Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Vegetation plays a key role in the global carbon cycle and thus is an important component within Earth system models (ESMs) that project future climate. Many ESMs are adopting methods to trace the size and succession-stage-structure of plants within demographic models. These models make it feasible to conduct more realistic simulation of processes that control vegetation dynamics. Separately, increasing understanding of the ecophysiological processes governing plant water use, and the need to understand ecosystem responses to drought in particular, has led to the adoption of physical plant hydrodynamic schemes within ESMs. In this study, we report on a new hydrodynamics (HYDRO) model incorporated in the Functionally Assembled Terrestrial Ecosystem simulator (FATES). The size and canopy structured representation within FATES is able to simulate how plant size and hydraulic traits affect vegetation dynamics and carbon/water fluxes. To better understand this new model system and its functionality in tropical forest systems in particular, we conducted a global parameter sensitivity analysis at Barro Colorado Island, Panama. We assembled observations of plant hydraulic traits for stomata, leaves, stems, and roots, and determined the best-fit statistical distribution for each trait. Our model analysis showed that the taper component determining hydraulic conductivity tapering from trunk to branch, the water potential leading to 50 % loss (P₅₀) of stomatal conductance, the maximum hydraulic conductivity for the stem, and the fraction of total hydraulic resistance in the above ground section are the top 5 traits determining the simulated water potential and loss of conductivity for different plant organs. For the risk of hydraulic failure and potential tree mortality, we found that ensemble members with high risk of mortality generally have a higher taper exponent and a higher xylem conductivity, less negative P₅₀ for stomata conductance, and more negative P₅₀ for stem and transporting roots. We expect that our results will provide guidance on future modeling studies using plant hydrodynamic models to predict the forest responses to droughts, and future field campaigns that aim to better parameterize the plant hydrodynamic model.