Tackling trade-offs? Understanding how trait coordination for drought- and shade-tolerance regulates seedling dynamics in fragmented wet tropical forests

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Tropical forests house over a third of Earth’s biodiversity and play a critical role as global carbon sinks, but stand threatened by climate change and fragmentation due to human land-use. It is therefore an urgent concern to disentangle the mechanisms by which fragmentation alters tropical forest dynamics in a drought-prone future climate. The coordination of plant tolerance to multiple stresses is a fundamental question in ecology with pervasive consequences for species distributions, community composition and ecosystem function. Two long-standing conflicting hypotheses propose that tolerance to drought and shade - two key dimensions of plant performance - correlate positively (synergy hypothesis) or trade-off (trade-off hypothesis). A third possibility is that shade and drought responses are independent (independence hypothesis). Results concerning the coordination of drought- and shade-tolerance remain conflicting.In this proposed research, our overarching goal is to provide fundamental insights into how the coordination of drought- and shade-tolerance influences seedling dynamics in wet tropical forests subject to global change. Specifically, we aim to understand (1) edge effects on light and water availability in fragmented forests, (2) how gradients of light and water availability correlate with seedling distributions and community composition, (3) interspecific variation in seedling performance (survival and growth) responses to light and water availability, (4) the trait-based mechanisms underlying seedling responses to water and light availability, and (5) the coordination between drought- and shade-tolerance of species and its consequences for regeneration dynamics of fragmented forests in a changing climate.Our fully integrated Indo-German work programme will be conducted in a human-modified forest complex in the central Western Ghats (India). We will combine observational and experimental approaches to integrate detailed characterizations of microsite conditions and assessment of seedling performance and community composition in a network of seedling plots, with comparative assessments of 18 relevant physiological, anatomical and morphological functional traits for seedlings of 25-30 tree species. This unprecedented data set will allow for a cutting-edge evaluation of the coordination of species drought- and shade-tolerance and its consequences for forest regeneration. Overall, our study will address fundamental knowledge gaps in trait-based ecology, advance our understanding of forest response to global change, and generate science-based inputs for restoration efforts geared towards supporting vulnerable species or selecting species that can mitigate the impacts of global change on biodiverse tropical forests.