinued untilfive plots hadbeen mus tectorum was best developed in sampled. Sampling was based on shru- Both exposed and understory ve- lavas with good vegetation develop- bs, not microsite differences. getation occupied a chronosequence se- ment (Tab. 4). Centranthus ruber was veral centuries long. Exposed vegeta- common across all sites and ages, but tion was sampled on sites initiated du- was best developed on U1886, which STATISTICAL METHODS ring episodes dated from 1536 to 1892. has a rough surface to permit its esta- Understory vegetation was sampled on blishment. This species and other pe- Sites were described using rich- sites initiated from 1169 (or earlier) to rennials as Rumex scutatus form poc- ness, the information theory [H’ = - Σpi 1886. Shrubs alter microclimate condi- kets of vegetation between blocks on ln p], and Simpson’s Index comple- i tions to favor growth and development the flow. Its absence beneath Quercus 2 ment [D = 1- Σpi ], each calculated at of asomewhatdifferentsuiteof species suggestedthatitcanpersistinachrono- several scales. The mean number of than those that colonize bare lavas. sequence until dense shade, deep tan- species (richness) was determined for nin-rich litter and deeper soils are for- quadrats, plots, and samples. med. Grasses, Asteraceae, annuals, and We used nonmetric multiple di- FLORISTICS other small herbs dominated the lavas mensional scaling (NMS; MCCUNE & ofE1780andE1636.ThelavasofE1536 MEFFORD, 1999) to visualize relation- Therophytes(59%)andHemicryp- were relatively barren, and were cha- ships among plots and samples. Preli- tophytes (25.6%) dominated the sam- racterized by Rumex multifidus, Isatis minary analysis showed both sets to be pledflora.Theremainingvascularplants tinctoria, and an assortment of annuals two-dimensional (cf. MCCUNE &GRA- wereChamaephytes (7.7%),Geophytes includingAiracupaniana.Speciestypi- CE,2002).Thefinalanalysiswasstarted (2.6%)and Phanerophytes(5.1%).The- cal of exposed lavas were best deve- from the best configuration, based on rophyte dominance results from the loped in sites of intermediate age the least stress. Varimax rotation maxi- Mediterranean climate, the immature (1780 and 1636), but were sparse or mized the degree to which patterns were soil, and nearby human disturbances. absent from both younger and older aligned with the axes. Of the Therophytes, 20.5% were native lavas. Detrended correspondence analy- xerophilous species common to expo- Understory vegetation was related sis (DCA; MCCUNE &MEFFORD, 1999) sed habitats (Helianthemetea guttatae floristically to adjacent exposed vege- was used to compare ground layer ve- class) and an additional 23.1% were tation. Shrubs establish in cracks while getation and to determine heterogenei- ruderal-nitrophilous species (Stellarie- the ground layer can establish on surfa- ty. The eigenvalues from DCA at each tea mediae class) that spread on lava ces with or without cracks. As a result, site and scale estimated overall varia- after human disturbances. The Hemi- the establishment of shrubs does not tion.Speciesturnover(diversity)along criptophytes and the Chamaephytes depend upon the establishment of her- floristic gradients was estimated using established in cracks on a’a lavas be- bs,soweshouldexpectsimilaritybetwe- half-changes in the floristic composi- cause they possess extensive root sy- en the understories and the exposed tion. stems. Such species as Rumex scutatus, ground layers. However, the presence The relationships among quadrats Centranthus ruber, and some N-fixing of shrubs clearly alters the microclima- ofaplot,plotsofasample,andsamples Phanerophytes (Genista aetnensis and te, thus permitting species adapted to of a site were calculated by percent Spartiumjunceum)arecommonthepio- shade and more favorable conditions to similarity(PS=200Σmin (Xik,Xjk)/Σ(Xik neers on Mount Etna, as are species of expand at the expense of more stress- + X ), where X = cover of species k in jk these plant functional types elsewhere tolerant species. Centranthus, Rumex samples i, j). Calculations (KOVACH, (TSUYUZAKI, 1991). Once woody vege- scutatus, Isatis tinctoria, Geranium ro- 1999)usedthepercentcover(quadrats) tation has established, forest species bertianum, and grasses dominated site or mean percent cover (plots and sam- such as Geranium robertianum (Quer- U1886. Many of these species were ple). co-Fagetea class)caninvadeanddomi- common on young, exposed lavas.Bro- 228 BENSETTITI A., BIORET F., BOULLET V., PEDROTTI F., Centenaire de la Phytosociologie

