Hyperspectral remote detection of niche partitioning among canopy trees driven by blowdown gap disturbances in the Central Amazon

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Abstract
Advanced recruitment and neutral processes play important roles in determining tree species composition in tropical forest canopy gaps, with few gaps experiencing clear secondary successional processes. However, most studies are limited to the relatively limited spatial scales provided by forest inventory plots, and investigations over the entire range of gap size are needed to better understand how ecological processes vary with tree mortality events. This study employed a landscape approach to test the hypothesis that tree species composition and forest structural attributes differ between large blowdown gaps and relatively undisturbed primary forest. Spectral mixture analysis on hyperspectral satellite imagery was employed to direct field sampling to widely distributed sites, and blowdown plots were compared with undisturbed primary forest plots. Tree species composition and forest structural attributes differed markedly between gap and non-gap sites, providing evidence of niche partitioning in response to disturbance across the region. Large gaps were dominated by classic Neotropical pioneer genera such as Cecropia and Vismia, and average tree size was significantly smaller. Mean wood density of trees recovering in large gaps (0.55 g cm−3) was significantly lower than in primary forest plots (0.71 g cm−3), a difference similar to that found when comparing less dynamic (i.e., tree recruitment, growth, and mortality) Central Amazon forests with more dynamic Western Amazon forests. Based on results, we hypothesize that the importance of neutral processes weaken, and niche processes strengthen, in determining community assembly along a gradient in gap size and tree mortality intensity. Over evolutionary time scales, pervasive dispersal among colonizers could result in the loss of tree diversity in the pioneer guild through competitive exclusion. Results also underscore the importance of considering disturbance processes across the landscape when addressing forest carbon balance.

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