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 has 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 affects 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.
Highlights
Bedrock structure and composition influence the ecosystem productivity (Hahm et al, 2014; Jiang et al, 2020) through its effect on plant nutrient and water uptake (Ding et al, 2021)
This study tested the impact of 455 this omission in a state-of-the art Land surface models (LSMs), Community Land Model version 5.0 (CLM 5), by comparing the simulated transpiration response of three different bedrock configurations
The two additional configurations mimic the effect of a deeper impermeable layer as well as the impact of an overlying weathered material on the impermeable bedrock. This overlying weathered bedrock is parameterized assigning a low clay content to account for its water holding capacity while the larger sand content enables quick drainage of water percolating from the soil
Summary
Bedrock structure and composition influence the ecosystem productivity (Hahm et al, 2014; Jiang et al, 2020) through its effect on plant nutrient and water uptake (Ding et al, 2021). Physical and chemical weathering processes allow the formation of cracks in the bedrock (Pope, 2015), increasing the presence of water reservoirs that are commonly neglected in ecological 25 analyses at sites characterized by thin soils and seasonal droughts (Sternberg et al, 1996). These reservoirs are highly dynamic, storing water from winter precipitation (Vrettas and Fung, 2017) and sometimes holding even more moisture than the overlying soil (Rempe and Dietrich, 2018). Plants growing in seasonally dry environments rely on deep water reservoirs often available within the 30 weathered bedrock (Barbeta et al, 2015; Ding et al, 2021; Sternberg et al, 1996). Due to the limited field data, detailed knowledge about the structure of weathered bedrock and its effect on the overlying vegetation remains poorly understood
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