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.

1. 大型水电站大坝上游形成的岛屿是评估生境破碎化对生物多样性影响的理想实验景观。本研究针对巴西亚马孙中部4437平方千米的巴尔比纳水电站水库（Balbina Hydroelectric Reservoir）形成后已隔离26年的34个森林岛屿，探究了样地、斑块及景观尺度变量对植物区系多样性格局的影响。此外，本研究还在相邻大陆区域选取了3个处于相似海拔梯度的未受干扰连续森林样地进行采样。
2. 本研究在总计87个0.25公顷的森林样地中，对所有胸径（DBH, Diameter at Breast Height）≥10厘米的活体树木进行物种水平鉴定，并系统整理了每个树种的功能性状。随后，本研究分析了种-面积关系（SARs），以及斑块和景观尺度指标对树木群落在分类多样性与功能多样性维度上的异质性格局的额外影响。
3. 尽管种-面积关系呈现显著的正向趋势，但边缘介导的森林干扰是影响岛屿内物种组成与多度的最核心驱动因子。研究结果显示，岛屿样地内的非随机植物区系更替遵循可预测的模式，不同生活史性状会对不同功能群的局域存续产生抑制或促进作用。距边缘的距离调控了由风倒与间歇性地表火引发的树木死亡概率，这一过程直接导致小型岛屿内的物种周转速率加快，功能群结构发生单向变化——速生轻质木材的先锋树种在这些小型岛屿中大量取代了原始林植物区系的重木质树种。
4. 综合分析——本研究以共享统一开阔水体基质、大小各异的亚马孙“纯正”森林岛屿群为研究对象，在所有岛屿均经历26年隔离的同步背景下，厘清了生境丧失与岛屿化对树木群落的影响。面积效应通过对边缘效应的响应得以体现：小型岛屿内的树木更易受到边缘相关地表火和风倒的威胁。此外，森林边缘效应可通过先锋种群快速增殖的过程，成为驱动巴尔比纳群岛内岛屿间非随机植物区系更替的强因子，大幅改变岛屿树木群落的分类与功能组成。最后，本研究结果表明，水电站大坝引发的森林破碎化所带来的不利影响，远强于嵌入陆地植被基质中的森林斑块所产生的影响。