Seasonal origins of soil water used by trees

Abstract

Abstract. Rain recharges soil water storages and either percolates\ndownward into aquifers and streams or is returned to the atmosphere through\nevapotranspiration. Although it is commonly assumed that summer rainfall\nrecharges plant-available water during the growing season, the seasonal\norigins of water used by plants have not been systematically explored. We\ncharacterize the seasonal origins of waters in soils and trees by comparing\ntheir midsummer isotopic signatures ( δ2H ) to seasonal isotopic\ncycles in precipitation, using a new seasonal origin index. Across 182 Swiss\nforest sites, xylem water isotopic signatures show that summer rain was not\nthe predominant water source for midsummer transpiration in any of the three\nsampled tree species. Beech and oak mostly used winter precipitation, whereas\nspruce used water of more diverse seasonal origins. Even in the same plots,\nbeech consistently used more winter precipitation than spruce, demonstrating\nconsistent niche partitioning in the rhizosphere. All three species xylem\nwater isotopes indicate that trees used more winter precipitation in drier\nregions, potentially mitigating their vulnerability to summer droughts. The\nwidespread occurrence of winter isotopic signatures in midsummer xylem\nimplies that growing-season rainfall may have minimally recharged the soil\nwater storages that supply tree growth, even across diverse humid climates\n(690–2068\u2009mm annual precipitation). These results challenge common\nassumptions concerning how water flows through soils and is accessed by\ntrees. Beyond these ecological and hydrological implications, our findings\nalso imply that stable isotopes of δ18O and δ2H in plant\ntissues, which are often used in climate reconstructions, may not reflect\nwater from growing-season climates.