Tab. 3 - Mean cover of species with a mean cover > 1% in at least one plot. Genista Isatis,Poabulbosa, Sedum tenuifolium, aetnensis, Spartium junceum and Quercus ilex were excluded because canopies were Geranium robertianum, and Microme- uniformly high (80 to 99%). Species listed by in order of DCA Axis 1 (all plots). ria, a species common in all older un- T=Therophytes;H=Hemicryptophytes,Ch=Chamaephytes,G=Geophytes,P=Phanerophyte. derstories. U1537, which was located GF=growth-form. on a flow that was partially covered by the lava of 1886, had a sparse canopy, with grasses, Centranthus, Isatis, several common forbs such as Crepis leon- todontoides, Sedum tenuifolium, Silene gallica, Galium aparine, andGeranium robertianum. This composition suggested some disturbance and little sup- pression by shade. The vegetation un- derQuercus (U1169) was distinct from theothersitesandincludedspeciesfrom the class Querco-Fagetea such as He- dera helix, Geranium molle, G. robertianum, and scattered legumes. Com- mon native species included Microme- riaand Rubia peregrina, a species from the class Quercetea ilicis.

STRUCTURE

Plots were analyzed by linear regression of plot age with the structural feature in question: species richness, percent cover, information theory diversity (H’), and the reciprocal of Sim- pson’s index (D). Patterns in richness were similar at each scale, so only plot values were analyzed by regression. Site richness (total number of species) was not linearly related to age in either exposed or understory plots (Tab. 4). Richness peaked in the intermediate site (1636) and was lower in younger and older sites. The pattern was similar Tab. 4 - Richness and cover in ground layer communities. All quadrats are included in in understory sites. Understories were calculations, even those without vascular plants. Date is the year of the lava flow; N is the richer in species in young sites, and in number of quadrats with vascular plants of 150 sampled; parentheses give the size of the lavas from 1537, but low beneathQuer- sample unit. There were five plots per sample of a site. cus (1169). There were significant differences between substrates ateach scale, but the differences were not related tosubstrateage.Plotrichnessdisplayed a strong quadratic relationship to age (r2 =0.64;P<0.0001).Thereweretwiceas many species per plot in samples that were 100 years younger. Richness beneath the sparse crowns of Genista and Spartium increased significantly with age at each scale. If Quercus samples are excluded, there is a significant linear increase in plot richness with age (r2 = 0.65; P < 0.0001). The relationship including Quercus is quadratic (r2 = 0.76;P<0.0001).Quercus samples had low richness at each scale and were Note: exposed and understory values were analyzed separately; superscripts indicate floristically distinct from other com- commonmembershipingroupsdeterminedaftersignificantANOVA(P<0.01)byBonferroni munities (Tab. 3). tests (P<0.05). Vegetation cover in exposed sites wasnotlinearlyrelatedtositeage(Tab. mus madritensis, Geranium robertia- des, and Galium aparine dominated 4). Percent cover demonstrated a signi- num, Briza maxima, Isatis, Cerastium U1766. U1636 was diverse, and contai- ficant quadratic relationship (r2 = 0.61; semidecandrum, Crepis leontodontoi- ned Centranthus, R. bucephalophorus, P < 0.0001). Cover of the understories BRAUN-BLANQUETIA, vol. 46, 2010 229 was similar to each other, except for thosebeneath Quercus,whichdatefrom 1169 or earlier. The quadratic relationship among all understory cover was strong (r2 =0.72;P<0.0001),butexclu- ding 1169 lavas eliminated any significant relationship. H’ changed significantly on both exposed and understory plots at each scale (Fig. 2). Quadrats of E1892 had significantly lower H’ than most older sites because many quadrats had only one species. The highest H’ values on exposed lavas at each scale were depo- sitedin1780andin1636.ThelowestH’ was on E1536, perhaps due to adverse surface conditions. Only the quadratic regression was significant (r2 = 0.59; P <0.0001).UnderstoryH’increasedwith age (r2 =0.29;P<0.0001),butthedense canopy of the 1169 plots reduced H’ dramatically. Dominance changed significantly Fig. 2 - Vegetation diversity (H’) on nine young lava sites. “Quadrats” is the mean value in both exposed and understory vegeta- for each quadrat (from 75 to 150) at a site; “plots” are the mean value for plots at a site; tion(Fig.3).Atthequadratlevel,expo- “sample” the mean value for each sample at a site. Values were analyzed by one-way sed sites increased in D between 1892 ANOVA, with exposed and understory vegetation analyzed separately. At each level, and 1636, but the 1536 samples were letters over the bars indicate group membership as determined by Bonferroni comparisons less diverse. At the plot level, exposed (P<0.05). lavas differed significantly, but the linear regression was poor (r2 = 0.08; P < 0.02). The quadratic relationship pre- dictedDbasedonagetoamuchgreater degree (r2 = 0.34; P < 0.0001) demon- strating that structural changes are not related primarily to lava age. The understories were relatively similar, but thequadraticregressionwassignificant (r2 = 0.57; P < 0.0001). When Quercus plots were excluded, there was a linear relationship of D with age (r2 = 0.45; P < 0.0001).

