Development of vegetation patterns in early primary succession

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<ul><li><p>Development of vegetation patterns in early primary succession</p><p>Brynds Marteinsdottir, Kristn Svavarsdottir &amp; Thora Ellen Thorhallsdottir</p><p>AbstractQuestion: We investigated colonisation lters inearly plant community development on a glacialoutwash plain. We asked if these were related toseed limitation or to a lack of safe sites, if topogra-phical heterogeneity affected species patchiness andhow species life cycles inuence successional trajec-tories.Location: An outwash plain (Skeijararsandur) insoutheast Iceland.Methods:We identied surface heterogeneity at twodifferent scales, ca. 1015 cm (larger stones andestablished plants) and ca. 50m (shallow depres-sions representing dry river beds) at two study sites.We quantied species cover, owering plant density,seed production, seed rain, seed bank density, seed-ling emergence and seedling survival from June 2005to June 2007 for the whole plant community, andmeasured seed production for ve species.Results: Mean vegetation cover was o2.5% at thesites. Low emergence rates and high seedling mor-tality were the two main recruitment lters. Only1.4% of seedlings emerging in 2005 survived into the2007 growing season. Topographical heterogeneityhad little effect on plant colonisation. High annualvariation was recorded, and the two study sites (ca.2 km apart) differed in their colonisation success. Ofthe ve species, establishment of Cerastium alpinumand Silene uniorawas most limited by lack of seeds,whereas establishment of Luzula spicata, Poa glaucaand Rumex acetosella was most limited by safe sites.Conclusions: We conclude that colonisation pro-cesses and patterns in early primary succession on</p><p>Skeijararsandur were largely inuenced by stochas-tic factors.</p><p>Keywords: Iceland; Microsite; Outwash plain; Re-cruitment; Seed bank; Seedling survival; Seed rain;Topography.</p><p>Nomenclature: Kristinsson (2001, 2009)</p><p>Introduction</p><p>Plant colonisation and vegetation developmentmay be directed and rate-limited by two differentprocesses. The rst is related to the size and speciescomposition of the seed pool and probabilities ofdispersal and arrival onto the site. The second is themore deterministic environmental ltering reectingsuitable microsites for germination and growth (e.g.Eriksson &amp; Ehrlen 1992; del Moral &amp; Bliss 1993).The absence of a species at a site or a seral stage maytherefore be due as much to its absence from theseed pool as to its inability to establish (Fastie 1995).Successful early establishment of a population invirgin terrain can have long lasting implications forsuccessional trajectories (del Moral &amp; Lacher 2005).Those rst colonists may act as nascent foci (sensuMoody &amp;Mack 1988), and become a local source ofseeds. They may facilitate establishment of othercolonists by creating shelter (Castellanos et al. 1994;Franks 2003) and increasing subsurface biologicalactivity (Cargill &amp; Chapin 1987; Blundon et al. 1993;Callaway 1995).</p><p>During the initial stages of primary succession,seed input comes mostly through long-distance dis-persal. With time, a seed bank builds up and localseed production is believed to gradually take over asthe major seed source (del Moral et al. 2005). Al-though the importance of an available seed pool inprimary succession has been emphasised in manystudies (Titus &amp; del Moral 1998), few have at-tempted simultaneously to quantify the seed rainand seed bank and assess their spatial components(but see Wood &amp; del Moral 2000; Bossuyt &amp; Hermy2004).</p><p>In harsh, early successional environments, sur-face heterogeneity at various scales in the landscapecan create microsites that are differently favourablefor plant establishment. Depressions, large stones</p><p>Marteinsdottir, B. (corresponding author, Bryndis.</p><p>Marteinsdottir@botan.su.se): Institute of Biology,</p><p>University of Iceland, Sturlugotu 7, 101 Reykjavk,</p><p>Iceland. Present address: Department of Botany,</p><p>Stockholm University, SE-106 91 Stockholm, Sweden.</p><p>Svavarsdottir, K. (Kristin.svavarsdottir@land.is): Soil</p><p>Conservation Service, Keldnaholt, 112 Reykjavk, Ice-</p><p>land</p><p>Thorhallsdottir, T.E. (theth@hi.is): Institute of Biol-</p><p>ogy, University of Iceland, Sturlugotu 7, 101</p><p>Reykjavk, Iceland.</p><p>Journal of Vegetation Science 21: 531540, 2010DOI: 10.1111/j.1654-1103.2009.01161.x&amp; 2010 International Association for Vegetation Science</p></li><li><p>and established plants can facilitate the colonisationprocesses by creating shelter, retaining moisture andtrapping seeds (Wood &amp; del Moral 1987; Jumppo-nen et al. 1999; Franks 2003).</p><p>All the factors mentioned above are important forplant establishment, but have seldom been simulta-neously studied. In this paper, seed rain, soil seedbank, emergence of seedlings, seedling survival andvegetation cover were all considered and quantied.