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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 si- tes (Tab. 1). Latitude and longitude were determined by GPS, and plotted on a topographic map, from which ele- vations were determined (Fig. 1). There is a vegetation mosaic for- med in response to lava age, surface morphology, microsites, and later di- sturbance (POLI MARCHESE &GRILLO, 2000b). Stereocaulon vesuvianum and other lichens and mosses colonized bar- ren lavas that were formed since 1910. These species form soil that fills crevi- cesandfacilitateinvadingvascularplan- ts.Centranthusruber and Rumex scuta- tus 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 aet- nensis and Spartium junceum had esta- blished 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 homoge- neous with a known age and little di- sturbance. 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: