Coupled whole-tree optimality and xylem-hydraulics explain carbon allocation under drought

Abstract:

Carbon allocation strategies of trees plays a critical role in carbon terrestrial storage and the carbon exchange between the atmosphere and the biosphere. Allocation strategies respond to resource limitation and physiological activity, which under drought-stress correspond to water-availability and trees’ water-use. We present the Tree Hydraulics and Optimal Resource Partitioning (THORP) model which predicts the dynamic allocation of carbon between stems, leaves, and multiple root pools distributed by depth at subdaily time-steps through an optimality framework that maximizes carbon assimilation. Allocation is optimized through considerations of both water-availability and water-use through mechanistic representation of the soil-plant-atmosphere continuum (including plant hydraulics, Sperry et al.’s Gain-Risk optimization algorithm for stomata, multi-layered root-water uptake, and soil- and ground-water hydrology), and growth rates are derived from principles of phloem-transport and turgor-driven growth. In agreement with observations, THORP predicts elevated root biomass and reduced shoot biomass under increasing water-stress. Rain-fed trees grew shallow roots that deepened as precipitation weakened. Groundwater-fed trees grew roots that tracked the groundwater table independent of the above-ground environment. Soil- and groundwater storage buffered against water-stress to maintain whole-tree allometry. Growth rates declined under water-stress, and how quickly growth shut down depended on trees’ water regulation (i.e. isohydric- or anisohydric behavior).