Data from: Edge-mediated compositional and functional decay of tree assemblages in Amazonian forest islands after 26 years of isolation

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.