Community assembly along proglacial chronosequences in the high Arctic: vegetation and soil development in north-west Svalbard

1 Community assembly is described for two contrasting high Arctic chronosequences representing glacial regression of up to 2000 years on Svalbard. The chronosequences included a nutrient-poor glacier foreland (Midtre Lovenbre) and a series of nutrient-enriched islands (Loven Islands) progressively released from below a tidewater glacier. 2 Soil development and community assembly paralleled proglacial sequences elsewhere but time scales were extended and mature vegetation types comprised species-poor prostrate communities. 3 Initial colonization by Cyanobacteria stabilized soil surfaces and raised nutrient status. Cyanobacteria formed the dominant ground cover (up to 34%) for 60 years, after when they declined. 4 Vascular plants established slowly and represented minor components of ground cover for the first 100 years. Earliest colonizers were often species with ectomycorrhizal associations, followed by mid-successional species that tended to disappear as ground cover increased. Some species present in the mature vegetation at the oldest sites, established only after 60+ years. 5 Species richness of vascular plants increased for c . 100 years, beyond when only occasional species were added. Bryophytes became increasingly dominant with time. 6 Soil development on the Midtre Lovenbre and Loven Island chronosequences was similar after 100 years. Differences subsequently developed, with organic horizon depth, percentage organic matter and water content on the older Loven islands significantly greater than at equivalent Midtre Lovenbre sites. This was associated with increased bryophyte cover but lower vascular plant species richness. One explanation is a slightly more favourable microclimate, coupled with nutrient input from nesting birds. 7 Communities progressively recruit from a limited pool of effectively dispersed species, each with particular ecological requirements that determine their point of entry into the community. A measure of determinism by default is suggested in the way communities assembled. 8 Under climate warming, in the absence of nutrient enrichment, community development will accelerate but will be constrained by nutrient limitations and a restricted species pool. Where nutrients are less limiting, acceleration towards a moss-dominated community is expected, with a lower species richness of vascular plants.