Water availability to plants is one of the most important drivers of plant ecology, from species distribution to ecosystem functioning. This availability is defined by water input (via precipitation), water loss (via evapotranspiration), and, critically, water retention and storage (given by soil properties and topography). The large majority of studies of plant ecology focus on the first two drivers, which compose the climate, but tend to overlook the local water availability modulated by soil and topography (as might be captured, for example, in the water table depth - WTD, a key index of local water availability). Results presented here are based on intensive and extensive studies of ~ 100 plots distributed over ~ 1400 km across the heart of the Amazon, plus remote sensing, to understand the importance of water table depth to forest structure and function.
Results/Conclusions
We show how this hydrological component – WTD - determines several patterns of plant distribution, acting as a filter of plant traits and thus affecting forest structure, composition, dynamics and ecosystem functioning over the entire Amazonian basin. In summary, forests over shallow water table depth (shallow WTDFs) have higher beta diversity, a distinct species composition, more resource acquisitive and hydrologically vulnerable trait composition, shorter canopies and lower biomass than deep WTDFs. During “normal” years, shallow WTDFs have higher mortality rates and lower growth rates than deep WTDFs, but during droughts these forests are buffered, having comparatively lower mortality and higher growth, and increased ecosystem productivity (represented by remotely sensed Enhanced Vegetation Index). Over 30% of Amazon forests have shallow water tables yet are under-represented in networks of permanent forest plots, and essentially absent from long-term eddy flux sites. We present evidence favoring the hypothesis that controversial debates about the resilience of tropical forests to drought arise in part because the neglected shallow WTDFs are more resilient to drought than other forests, and represent hydrologic refugia from drought, due to differences in underlying soil water regimes and associated functional composition. This challenges the perspective of widespread negative effects of climate change on Amazonian forests.