SurEau: a mechanistic model of plant water relations under extreme drought

Abstract

Key message A new process-based model,SurEau, is described. It predicts the risk of xylem hydraulic failure under drought. Context The increase in drought intensity due to climate change will accentuate the risk of tree mortality. But very few process-based models are currently able to predict this mortality risk. Aims We describe the operating principle of a new mechanistic model SurEau that computes the water balance, water relations, and hydraulics of a plant under extreme drought. Methods SurEau is based on the formalization of key physiological processes of plant response to water stress. The hydraulic and hydric functioning of the plant is at the core of this model, which focuses on both water flows (i.e., hydraulic) and water pools (i.e., hydric) using variable hydraulic conductances. The model considers the elementary flow of water from the soil to the atmosphere through different plant organs that are described by their symplasmic and apoplasmic compartments. For each organ, the symplasm is described by a pressure-volume curve and the apoplasm by its vulnerability curve to cavitation. The model is evaluated on mature oak trees exposed to water stress. Results On the tested oak trees, the model captures well the observed soil water balance, water relations, and level of embolism. A sensitivity analysis reveals that the level of embolism is strongly determined by air VPD and key physiological traits such as cuticular transpiration, resistance to cavitation, and leaf area. Conclusion The process-based SurEau model offers new opportunities to evaluate how different species or genotypes will respond to future climatic conditions.

Leaf Shedding and Non-Stomatal Limitations of Photosynthesis Mitigate Hydraulic Conductance Losses in Scots Pine Saplings During Severe Drought Stress

“…If estimates of gs SOX are lower than a predefined minimum leaf conductance, representing leaf leakiness once stomata are fully closed (g min , in mmol m −2 s −1 ; here 2 mmol m −2 s −1 ), we considered gs equal to g min , otherwise gs = gs SOX , following Duursma et al (2019). Note that in the approach, g min integrates leaf water losses both because of imperfect stomatal closure and leaf cuticular conductance, considering a well-coupled canopy and low wind speed conditions (e.g., Cochard et al, 2021).…”

“…Nevertheless, we are aware that other potential acclimation processes such as changes in rooting depth, root distribution, or soil-to-root conductance (e.g., Mu et al, 2021), as well as changes in leaf distribution or traits, such as leaf thickness and stomatal density, affect whole plant conductance. Also, shortterm responses, such as an increase in leaf cuticular conductance (g min ) in response to rising temperature (e.g., Cochard et al, 2021), are not addressed in this version of SOX+, which leaves room for model improvement. Furthermore, the SOX+ model has, so far, been tested only for small trees growing under controlled conditions.…”

“…This comes at a cost of reduced leaf permeability to CO 2 , which limits C assimilation (Martorell et al, 2014;Reich et al, 2018). Much research has focused on modeling stomatal conductance (gs) to water deficit (Damour et al, 2010;Mencuccini et al, 2019), as well as tree internal water balance after stomatal closure (e.g., Martin-St. Paul et al, 2017;Cochard et al, 2021). To date, many approaches exist that combine gs responses to decreasing soil water content (SWC) and increasing vapor pressure deficit (VPD).…”

Bark Transpiration Rates Can Reach Needle Transpiration Rates Under Dry Conditions in a Semi-arid Forest

“…This so-called hydraulic safety margin, i.e., the difference between the level of water stress experienced by a species in the field and the level of water stress leading to hydraulic failure, is generally higher in gymnosperms than in angiosperms, but most plant species live on the verge of hydraulic failure with surprisingly small hydraulic safety margins ( Choat et al, 2012 ). In the case of extreme heat and drought, the hydraulic safety margin may be even further reduced due to unregulated water loss through leaf cuticles and stem bark after stomatal closure ( Cochard, 2019 ; Cochard, et al 2021 ).…”

“…In the latter case, it is more accurate to talk about residual or minimum transpiration after maximum stomatal closure, as the stomata may leak even when they are seemingly closed ( Burghardt and Riederer, 2003 ; Dayer et al, 2020 ; Machado et al, 2020 ). Minimum leaf transpiration is increasingly recognised to play an important role during heat waves ( Kala et al, 2016 ) and in models of plant drought response ( Blackman, et al, 2016 ; Cochard et al, 2021 ; Martin-StPaul, et al, 2017 ). Meta-analyses with various plant life forms show that the minimum leaf conductance after presumed stomatal closure does not seem to differ significantly between deciduous trees and evergreen conifers ( Duursma, et al, 2019 ; Schuster, et al, 2017 ).…”