COMMUNITY PATTERNS-NMS

The 24 samples (Quercus samples excluded) were analyzed by NMS to clarifythefloristicpattern.Sampleswere composed of the mean of 50 quadrats (10 in 5 plots). The observed stress of 9.7 (instability < 0.06) provided a use- Fig. 3 - The complement of Simpson’s index (D) on nine young lava sites. Analysis able analysis. described in Fig. 2. The floristic pattern on exposed samples was poorly related to site age TURNOVER the variation among plots of a sample, (NMS-2;Fig.4).Thepatternwassigni- measured by standard deviations of the ficantly different from random and any Turnover is the floristic variation DCA scores, should decline. inferenceswouldberobust(stress<9.5 betweensamplesandistermedß-diver- The first DCA axis revealed no instability < 0.07). Exposed plots were sity. We examined turnover at the plot trendsofHCwithageorbetweenexpo- distinct from understory plots. The flo- scale with DCA because this method sed and understory plots. Understory ristic gradient, which describes patterns artfully measures floristic variation. plots also showed little pattern. Here, of accumulating species richness and Turnover was estimated by half-chan- turnoverwasinfluencedbyfactorsother cover, is not correlated to substrate age ges (HC) in DCA scores of the ordina- than age, in particular the surface mor- for either exposed or understory plots. tion(Tab. 5) when 120 plotswere analy- phology. If sites were arranged by the Within each group, there was a correla- zed (excluding U1169). If sites were degreeofvegetationdevelopment,then tion between the ordination and the de- more homogeneous with age, then tur- exposed sites demonstrated adecline in gree of plot development. nover should bereduced with time, and variation with time. 230 BENSETTITI A., BIORET F., BOULLET V., PEDROTTI F., Centenaire de la Phytosociologie

