Abstract
Trees in seasonal climates may use water originating from both winter
and summer precipitation. However, the seasonal origins of water used by
trees have not been systematically studied. We used stable isotopes of
water to compare the seasonal origins of water found in three common
tree species across 24 Swiss forest sites sampled in two different
years. Water from winter precipitation was observed in trees at most
sites, even at the peak of summer, although the relative representation
of seasonal sources differed by species. However, the representation of
winter precipitation in trees decreased with site mean annual
precipitation in both years; additionally, it was generally lower in the
cooler and wetter year. Together, these relationships show that
precipitation amount influenced the seasonal origin water taken up by
trees across both time and space. These results suggest higher turnover
of the plant-available soil-water pool in wetter sites and wetter years.