Fine root distribution and biomass allocation in Amazon forests with shallow water tables

Shallow water table (WT) forests represent more than 50% of Amazonian land areas and exhibit different characteristics and functioning compared to the more widely studied deep WT forests. Yet, less is understood about the determinants of biomass structure and allocation in these particular hydrological environments. Here we investigated how limitation imposed by soil water, soil properties and nutrient availability influence the distribution of fine root biomass and the allocation of belowground and aboveground biomass in shallow WT forests, and the relationships with their stand structure. We used extensive ground-based data on soil properties, soil nutrients, WT monitoring, and direct biomass measurements along a 600 km transect of shallow WT forests in the central Amazon region. Our results show that soil water excess, due to a wetter climate or a consistently shallow WT, restricts fine-root distribution to the upper soil layers. This restriction, alone or combined with phosphorus limitation, leads to a higher biomass allocation belowground, associated with stand structures characterized by a higher density of smaller trees. Opposite patterns are found where the soil experiences no excess water or does so infrequently. Soil fertility contributes to biomass partitioning, but with WT regimes strongly modulating its effects. We conclude that soil water regime, having an overarching control on fine-root distribution and biomass partitioning, challenges the dominant perspective of vegetation carbon allocation being mostly affected by soil nutrients and climate. These findings improve our understanding of a large fraction of the Amazon Forest, with important implications for modeling and predicting its resilience to climate change.