Turnover also was estimated by similarity among plots. The samples of a site were compared with each of the samples of the comparison sites and analyzed by ANOVA. There was significant turnover in response to age and canopy (Tab. 6). PS in exposed sites ranged from 3.6% to 51.5%, indicating large turnover. There were no trends of similaritywithageamongexposedplots. Similarity declined regularly if sites werearrayedbystructure.Forexample, the youngest site (E1892) was most similar to the oldest sites (E1536A, B) because the development of these old sites was least. Understory similarities were always lowest when compared to U1169. There were no clear patterns between understory similarity and their age differential. The similarity of other Fig. 4 - Nonmetric multidimensional scaling (NMS) ordination of 120 plots from 8 sites. understories to U1169 increased with U 1169 was excluded because it was too distinctive. site age, suggesting thatunderstory tur- novertendstowardscompositionfound Tab. 5 - Turnover (HC) in ground layer vegetation and the variation in DCA scores when under denser canopies. Species such as plotswereexaminedinasingledataset(U1169excluded).SDisthevariationofthesample Micromeria, Geranium robertianum, DCA scores of the site. Galium aparine, Poa and Sedum tenuifolium were common beneath Quercus and denser Genista and Spartium canopies.

WITHIN-SITE HETEROGENEITY

As a site matures, species invade, expand, and interact. An initially hete- rogeneous site should become less variable because as biomass increases, microsite variation is subdued. We measured heterogeneity using DCA of the individual sites and floristic similarity Tab. 6 - Turnover estimated by floristic similarity among understory and among exposed within sites at three scales. samples. The ground layer vegetation was explored to examine floristic variation (Tab. 7). The eigenvalues shown for E1892 and E1536-A are arbitrary because so many quadrats had no species in common. The eigenvalues of exposed siteswere least at intermediate ages andhighestinyoungestandoldestsites. This variation is correlated to vegetation development, not substrate age. HC (turnover) was reduced slightly at thequadratlevel,butatplotandsample levels HC was minimal at intermediate ages. Understory vegetation variation increased with plot age, as did HC, at each scale. Rather than becoming more homogeneous, each there is evidence for greater differentiation within a site. ThelowvaluesinU1766aredueinpart tothelowersamplesize.Exposedvege- tation generally expressed greater va- Note: Comparisons were made among all understory sites and among exposed sites riation at a given scale than did the (ANOVA, P<0.05) followed by Bonferroni comparisons; values with the same superscript understories of a comparable age. The are within the same group. exception was quadrats in 1636, where BRAUN-BLANQUETIA, vol. 46, 2010 231 the understory vegetation was slightly Tab. 7 - Summary of DCA of individual sites on the first axis at three scales. Eigenvalue more variable than the exposed sites. estimates variance, HC is the number of half-changes on the gradient. Shrub plots were We determined percent similarity analyzed without shrubs. (PS) among allquadrats of a site, among 10quadratsineachoftheplotsinasite, andamongthefiveplotsineachsample atasite(Fig.5).ThePSofexposedsites differed significantly with site age at each scale (ANOVA, P << 0.001). However, in no case was there a simple relationship between substrate age and community similarity, due primarily to surface effects. PS at exposed sites was lower than that of the understory at comparably aged sites, except in 1636, wheretheunderstoryvalueswerelower. ShrubspermittedhigherPSthaninexpo- sed sites at each scale, presumably by creating a more homogeneous environ- Note: A=values could not be calculated due to low species richness in many quadrats and ment. Though PS among understory the number of times quadrats had no species in common. plots differed significantly, there was noparticularpattern.Quercus plotswere changed dramatically among the expo- shrub-dominated vegetation. The lava the least similar, but at smaller scales, sed sites, but changes were not linearly of 1892 lacked shrubs, while that of theseoldsiteswerecomparabletoother relatedtoage.Speciesaccumulatedover 1886 had a significant shrub popula- sites. Over this chronosequence, varia- time at different rates, leading to a com- tion.Theeffectofacanopyistoaccele- tion within a site remains more impor- plex mosaic of similar vegetation on rate the development of the understory tant than effects that could create ho- substratesofdifferentages.Twoexam- and to permit a more rapid accumula- mogeneity. ples are the similarity between exposed tion of vegetation cover. vegetation on lavas of 1892 and 1536, Observations of young lavas sug- and the similarities among understory gestthatspeciesinthischronosequence CANOPY EFFECTS vegetation on lavas of 1766 and 1537. invade sporadically and individualisti- In contrast, vegetation of nearly the cally. They establish episodically, in- Shrubs strongly affected species same age may differ strongly, even asi- fluenced stronglyby stochastic andcon- composition and thus altered the de from the low similarities among the tingent factors and by variations in soil trajectory of succession. We compared samples of a site (Fig. 5). For example, depth, surface texture and morphology. the five exposed sites to the five under- the lava of 1636 sustains both well- Priority effects (BELYEA &LANCA- story sites (Tab. 8). The youngest un- developed exposed vegetation and STER, 1999) may alter trajectories. Whi- derstory was mostsimilar to older exposed sites, while the youngest exposed siteshadverylittleincommonwithany understory. Exposed andunderstory vegetation had limited similarity. The presence of a dense evergreen canopy markedly altered the species understory composition, such that its highest simi- laritywastothatoftheadjacentE1536A site. U1169 does not appear to be the “target” to which all trajectories are currentlyaimed.Intermediatestagesdo occurinthegeneralarea,butnotwithin theimmediateareasampled(POLI MAR- CHESE &GRILLO, 2000a).

