Exploring the role of bedrock representation on plant transpiration response during dry periods at four forested sites in Europe

Abstract

Abstract. Forest transpiration is controlled by the atmospheric water demand, potentially constrained by soil moisture availability, and regulated by plant physiological properties. During summer periods, soil moisture availability at sites with thin soils can be limited, forcing the plants to access moisture stored in the weathered bedrock. Land surface models (LSMs) have considerably evolved in the description of the physical processes related to vegetation water use, but the effects of bedrock position and water uptake from fractured bedrock have not received much attention. In this study, the Community Land Model version 5.0 (CLM 5) is implemented at four forested sites with relatively shallow bedrock and located across an environmental gradient in Europe. Three different bedrock configurations (i.e., default, deeper, and fractured) are applied to evaluate if the omission of water uptake from weathered bedrock could explain some model deficiencies with respect to the simulation of seasonal transpiration patterns. Sap flow measurements are used to benchmark the response of these three bedrock configurations. It was found that the simulated transpiration response of the default model configuration is strongly limited by soil moisture availability at sites with extended dry seasons. Under these climate conditions, the implementation of an alternative (i.e., deeper and fractured) bedrock configuration resulted in a better agreement between modeled and measured transpiration. At the site with a continental climate, the default model configuration accurately reproduced the magnitude and temporal patterns of the measured transpiration. The implementation of the alternative bedrock configurations at this site provided more realistic water potentials in plant tissues but negatively affected the modeled transpiration during the summer period. Finally, all three bedrock configurations did not show differences in terms of water potentials, fluxes, and performances on the more northern and colder site exhibiting a transition between oceanic and continental climate. Model performances at this site are low, with a clear overestimation of transpiration compared to sap flow data. The results of this study call for increased efforts into better representing lithological controls on plant water uptake in LSMs.

The Role of the Intraspecific Variability of Hydraulic Traits for Modeling the Plant Water Use in Different European Forest Ecosystems

“…Although, the inclusion of water uptake from deeper soil reservoirs can also reduce the severity of simulated water stress and under‐estimation of transpiration rates in the model during summer periods, as shown by Jiménez‐Rodríguez et al. (2022), the need to regulate the extreme plant water consumption during un‐stressed wet periods prevailed.…”

“…However, the use of the same plant hydraulic parameterization across the selected PFTs (Table 2) does not reflect the conservative WUS expected at Mediterranean sites, such as FR-Pue and ES-Alt, which are inhabited by Q. ilex. These two sites have a strong atmospheric water demand but receive very little precipitation in summer (Allard et al, 2008;Lorenzo-Lacruz et al, 2010), resulting in extremely negative soil water potentials and severe plant water stress in the default model simulations (Jiménez-Rodríguez et al, 2022). Although, the inclusion of water uptake from deeper soil reservoirs can also reduce the severity of simulated water stress and under-estimation of transpiration rates in the model during summer periods, as shown by Jiménez-Rodríguez et al (2022), the need to regulate the extreme plant water consumption during un-stressed wet periods prevailed.…”

Incorporating Plant Access to Groundwater in Existing Global, Satellite‐Based Evaporation Estimates

“…Moreover, results are sensitive to data uncertainties, including initial GWL and soil properties. Last but not least, results here are constrained to the processes represented in GLEAM, which does not explicitly model, among others, human impacts such as pumping and irrigation, preferential flow, the impact of vapor pressure deficit (VPD) on evaporation, and processes related to (weathered) bedrock water, which has been proven to be an important water source for transpiration (Jiménez‐Rodríguez et al., 2022; McCormick et al., 2021; Rempe & Dietrich, 2018). These are all potential avenues for improvements in the future, but are considered outside the scope of this study.…”

Variations in Bedrock and Vegetation Cover Modulate Subsurface Water Flow Dynamics of a Mountainous Hillslope

“…While climatic forcing can be constrained through detailed weather observations, for example, high resolution ground‐based radar installations (Sokol et al., 2021), understanding the subsurface characteristics and processes that control how water flows in the subsurface remains a methodological and modeling challenge, especially for non‐diffusive processes (Beven & Binley, 2014), yet remains critical for predicting the hydrological response of watersheds to climate disturbances (Hartmann et al., 2017; Schreiner‐McGraw & Ajami, 2020). Particularly in mountainous catchments that provide essential input to the water supply of lowland basins (Viviroli et al., 2007), subsurface characteristics have been shown to pose strong control on the discharge characteristics (Apurv & Cai, 2020; Carroll et al., 2018; Rapp et al., 2020; Somers & McKenzie, 2020), and on plant health and dynamics (M. G. Anderson & Ferree, 2010; Callahan et al., 2022; Jiménez‐Rodríguez et al., 2022).…”