Water stress and carbon uptake in the tropics: surface temperatures as a new indicator

Abstract:

During photosynthesis, plants lose water from their leaves in the form of transpiration. Therefore, in periods of low soil supply or high atmospheric demand for water, plants tend to either decrease or shutdown photosynthesis in order to conserve water and reduce the risk of cavitation. When this occurs, carbon uptake can slow or cease, with implications for the atmospheric CO₂ growth rate. Due to the feedbacks between plant water-stress and the carbon cycle, it is necessary to first be able to identify these events, in order to then accurately quantify the resulting changes in carbon uptake. This is especially important in tropical rainforest regions, which have the highest biomass density of all forests.

Current water stress indicators typically hinge on the accuracy of evapotranspiration (ET) data, a flux which is very difficult to measure globally and is often estimated with assumptions, thus leading to high degrees of uncertainty. To get around this issue, here we use a new indicator of water-stress, a ratio of land surface/atmospheric temperatures, which both have much lower levels of uncertainty than ET products. Using this new indicator combined with random forest models, we are able to determine the leading environmental factors that cause vegetation water-stress throughout the tropical regions of Africa and the Americas, and then to quantify how water stress is reducing the ability of vegetation to uptake carbon across these continents.