How drought, waterlogging, and light availability shape patterns of tropical tree distributions, in French Guiana

Abstract

Tropical forests display striking patterns in species richness along environmental gradients. At local scales, spatial and seasonal variation in soil-water and light availability are strongly linked to performance differences between species, and therefore compositional turnover between habitats. Understanding ecological responses to rapid environmental change is an urgent necessity in tropical forests as future tree distributions are likely to alter along lines delimiting wet and dry habitats. However, it is currently unknown what controls turnover of species between wetter and drier habitats and to what extent regeneration is controlled by the interactive effects of water and light. As field gradients of drought, waterlogging, and light rarely occur independently from one another, or indeed from other important abiotic and biotic drivers, this confounding of variables make it particularly challenging to attribute observed distributions to environmental variation. I addressed these challenges by examining seedling and sapling performance under experimental and natural conditions of drought, waterlogging and light availability, utilising the contrasting wet and dry rainforest habitats of Paracou, French Guiana, South America, as the study system. I conducted a shadehouse experiment involving seedlings of eleven co-occurring tree species with contrasting distributional patterns among the wet and dry habitats of Paracou. Using survival modelling techniques, I examined survival times of 1532 seedlings and 20,000 individual observations to investigate species variation in survival along an experimental irradiance gradient and in three contrasting water availability regimes, ‘drought’, ‘waterlogged’, and ‘watered to field capacity’. I calculated tolerance indices for drought, waterlogging, and shade based on survival time predictions, then assessed indices for evidence of interspecific trade-offs. I found evidence of non-interactive effects of water and light on seedling survival and species tolerance indices revealed a negative relationship between drought and shade tolerance. These results suggest that, when nutrients are not limiting and in the absence of root competition, shaded tropical seedlings are impacted more strongly by drought than seedlings in higher light conditions. Then, I evaluated the roles of water and light availability in governing sapling survival in the field. I used data on the survival of 5374 individual saplings of 25 species, frequency of drought and waterlogging over 24 years, and detailed information on understory light availability. Utilising Generalised Linear Mixed Modelling techniques I showed that drought was the relatively more important factor determining differential species survival, as well as a likely important role of long-term soil instability associated with topographical water availability. By comparing survival responses of species with contrasting distributions among wet and dry habitats, I was able to infer that differential survival among wet habitat associated species may filter those species from drier habitats, whereas drier habitat associated species survive better than wet associated species in the driest habitats. Finally, I used Generalised Linear Mixed Modelling techniques to assess the ability of experimentally derived species tolerances of drought, waterlogging, and shade to predict sapling performance in the field. I found evidence that abundance in topographic habitats defined by variation in water availability did depend on differential tolerance of drought, and waterlogging, but not shade. I found no evidence that sapling survival was related to any of the indices. Studies based on whole-plant tolerances can directly link plant performance to variation in resource availability and have been credited with improving the understanding of species distributions by providing mechanistic and predictive links between observed distributions and environmental conditions. This study provides positive first steps towards using shadehouse derived tolerance indices to achieve explanatory and predictive power regarding sapling performance across topographic habitats. In summary, the results presented in this thesis strongly suggest that both water limitation and waterlogging are important factors limiting species performance and therefore structuring communities along environmental gradients. Under a scenario of increasingly dry weather, the forest of Paracou may see a turnover in species composition that would lead to more drought tolerant and fewer waterlogging and shade tolerant species. In this thesis, I have focussed on plant responses to locally contrasting soil-moisture availability. However, as plant responses at this scale are ultimately responsible for distributions at large as well as finer spatial scales, these results may be applicable more widely among neotropical and other tropical forests.