DISCUSSION

The focus of this study was to determine if, during succession on young lavas, vegetationconverges.In a relatively confinedgeographic area,can the chronosequence approach be used to assess this question? Alternatively, do even minor differences in site quali- Fig. 5 - Percent similarity at three scales. Quadrats at site = mean pair-wise comparisons ties and location lead to persistent vege- among all quadrats (75 to 150) in one site; Quadrats in site = the mean of pair-wise tation differences? comparisons of quadrats within each sample; Plots = the mean similarity among the plots Measures of vegetation structure at a site. Statistics are described in caption for Fig. 2. 232 BENSETTITI A., BIORET F., BOULLET V., PEDROTTI F., Centenaire de la Phytosociologie

Tab. 8 - Floristic similarity (percent) of ground layer vegetation samples: exposed sites vs. The rate of vegetation develop- understory sites. Values are the mean of the nine pair-wise comparisons among the three mentonyounglavasofMountEtnahas samples of one site and the three samples of the comparison site. been fitful. Sites of the same age can differ subsequently due to surface characteristics, nature and proximity to poolsofpotentialcolonists,and contingent factors. The chance establishment of large herbs (e.g. Centranthus or Isa- tis) or tall shrubs (e.g. Spartium) alters the chronosequence. As a result, each site experiences a unique vegetation trajectory. Given the importance of legumes and of landscape effects, we believe that while convergence among dominants (e.g. Quercus) may be ex- pected, understory differenceswillper- Note:Comparisonswere madebetweenallunderstorysitesandallexposedsites (ANOVA, sist. Early in the chronosequence, mi- P<0.05) followed by Bonferroni comparisons; values with the same superscript are not different. crosites diverge under the influence of different pioneers, and it is only later le the Quercus plots sampled in this curs (GLENN-LEWIN & VAN DER MAAREL, that there is the potential for even par- study showed no evidence of having 1992). However, for the process to be tial convergence. Because Quercus ilex once supported Genista or Spartium, completed, initial effects due to land- and Q. pubescens (s.l.) are both long- other nearby sites at lower elevations scape (cf. DEL MORAL &LACHER, 2005), lived species capable of exerting strong have Q. ilexthathasinvadedtheseshru- persistentofinitialcolonizers,andchan- influences in the understory, it is likely bs, and which sustains a more diverse ce (cf. DEL MORAL &ECKERT, 2005) that heterogeneity within a site will de- understory. It is quite possible that the must be extinguished. To the extent that cline.Increasingsimilarityandreduced Quercus studiedhere did invade shrubs such confounding factors remain, con- overallvariationreflectedbyNMSsup- that long ago disappeared. Therefore, vergence cannot occur, even if homo- port this assumption. However, becau- while species accumulate and structure geneity is increasing (WALKER & DEL se Quercus individuals, even within a developsthroughtime,theprocesspro- MORAL, 2003). small area, may establish at different ceeds at different rates on different la- Turnover (HC) in exposed plots times under varied circumstances, va- vas and is affected by many factors. As declined with vegetation development riation between samples may remain noted by FASTIE (1995) in relationship on DCA-2, but not with lava age. The high (as it did in this study). to succession at Glacier Bay, the pre- presence of shrubs tended to increase, The situation on the lower south sence or absence of a legume can sub- not decrease understory variation. The slopeofMountEtnasuggestsahypothe- stantially alter the subsequent succes- understory beneath Quercus was even sis that can be tested over a long period sion (cf. DEL MORAL &ROZZELL, 2005). more variable at these scales, and was using a modified chronosequence