Determining drought sensitivity of the Amazon forest: does plant hydraulics matter?

Abstract

Climate change is projected to cause significant shifts in precipitation patterns across the Amazon basin. This dissertation is designed to address key uncertainties surrounding our ability to predict the fate of the Amazon rainforest in a drier climate. The second chapter is an assessment of the ability of four leading dynamic vegetation models—CLM3.5, ED2, IBIS and JULES—to replicate observation from two long-term ecosystem-scale drought experiments in the eastern Brazilian Amazon. This analysis revealed that these four models can reliably predict plant and ecosystem carbon fluxes under the present climate, but still require substantial development for predicting the consequences of severe drought. These four models were not parameterized to mechanistically represent soil water-stress or the competitive differences in plant hydraulics that exist between tree species. Therefore, chapter three is a field-based study designed to quantify the range of variation in two plant hydraulic traits—xylem-P50 and turgor loss point (TLP)—that exists in mature tropical trees. The field measurements were made on four genera common to both experimental study sites. Each genus was categorized a prior into one of four plant functional types: early- versus late-successional that were each subdivided into drought-tolerant versus intolerant. Xylem-P50 and TLP occurred at water potentials that were 0.7 to 1.1 MPa and 0.75 MPa higher, respectively, in the drought-intolerant genera compared to the tolerant genera. In comparison, the early- versus late-successional genera showed no significant differences in xylem-P50 and TLP, thereby revealing two orthogonal axes of competition: one along a successional gradient and the other along a soil moisture gradient. The results from chapter three were then used to parameterize and test a new mechanistic water-stress formulation in the Ecosystem Demography (ED2) model, which became the basis of chapter four. With the new water-stress formulation, ED2 successfully replicates the observed reductions in total aboveground biomass in the drought experiments. It also more realistically captures the compositional and structural shifts that occur as a result of severe droughts. This dissertation makes an important contribution that advances the science of tropical forest drought ecology and enhances our ability to make reliable predictions about the fate of tropical forests in a future drier climate.