Will modeling demographic differences in xylem vulnerability and stomatal closure in tropical trees improve drought response predictions of tropical forests?

Abstract:

Climate change is projected to cause significant shifts in precipitation patterns across the Amazon basin; yet, there remains considerable uncertainty about the resilience of its forests under increasing water stress. Simulations by four leading dynamic vegetation models—CLM3.5, ED2, IBIS and JULES—revealed limited ability to replicate both the 20% decline in aboveground biomass and compositional shifts observed in two long-term ecosystem-scale drought experiments in the eastern Brazilian Amazon. These four models were not parameterized to mechanistically represent competitive differences in plant hydraulics that exist between demographic groups; which may in part explain why they performed poorly in the drought simulations. Therefore, we measured xylem vulnerability to cavitation and sensitivity of stomatal conductance to leaf water potential, as determined by turgor loss point (TLP), in order to parameterize a hydrodynamic pipe model for inclusion in the Ecosystem Demography (ED2) dynamic vegetation model. Measurements were made on four genera common to both experimental study sites. Each genus was placed a prior into one of four functional categories: drought tolerant versus intolerant and early versus late successional. We evaluated the hypothesis that these two traits would be significantly different between each of the plant functional groups and thus explain the observed differential mortality rates. P50 values for both xylem cavitation and stomatal closure (i.e. TLP) occurred at 0.5 to 1.0 MPa and 0.75 MPa, respectively, higher water potentials 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. These hydraulic trait values were then used to parameterized a hydrodynamic pipe model that effectively tracked the flow of water through the trees of each functional group and showed differential water-use between the drought tolerant and intolerant groups. The ecological implications during droughts of these differences in hydraulic traits can be evaluated fully once implemented in ED2, which currently tracks competitive dynamics between trees belonging to different demographic groups, but lacks a mechanistic formulation of plant water-use.