Plant water relations during drought and recovery in an experimental rainforest

Increasing drought in the tropics poses a major threat to rainforests. Therefore it is crucial to understand how individual plant species utilize water. In a large-scale drought experiment in a model rainforest (Biosphere 2, USA), we investigated the role of above- and belowground physiological responses to drought and recovery. Severe drought was concluded with a deep-water pulse highly enriched in 2H. This pulse allowed us to distinguish between deep and shallow rooted plants. Our study comprised five species, three canopy and two understory species. We assessed physiological responses using leaf water potential, sap flow, and high-resolution monitoring of leaf gas exchange, including concentrations and stable isotopes of H2O and CO2. We derived plant water uptake and leaf water use efficiency (WUEleaf) at high temporal resolution. The observed water use of species and plants was highly variable, resulting in varying trends of decreasing, relatively constant, or increasing WUEleaf across plant individuals. We hypothesize that these different responses were strongly related to the individual plants’ water access. We propose that individuals with constant WUEleaf were able to sustain their leaf gas exchange due to deep soil water access. Plants with increasing or decreasing WUEleaf depended primarily on surface soil water and had limited or no access to deep water. Our findings highlight the plasticity of water use strategies beyond species-specific strategies.