SYMP 21-4 - Understanding forest resilience to drought: the role of bedrock water storage

Background/Question/Methods
Droughts are increasing in severity and frequency, yet the biogeography of accompanying forest mortality is not entirely explained by above-ground drivers like plant density or climate anomalies. The role of the subsurface in mediating plant water supply has emerged as a significant yet difficult to quantify factor in determining forest outcomes following drought. Historically, plant water availability has been understood through the lens of soil water storage dynamics. This is due in part to widely available datasets of soil depth and hydraulic parameters at large spatial scales. However, mounting evidence suggests that woody plants routinely access water from weathered bedrock, which is challenging to study directly and difficult to map at large spatial scales. When, where, and why does access to bedrock water confer resilience to forests experiencing meteorological drought? Here, we synthesize insights gained from multi-year, hillslope-scale monitoring of bedrock water storage dynamics under drought and the response of overlying woody plant communities in seasonally dry California. We compare these findings to inferences made at regional scales from distributed water flux datasets in order to develop a framework for understanding how subsurface water storage capacity mediates forest sensitivity to precipitation variability.

Results/Conclusions
In upland landscapes where forests are concentrated, plant-available water storage capacity is governed by both soil characteristics and the extent of weathering into fresh bedrock underlying hillslopes. Where wet season precipitation always exceeds subsurface storage capacity, growing season evapotranspiration can be decoupled from year-to-year precipitation variability. In contrast, where wet season precipitation is insufficient to replenish storage deficits in dry years, plant communities are less resilient to meteorological drought, and forest survival depends on physiological responses to water stress. Knowledge of soil water storage properties is necessary but insufficient to explain observed forest responses and predict climate-change induced shifts: accounting for water storage in weathered bedrock is also required to understand how the subsurface mediates plant water supply.