Multi-Scale Understanding and Modeling of Plant Hydraulics

Abstract:

Mechanistic modeling of plant hydraulics has advanced in recent decades with a demonstrated capability to simulate water and carbon cycles, especially under water stress conditions. As these water stress conditions are expected to intensify in a warming climate, it is crucial to ensure land surface models (LSMs) for use in Earth system models (ESMs) are equipped with appropriate parameterizations of plant hydraulics. Most land surface models, including Noah-MP, employ an idealized “big-leaf” concept to regulate water and carbon fluxes in response to soil moisture stress through empirical soil hydraulics schemes (SHSs), which have been shown to cause significant uncertainties in carbon and water simulations. We present a novel plant hydraulics scheme (PHS) for Noah-MP (hereafter, Noah-MP-PHS), which employs a big-tree rather than a big-leaf concept, wherein the whole-plant hydraulic strategy is considered including root-level soil water acquisition, stem-level hydraulic conductance and capacitance, and leaf-level isohydricity and hydraulic capacitance. Evaluated against plot-level observations from the University of Michigan Biological Station and compared with the default Noah-MP, Noah-MP-PHS better represents plant water stress and improves water and carbon simulations, especially during periods of dry soil conditions. Noah-MP-PHS is able to reproduce different patterns of transpiration and stem water storage during a two-week dry-down period for two species with contrasting plant hydraulic behaviors, i.e., the “cavitation risk-averse” red maple and the “cavitation risk-prone” red oak. We further extend the plot-level and tree-level simulations to forest regions in the contiguous United States. PHS experiments with different hydraulic strategies show large impacts on the ET partitioning to transpiration and the total water storage anomaly (e.g., during the 2011 Texas drought period). Therefore, the implementation of plant hydraulics, along with realistic representations of plant hydraulic traits and associated hydraulic strategies, could reconcile observations and models of terrestrial water and carbon cycles.