</p><p>We studied early primary succession on Skei-jararsandur, a vast (1000 km2), homogeneous gla-cial outwash plain in southeast Iceland. Our aim wasto determine which factors control plant colonisa-tion and development of vegetation patterns early insuccession. We asked: (1) Are the colonisation lterson Skeijararsandur related to seed limitation or to alack of suitable sites for establishment? (2) Does to-pographical heterogeneity affect species patchiness?(3) How do species life cycles inuence successionaltrajectories?</p><p>Materials and Methods</p><p>Study sites</p><p>Two sites at a pioneering successional stagewere selected near the upper centre of Skeijar-arsandur, (site A: 631570N, 171090W, site B:631560N, 171120W), an outwash plain south of Skei-jararjokull, the largest outlet glacier of Vatnajokullicecap. The two study sites, about 2 km apart, weremostly level with a subtly undulating pattern of atsand depressions (mean depth 0.1m, mean diameter26m), representing old ood pathways. These sitesappeared similar and were chosen as representativefor very large sparsely vegetated areas of Skeijar-arsandur. The soil in the area is classied as vitrisol,with mineralogy dominated by volcanic glass (Ar-nalds 2004). It is infertile, with mean nitrogen andcarbon concentrations of 0.01% and 0.05%, re-spectively. The soil was 74% sand, 20% gravel and6% silt/clay, and soil conditions were similar be-tween ats and depressions and between the twostudy sites, although site B had slightly ner soilgrain size than site A (Marteinsdottir 2007).</p><p>The climate on Skeijararsandur is maritime,with cool summers and mild winters. Temperatureand precipitation values during the study periodwere obtained from the weather station Fa-gurholsmyri (651530N, 161390W) approx. 26 kmfrom the study sites. The mean annual air tempera-tures were 5.11C and 5.81C in 2005 and 2006,respectively, and the mean summer (June-August)</p><p>temperature was 10.61C in both years. Mean annualand summer precipitation was 1593 and 469mmrespectively in 2005, and 2070 and 433mm respec-tively in 2006 (Icelandic Meteorological Ofce2007). During the study period the growing seasonat Skeijararsandur lasted from approximately mid-May to early September.</p><p>Sampling</p><p>Within each study site, 10 plots (1010m) werelaid out, ve plots in depressions and ve on ats, re-presenting mesoscale environmental heterogeneity atthe sites. The plots were positioned in the closest de-pression/at to a random point determined usingrandom compass bearings and distances. In July2005, 12 (0.50.5m) quadrats were randomly placedwithin each study plot (240 quadrats in total) formeasurements of seedling emergence and survival.Within quadrats, seedlings were counted, identied tospecies or group (i.e. graminoids and Galium spp.)and marked with a coloured wire ring, using differentcolours for each sampling date. Due to high seedlingdensity, smaller quadrat size (0.30.5m) was used for46 samples at site B. Seedling emergence and/or sur-vival was recorded six times during the study, 1 Julyand 22 August 2005, 12 June, 17 July and 20 August2006 and 20 June 2007.</p><p>For estimation of seed rain at the study sites, oneseed trap was randomly positioned in August 2005(N5 20) and two traps in August 2006 (N540),within each of the 20 plots. The traps were emptied inearly and late September in both years and the sam-ples combined before analysis. Seeds were countedand assigned to species or higher taxa, such as car-yophyllaceous species and graminoids. The seed trapswere 0.20.2m Astroturf grass mats anchored to theground with wire pins in each corner. This measure-ment only gives an abrupt estimate of the seed rain inthe area. Even though synthetic grass mats of this kindhave been used with good efciency in various seedrain studies in arctic regions (e.g. Molau &amp; Larsson2000), they can underestimate the seed rain of grami-noids and herbs (Larsson &amp; Molau 2001). The seedrain was also only measured in late summer whenmost species in the area shed their seeds. Late summershedding of seeds has been shown to be much greaterthan winter shedding (Ryvarden 1971, 1975), but thismay be offset by higher probabilities of long-distancedispersal in winter. Despite limitations of the seed rainestimates in this study, our measurements indicate theextent and composition of the local seed rain and al-low us to compare seed rain between topographies,sites and years.</p><p>532 Marteinsdottir, Brynds et al.</p></li><li><p>In early August 2006, 10-m transects were laidout, adjacent to each study plot (in the same de-pressions and ats). Vegetation cover was deter-mined along transects using the point interceptmethod. Five quadrats (0.50.5m), with 100 reg-ularly distributed points, were laid out at regularintervals and the rst hit recorded for each species ateach point. Vascular plants were identied to spe-cies, while bryophytes and lichens were eachrecorded as separate taxa. In addition, all vascularplant species were listed at both sites.