Influence of the dry event induced hydraulic redistribution on water and carbon cycles at five AsiaFlux forest sites: A site study combining measurements and modeling

Plant Hydraulic Stress Strategy Improves Model Predictions of the Response of Gross Primary Productivity to Drought Across China

“…However, as we noted in the methods section, nighttime stomatal conductance tends toward zero in CLM5 simulation. The model deficiency in reproducing nighttime stomatal conductance and transpiration could lead to bias in simulations of HR and subsequently ecosystem GPP (Figures 11 and 13; Fu, Lee, et al, 2018; Wu et al, 2020).…”

“…Compared to SMS configuration, the water transported by plant root from deep to shallow soil layers (i.e., upward HR) in PHS supported plant survival and growth during drought events, which increased leaf stomatal conductance (Figure S4) and plant transpiration (Figure S5), and simultaneously promoted carbon uptake via photosynthesis and GPP. In contrast, downward HR that usually occurred during rainfall events transported water from shallow to deep soil layers, which might cause decrease of GPP, although downward HR could also help plant to survive during the following dry events/season (Fu, Lee, et al, 2018; Scott et al, 2008; Wu et al, 2020). The positive correlation between the PHS‐induced GPP increase and annual precipitation amount in the DRY region (Figure 10d) was caused by the fact that more water was available for upward HR when the limited annual precipitation increased from ~100 to ~200 mm yr −1 .…”

“…Journal of Geophysical Research: Atmospheres terization of CLM5, the PHS configuration implements a hydraulic gradient of RWU, which allows for HR and compensatory RWU, and thus results in PHS using a larger portion of the soil column to buffer shortfalls in precipitation (Kennedy et al, 2019). Sharp increases in or high values of upward HR were modeled during dry events in various climatic regions of China (Figure 13) as well as for forested sites in northwestern and southeastern China (Wu et al, 2020). Compared to SMS configuration, the water transported by plant root from deep to shallow soil layers (i.e., upward HR) in PHS supported plant survival and growth during drought events, which increased leaf stomatal conductance ( Figure S4) and plant transpiration ( Figure S5), and simultaneously promoted carbon uptake via photosynthesis and GPP.…”

“…However, SMS functions are considered a major driver of uncertainty in leaf gas exchange in LSMs (Trugman et al, 2018) and may systematically overestimate the effects of soil moisture drought on GPP (Restrepo‐Coupe et al, 2017). Previous modeling studies have incorporated various representations of plant hydraulic theory (e.g., segmentation into root, stem, and leaf hydraulic elements) into LSMs to simulate water flow through the soil‐plant‐atmosphere continuum (Sperry et al, 2017; Wu et al, 2020; Xu et al, 2016). Recently, a new plant hydraulic stress (PHS) configuration was developed to replace SMS in Version 5 of the Community Land Model (CLM5).…”

“…PHS allows for hydraulic redistribution (HR) and compensatory RWU, which means that roots can use a large portion of the soil column to buffer shortfalls in precipitation. Previous studies have incorporated various HR schemes into CLM4.5 and reported significant effects of HR on ecosystem carbon flux during dry seasons and dry events in wet seasons at the local scale (Fu et al, 2016; Fu, Lee, et al, 2018; Fu, Wang, et al, 2018; Wu et al, 2020). Reflecting HR in PHS is an important new feature of CLM5 (Kennedy et al, 2019), but the effects of HR on ecosystem carbon fluxes in different climate zones and during years with different precipitation features (e.g., an extreme dry year, a rainy year, a year with moderate annual precipitation but including serious dry events) remain unclear.…”