DOI: 10.5061/dryad.2v8f9
title: Data from: Edge-mediated compositional and functional decay of tree assemblages in Amazonian forest islands after 26 years of isolation

description: 1.Islands formed upstream of mega hydroelectric dams are excellent
 experimental landscapes to assess the impacts of habitat fragmentation on
 biodiversity. We examined the effects of plot-, patch- and landscape-scale
 variables on the patterns of floristic diversity across 34 forest islands
 that had experienced 26 years of isolation since the creation of the 4,437
 km2 Balbina Hydroelectric Reservoir of central Brazilian Amazonia. In
 addition, three undisturbed continuous forest sites in neighbouring
 mainland areas were also sampled across a comparable elevational gradient.
 2.We identified all live trees ≥10 cm DBH at species level within a total
 of 87 quarter-hectare forest plots and conducted a comprehensive
 compilation of functional attributes of each tree species. We then
 examined species-area relationships (SARs) and the additional effects of
 patch and landscape scale metrics on patterns of tree assemblage
 heterogeneity, both in terms of taxonomic and functional diversity.
 3.Despite a clearly positive SAR, edge-mediated forest disturbance was the
 single most important driver of species composition and abundance within
 islands. Our results suggest that non-random floristic transitions within
 island plots followed a predictable pattern, with different life-history
 traits either penalizing or rewarding local persistence of different
 functional groups. Distance to edges mediated the probability of tree
 mortality induced by windfalls and episodic surface fires, clearly
 resulting in faster species turnover and unidirectional changes in guild
 structure within small islands where light-wooded fast-growing pioneers
 largely replaced heavy-wooded species of the old-growth flora. 3.Synthesis
 ─ Following a simultaneous 26-year post-isolation history, we disentangle
 the effects of habitat loss and insularization on tree assemblages within
 a large set of Amazonian ‘true’ forest islands, of variable sizes, sharing
 a uniform open-water matrix. Area effects are expressed via a response to
 edge effects, with trees in smaller islands being more vulnerable to
 edge-related surface fires and windthrows. Additionally, forest edge
 effects can be a powerful driver of non-random floristic transitions
 across islands within the Balbina archipelago via a process of rapid
 pioneer proliferation, drastically affecting both the taxonomic and
 functional composition of insular tree communities. Finally, our results
 indicate that detrimental effects of forest fragmentation induced by
 hydroelectric dams are considerably stronger than those of forest patches
 embedded within a terrestrial vegetation matrix.
description: tree species
 dataData were collected in
 2012 in 87 one-quarter hectare forest plots within the Balbina
 Hydroeletric Dam. The first column indicates the scientific name of each
 species recorded on at least one forest plot. The first line indicates
 forest plot identification. See the 'metrics data' for
 information of the island name according to each forest plot
 identification. The matrix contains the presence/absence data of each tree
 species within each of those 87 forest plots.Data_sps.xlsmetrics dataPlot, patch and landscape metrics of each surveyed forest plot within the Balbina Hydroeletric Dam. Abbreviated headlines are as follows: 'local' = island name; 'plot_ID' = plot number identification; 'area' = island area in hectares; 'slope' = the difference between maximum and minimum plot elevations based on the Shuttle Radar Topography Mission; 'windstorm' = the angular difference (0 - 90°) between the main axis of each rectangular plot and the median angular direction of prevailing strong winds recorded prior to and during convective windstorms; 'edge_mean' = the mean linear distance between the five midpoints of every 50-m section of each plot and the nearest points along the island perimeter; 'fire' = the understorey burn or fire severity measured according to a composite ordinal score (0–3); 'prox' = modified McGarigal et al. (2012) proximity index by considering both the aggregate area and distance to any land mass within 500 m of each island; 'dist_mainland' = the shortest linear distance from each island to the nearest mainlandData_metrics.xls