Introduction

The Amazon rainforest has long been recognized as a heterogeneous vegetation mosaic including flooded and unflooded (terra-firme) forests. More recently, it was recognized that about 50% of terra-firme forests grow on shallow water tables (WT) connecting those two main forest types through a more subtle waterlogging gradient. Previous studies from our group have shown that WT has important effects on forest structure and composition in the Amazon basin at local scales, selecting resource-acquisitive and hydraulically vulnerable tree species, and being associated with higher compositional turnover. But how those local patterns translate to spatial patterns of forest structure and composition across scales is unknown.

Hypothesis

We hypothesize that shallow water table forests are less diverse, shorter, thinner, and stock less biomass as a result of seasonal waterlogging and limited growth season.

Methods

We collect/compile forest structure and composition data for over 500 forest plots distributed across the entire Amazon basin. WT depth measures for each plot were obtained from field measurements or estimated from large-scale hydrological models or remote-sensing products. Forest structure was analyzed using regressions and structural equations modeling. Composition analyses were performed using multivariate analysis.

Results

At the whole Amazon basin scale, tree richness decreases in shallow WT forests within wet climates. Shallow WT forests store 18% less biomass than deep WT forests, even when under drier climates. The influence of WT on species composition was less clear and only observable in two of four Amazonian biogeographical regions. Within these same regions, species turnover increases in shallow WT depths. Controlling by climate and floristic composition, shallow WT forests are shorter and thinner.

Conclusions

Amazonian forest composition and structure are related to water table depth across scales. Shallow water tables act as a strong environmental filter with impacts to tree diversity and growth. Shallow water table forests may buffer forests against droughts, but at the cost of tree diversity, growth, and carbon sequestration under average historical climatic conditions. Future research should focus on unravelling the context-specific response of forest diversity and composition to local WT depth fluctuation and exploring the causes of the high species turnover within shallow water table forests. In both contexts (shallow and deep WT), functional biogeography studies can be promising but are currently limited by the availability of trait data on shallow water table forests and species, something we hope to change in the next years.