New Representation of Plant Hydraulics Improves the Estimates of Transpiration in Land Surface Model

Abstract

Transpiration represents more than 30% of the global land–atmosphere water exchange but is highly uncertain. Plant hydraulics was ignored in traditional land surface modeling, but recently plant hydraulics has been found to play an essential role in transpiration simulation. A new physical-based representation of plant hydraulic schemes (PHS) was recently developed and implemented in the Common Land Model (CoLM). However, it is unclear to what extent PHS can reduce these uncertainties. Here, we evaluated the PHS against measurements obtained at 81 FLUXNET sites. The transpiration of each site was estimated using an empirical evapotranspiration partitioning approach. The metric scores defined by the International Land Model Benchmarking Project (ILAMB) were used to evaluate the model performance and compare it with that of the CoLM default scheme (soil moisture stress (SMS)). The bias score of transpiration in PHS was higher than SMS for most sites, and more significant improvements were found in semi-arid and arid sites where transpiration was limited by soil moisture. The hydraulic redistribution in PHS optimized the soil water supply and thus improved the transpiration estimates. In humid sites, no significant improvement in seasonal or interannual variability of transpiration was simulated by PHS, which can be explained by the insensitivity of transpiration demand coupled to the photosynthesis response to precipitation. In arid and semi-arid sites, seasonal or interannual variability of transpiration was better captured by PHS than SMS, which was interpreted by the improved drought sensitivity for transpiration. Arid land is widespread and is expected to expand due to climate change, thus there is an urgent need to couple PHS in land surface models.

Elevated Wildfire and Ecosystem Carbon Loss Risks Due to Plant Hydraulic Stress Functions: A Global Modeling Perspective

“…The present study further showed that the global fire count increment induced by PHS functions could be attributed to the elevated ecosystem water loss via plant transpiration and the resultant drier environment from the global perspective (Figure 8). Consistent with our results, the PHS configuration has been proven to increase plant transpiration at 81 FLUXNET sites across the globe [20] and over four different climatic subregions of China [21]. The PHS configuration uses water stress on leaf water potential to restrict transpiration; stress on transpiration can be induced by decreasing atmospheric moisture (e.g., increasing vapor pressure deficit) and soil water supply, and the latter was also regulated by root hydraulic redistribution (HR) processes [19].…”

“…It is worth mentioning that the PHS configuration in CLM5 allows for hydraulic redistribution (HR) and compensatory root water uptake. Previous research has confirmed that the incorporation of PHS configuration into CLM5 improved the estimates of ecosystem water and carbon fluxes [19][20][21]. Moreover, the PHS-incorporated CLM5 model could well reproduce the global wildfire carbon emission derived from the satellite products [22], and it has advantages against other LSMs, because the CLM5 uniquely considered the seasonality of crop fires and simulated the burning of plant tissue and litter from peat fires and the drought-linked tropical deforestation and degradation fire [23].…”

“…Previous land surface models, including the earlier versions of CLM, roughly related a metric of soil moisture to transpiration and neglected the real plant hydraulic functions [18,24]. More mechanistic representations of stresses and vegetation water use dynamics, i.e., the plant hydraulic stress (PHS) configuration, have been incorporated in the CLM5 and been suggested to improve the ecosystem water fluxes estimation [19][20][21]. Plant hydraulic stress (PHS) configuration is switched on by default and can also be turned off in CLM5.…”

Parallels between drought and flooding: An integrated framework for plant eco‐physiological responses to water stress

“…A natural application for our model framework is its incorporation into land‐surface models. So far, the incorporation of plant hydraulics into land‐surface models has been used to investigate the feedback between climate and vegetation in terms of water cycle during non‐stressed conditions and drought, yet mechanistic plant responses to excessive soil water content are still lacking in these models (Li et al, 2021 ; Nguyen et al, 2020 ). With climate change expected to enhance surface evaporation and atmospheric vapor accumulation, the intensity and duration of droughts as well as the chances of flooding are both increased, underlining the demand for a framework capable of integrating both drought and flooding responses of plants (Trenberth, 2011 ).…”

Parallels between drought and flooding: an integrated framework for plant eco-physiological responses to water stress

“…A natural application for our model framework is its incorporation into land-surface models. So far, the incorporation of plant hydraulics into land-surface models has been used to investigate the feedback between climate and vegetation in terms of water cycle during non-stressed conditions and drought, yet mechanistic plant responses to excessive soil water content are still lacking in these models (Li et al, 2021;Nguyen et al, 2020). With climate change expected to enhance surface evaporation and atmospheric vapor accumulation, the intensity and duration of droughts as well as the chances of flooding are both increased, underlining the demand for a framework capable of integrating both drought and flooding responses of plants (Trenberth, 2011).…”