Trees use predominantly summer water in a pre-Alpine catchment

Understanding the spatial and temporal origin of the water sources exploited by plants for transpiration is highly relevant for improving forest and water resource management under future drought conditions. However, little is known about the role of local factors (e.g., wetness conditions and topography) on the temporal origin of soil and plant waters. In this study, we used a 6-year isotopic dataset to investigate the seasonal origin of the water sources in a small headwater catchment in the Italian pre-Alps, assess how the seasonal origin of soil and plant water varies under different wetness conditions (i.e., based on soil moisture data), and determine the role of topography (riparian zone vs. hillslope) and wetness conditions on water uptake by beech and chestnut trees. Our results, based on the analysis of the seasonal origin index (SOI), highlighted a distinct temporal variability for all water sources, except for groundwater. A quick turnover of the waters in the catchment indicates that precipitation quickly replenishes the soil, is available for plant water uptake, and contributes to stream runoff. Interestingly, we found that beech and chestnut trees use water derived from summer precipitation, with negligible differences in the water uptake of riparian and hillslope trees. Seasonality in water fluxes and isotopes in precipitation play a major role on SOI values of soil water and plant water compared to soil moisture. These results imply that in our catchment, during the growing season, trees and the stream primarily use young waters, even during dry years.