Limited Soil Water Recharge in Summer Affects Seasonal Isotopic Signatures of Tree Xylem Water

Recent studies have reported that xylem waters in forest trees during the summer growing season are often isotopically similar to winter, rather than summer, precipitation. Several hypotheses have been proposed to explain this observation, which can be viewed as a form of ecohydrological separation, but empirical tests spanning space and time remain limited. We hypothesise that variations in the seasonal origin of precipitation found in tree xylem waters can be explained by seasonal differences in soil water recharge. We test this hypothesis by evaluating to what extent climatic aridity can explain seasonal signals in extensive surveys of the isotope ratios of tree xylem waters across Switzerland. Using these tree xylem stable water isotope data from previous studies, we show that (i) there is only limited summer precipitation replenishing soil water storage in places where evaporative demand in summer is high and that (ii) seasonal differences in PET/P can broadly predict the seasonality of soil water recharge and tree water uptake of spruce and beech trees.