Role of summer precipitation in plant water uptake in a pre-Alpine catchment

Understanding the spatial and temporal origins of water used by plants for transpiration is crucial for improving forest and water resource management under future drought conditions. However, the impact of local factors such as wetness conditions and topography on the temporal origin of soil and plant waters remains largely unexplored.

In this study, we utilized a 6-year isotopic dataset to investigate i) the seasonal origin of water sources in a small headwater catchment in the Italian pre-Alps, ii) the seasonal origin of soil and plant water under different wetness conditions (based on soil moisture data), iii) the influence of topography (riparian zone vs. hillslope) and wetness conditions on water uptake by beech and chestnut trees.

The sampling campaigns were carried out in the Ressi catchment, which has a 2.4-ha area, steep hillslopes and a narrow riparian zone. The climate is humid and temperate, and the catchment is mostly covered by a forest mainly composed of beech, chestnut, maple and hazel trees. Water samples for isotopic analysis (δ²H and δ¹⁸O) were taken from precipitation, stream water, shallow groundwater, soil, and twigs from beech and chestnut trees. Samples were taken approximately bi-weekly during the growing season, whereas precipitation, stream water and shallow groundwater were collected monthly from October to May. Bulk soil water and plant water were extracted by cryogenic vacuum distillation before the isotopic analysis.

Our results, based on the estimation of the seasonal origin index (SOI), showed distinct temporal variability for all water sources, except groundwater. The rapid turnover of water in the catchment indicates that precipitation quickly replenishes the soil, becomes available for plant water uptake, and contributes to stream runoff. Interestingly, we found that both beech and chestnut trees primarily use water derived from summer precipitation, with minimal differences in water uptake between riparian and hillslope trees. The seasonality of water fluxes (i.e., precipitation and evapotranspiration) and isotopes in precipitation have a more significant impact on SOI values of soil water and plant water compared to soil moisture.

These findings suggest that in the Ressi catchment, during the growing season, trees and the stream primarily utilize young waters, even during dry years. This research contributes to our understanding of plant water use strategies and their implications for forest and water resource management under changing climate conditions.