</p><p>To analyse the seed bank, ve 10-m transectswere laid out in depressions and ve on ats at eachsite and seed bank data obtained by the seed germi-nation method (Baskin &amp; Baskin 1998). Along eachtransect, ve soil cores were collected at each ofthree microsites, representing the microscale topo-graphical variation at the sites: (1) the lee side of acushion plant (520 cm in diameter), (2) the lee sideof a stone (520 cm in diameter, max. height 10 cm)and (3) exposed controls, neither close to a stone nora cushion plant. The position of the cores was de-termined by choosing the closest appropriatemicrosite to a random point. A total of 300 soil coreswere collected in early May 2005 using an auger of5.3 cm in diameter down to 5-cm depth. The sampleswere spread as a maximum 2-mm thick layer ontrays lled with sterile peat (pH 5.5). The trays wererandomly placed in a greenhouse with additionalUV light during the day for the rst two months.Soil moisture was maintained by regular watering.No seedlings appeared in control trays (sterile peat).Seedlings were assigned to a species or in case ofCaryophyllaceae, Galium spp. and graminoids, tohigher taxa as they could not be identied at thespecies level. Counts were conducted weekly duringthe rst 10 weeks, but in alternate weeks thereafter.The study was terminated after 16 weeks, when nonew seedlings had emerged for six weeks.</p><p>For estimation on seed production, ve com-mon perennial species, Luzula spicata, Cerastiumalpinum, Silene uniora, Rumex acetosella and Poaglauca, were chosen to represent the community.These species are among the most common in thearea (collectively 49% of the total vegetation cover)and represent the two most important functionalgroups, graminoids and herbs. The above specieswill hereafter be referred to by their generic names.As abundance of owering individuals was low, nodistinction was made in seed production betweentopographies or sites. For each species, 15 oweringindividuals, growing within or close to the samplingplots were selected from the sites. In mid-July 2006,after stigmas had withered, inorescences of Luzula,</p><p>Cerastium and Silene were carefully bagged with1-mmmesh white nylon material. The bags were tiedto thin metal rods that arched at right angles overthe plant so the inorescence did not bear the weightof the bag. This procedure was carried out on Ru-mex and Poa in early August. All seed bags werecollected in late September. Mature seeds, abortedseeds and unripe seeds were separated and counted.Pre-dispersal seed predation was estimated forLuzula, Poa, Rumex and Cerastium when pupae,larvae or holes after larvae were observed in fruits.For Silene, the procedure described by Pettersson(1994) was followed so only individuals with swollenovules were bagged, and seed predation recorded inthe eld when a fruit was found to be completelyempty. The density of the bagged species and num-ber of owers or inorescence per individual wasrecorded in the 10 study plots at each site to obtainestimations of number of seeds perm 2.</p><p>To evaluate the relative importance of local seedproduction in the area and the inuence of differentlters on the colonisation of those ve species, a lifecycle diagram was constructed. Survival percentageswere estimated as follows: number of ovules perm 2 was estimated from the mean number of ovulesper ower or inorescence and the mean number ofowers or inorescence perm 2. Seed/ovule ratioswere obtained by comparing number of matureovules with the number of all ovules. Dispersed seedperm 2 was obtained by multiplying the number ofmature ovules perm 2 with the percentage of intactfruits. Germination percentage was calculated bydividing seedling density in 2005 by the number ofseeds in the spring seed bank in 2005. Survival wasthe percentage of seedlings emerging in 2005 thatwere alive at the end of the 2006 growing season.</p><p>Statistical analyses</p><p>Generalised linear models (GLM) with Poissonerror were used to test for soil seed bank differencesbetween sites, topography and microsites. Multiplecomparisons among microsites were done using at-test based on the best model, with Bonferroniadjusted signicance values (P5 0.025). For dif-ferences in number of seedlings between sites, topo-graphies and years, a GLMwith quasi-Poisson error(due to over-dispersion) was used. The same ap-proach was used to test for seed rain differencesbetween site, year and topography. The seed rainand seedling emergence data had high variation be-tween years, so each year was analysed separately.GLMs with quasi-binomial error (due to over-dispersion) were used to analyse the effect of site and</p><p>Vegetation patterns in early primary succession 533</p></li><li><p>topography on rst-year survival (from seedlingemerging to the rst sampling date in the next sum-mer), total survival (from seedling emerging in 2005until the last sampling date in 2007) and for com-parison between years in rst-year survival. Modelslooking at seedling emergence and survival includedcohorts as a covariate. The effect of site and topo-graphy on total vegetation cover was analysed in thesame way. In the GLMs, saturated models includedall variables as well as the interaction between them.To test for signicance of each variable, the ts offull and reduced models were compared in a hier-archical manner using an F-test for models...</p></li></ul>