Water cycling (pools and movement) through an enclosed tropical forest in response to drought.

Tropical rain forests are greatly dependent on water supply and are highly efficient in water cycling. Soil infiltration rates as well as tree transpiration rates are high in these often seasonally dry ecosystems. Both deforestation and climate change have been shown to cause drought stress in tropical forests, the former through the increase of runoff and reduction in evapotranspiration, the latter mainly through the reduction in precipitation and transpiration.

Although great efforts have been made to determine the ecosystem and species responses to variable water supply, many processes determining how tree species in tropical ecosystems impact and are impacted by the water cycle (water uptake and redistribution, and stem storage) remain poorly understood. Water movement through trees, as measured by a D₂O pulse label in the rainwater, was found to be high variable and species dependent in a previous experiment in the Biosphere 2 tropical rainforest (Evaristo et al. 2019). We hypothesized that differential rooting depth and/or stem water storage could be the main causes for the difference in water label transport through the trees.

Our study is part of a large-scale experiment in the Biosphere 2 tropical forest that uses isotope labeling (¹³C and D) to trace C- and water-cycle processes underpinning ecosystem responses to drought from a molecular to an ecosystem-scale level. Here, we focus on the water cycling of this ecosystem and how it is impacted by controlled drought and rewetting conditions. Detailed continuous measurements of both the water pools (soil and stem) and movement (stems, atmospheric fluxes) will be used to determine individual tree (including different species) and whole ecosystem responses to drought. These data will be presented in light of their implications for tropical forest water movement and drought vulnerability.

Reference

Evaristo J, Kim M, van Haren J, Pangle LA, Harman CJ, Troch PA, McDonnell JJ, (2019) Characterizing the fluxes and age distribution of soil water, plant water and deep percolation in a model tropical ecosystem. Water Resources Research, 55(4), 3307-3327.