Effects of litter layer water retention along an elevation gradient

Forest floor litter layers can temporarily store and evaporate substantial fractions of annual precipitation, thereby reducing the amount of water that infiltrates into soils and becomes available for plant uptake. Yet litter retention and evaporation are commonly omitted from forest water-balance assessments. Here we synthesize evidence for these litter-layer effects across elevation gradients and contrasting climates using two complementary datasets: (i) long-term observations from Waldlabor Zürich and (ii) a pan-Alpine sampling campaign (>400 plots) combined with laboratory measurements of litter water storage and drying dynamics. Climate-chamber drying experiments indicated mean litter water retention times of ~6 days for broadleaf litter and ~10 days for needle litter, consistent with field observations across the European Alps. We used these experiments to parameterize sensitivity tests (half-life storage decay) and to drive a simple daily bucket model. Across the Alps, the litter layer temporarily stored roughly ~10-20% of annual precipitation, while litter evaporation accounted for ~15-25% of annual evapotranspiration, with magnitudes varying by elevation and litter type. Together, these results show that the litter layer is a small but hydrologically relevant reservoir that shifts the timing and partitioning of water fluxes in mountain forests.