con- Species turnover clearly occurred sodistinct thatwedid not analyzethese cept.Proper comparisonsarenotbetwe- on exposed sites. Many species that samplesalongwithotherplots.Therate en samples basedon their calendar age, were initially sparse in E1892 sites were of succession appears to vary on each but on their relative age, determined by common on older substrates. The pre- lava flow, leading to nonlinear changes measures of vegetation development. sence of shrubs, whether established in turnover with age. Completely deterministic succession is prior to, simultaneous with, or after Thereislittleevidence for conver- unlikely to occur because the effects of groundlayer, hasalteredthe herblayer, gence in this chronosequence if one early, often stochastic or contingent butdifferentdominantsoccurredindif- uses chronological age. However, if we eventspersistindefinitely.Duringalong ferent plots of a site. The composition applytheconceptofdevelopmentalage, chronosequence new factors are intro- of shrub understory appears less varia- there are some indications of partial duced that affect young sites that never ble when assessed by NMS (Fig. 4). convergence.Similarityamongquadrats influenced older ones. For example, Shrubs reduce options available to un- atasite,withinasample,and,toalesser disturbances due to roads, quarries, and derstory species and enhance local ho- degree within plots, increased with de- pastures alter the landscape; new spe- mogeneity. However, variation at the velopmental age (Fig. 5), but under- cies that potentially can alter growing sample scale remains similar to that of storyvegetationdemonstratednotrend. conditions are transported to the site; exposed sites. Variation patterns provi- We found no evidence for floristiccon- and climate patterns shift. However, de no evidence for convergence on any vergence at the level of a sample. there are some deterministic processes but the smallest scale. One small area Similarity between exposed sites that do mute heterogeneity, limit the mightdevelophomogeneityundershru- declines not with their age differential, species capable of persisting, and can bs, but substantial between-sample va- but with differences in development. promote convergence. Legumes (e.g. riationpersistedevenintheoldestsites. For example, E1892 is most similar to Lupinus, Vicia)alter soilnitrogen,whi- Shrubs increased the homogeneity of E1536A, which is the sample most si- le tall vegetation modifies the site to understory vegetation and changed spe- milar to it in cover and measures of favor mesophytic species. The diversi- ciescomposition. Shrubs increasedsha- dominance (Tab. 6). The PS of U1886 ty and abundance of annuals are redu- de and soil organic matter and altered to other understories declines not with ced in favor of persistent herbaceous the competitive balance among under- age but with structure. These results perennials. storyspecies.Overstorydominancemay suggest that within this small area, spe- What we observed on Mount Etna be the principal mechanism by which cies composition will tend to develop is that several communities assemble convergence to a single association oc- along somewhat similar trajectories. during the firstfew centuries on similar BRAUN-BLANQUETIA, vol. 46, 2010 233 substrates. Each is the result of unique sembly rules within a contingent tion development. Vegetatio, 4: combinations of variable substrate mor- ecology. Oikos, 86: 402-416. 412-417. phology, landscape factors, competiti- BJÄRNASON A.H., 1991 - Vegetation on ELLIS E.E., 2004 - Gradients in hetero- ve interactions, and stochastic proces- lava fields in the Hekla area, Ice- geneity and structure on lahars, ses. Collectively the vegetation forms a land. Acta Phytogeograp. Sueci- Mount St. Helens, Washington, mosaic. At a given site, the vegetation ca, 77: pp. 84. USA. 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