Disentangling hydrological responses of forest ecosystems to impulses of precipitation, VPD and solar radiation

Water fluxes in the critical zone are driven by various environmental variables. For example, soil water is rapidly replenished during precipitation events and is slowly emptied during periods of transpiration at rates which are mainly driven by diurnal solar radiation and vapor pressure deficit. Precipitation, solar radiation and vapor pressure deficit correlate, which complicates proper disentanglement of their individual effects on tree physiology and tree-mediated water fluxes. Here, we use Ensemble Rainfall-Runoff Analysis (ERRA) to disentangle how different environmental variables contribute to ecosystem water fluxes (net soil water and tree water recharge and sapflow) measured at the ‘WaldLab Forest Experimental Site’ in Zurich. The methodology is data‐driven and relies on non-linear and non-stationary deconvolution of time series to infer impulse-response functions. These impulse-response functions quantify the intensity and the time lag in the responses of tree-mediated water fluxes to precipitation, solar radiation and vapor pressure deficit, and account for any covariation effects among these drivers. The results are based on five years of sub-daily sapflow and dendrometer measurements on three beech and spruce trees, respectively, and show the immediate response of tree water fluxes at the field site. Notably, the response of sapflow and tree water recharge towards solar radiation is more pronounced then the response towards vapor pressure deficit, reflecting the higher importance of radiation (a physiological necessity) compared to vapor pressure deficit (a hydraulic boundary condition) in driving transpiration. Beech and spruce trees differ in the duration of the response, with spruce trees showing responses lasting longer then beech, reflecting the higher hydraulic capacitance of spruce trees. Our study highlights how this novel impulse-response approach helps identifying soil-plant-atmosphere relations that complement our understanding of how forest ecosystems work.