Seasonality of tree water uptake explained by amount and timing of soil water refill

The hydrologic cycle in Switzerland relies on inputs from winter precipitation, whereby snowmelt plays an important role replenishing soil water, streams and aquifers. Previous studies found that plant water uptake during the summer growing season is dominated by winter precipitation. Here we use stable water isotope data of xylem and soil water from four experimental catchments and from two snapshot sampling campaigns during the growing season to assess what drives seasonal patterns in xylem water signatures. We unveil divergent trends in the seasonal origin of waters used by trees compared to water flowing into nearby streams and aquifers. In low-elevation catchments characterized by little snowfall, summer precipitation predominantly refills streams and aquifers, while winter precipitation feeds vegetation water uptake. Conversely, higher-elevation catchments exhibit an opposite pattern, where winter precipitation primarily recharges streams and aquifers, and vegetation water uptake is driven by summer precipitation. We propose a theoretical framework to elucidate the divergence in vegetation water uptake from stream and groundwater recharge. This framework offers insights into the intricate relationships governing water availability in terrestrial ecosystems in different elevations across the Swiss Alps.