Figure 3: Water retention in the forest-floor litter layer after three precipitation events of 5.8 mm (a) , 7.5 mm(b) and 8.8 mm (c) , estimated from grab samples of broadleaf (beech) and needle (spruce) litter (orange and green, respectively) at different timesteps before and after the precipitation event (n = 20 for each timestep in each event). The litter retained precipitation for up to 48 hours, despite starting at less than half of saturated water content (indicated by the dashed lines). Water content at 40 or 48 hours can be less than the pre-event water content, because the pre-event samples were not completely dry.
The retention capacity of forest‑floor litter was also assessed by monitoring changes in the water content of the underlying soils in two pairs of 1 m2 plots, each pair consisting of one plot with a broadleaf litter layer and one plot where we removed the litter regularly (see Figure 4a). Qualitatively, soil sensors at 10 cm depth in the litter-covered plots detected fewer precipitation events, as indicated by fewer soil moisture responses to rainfall in the black and grey lines (10 cm depth – with litter) in Figure 4b. Losses from evaporation and drainage, however, appeared larger in the plots without litter, as suggested by the steeper soil-moisture recession (see the red line in Figure 4b) during two periods without precipitation in May 2020. The observations in Figure 4, spanning 29 April until 13 June 2020, were used to estimate the soil water balance within the plot by assuming that the water content at 10 cm depth was representative for the 0-10 cm soil layer. During the observation period, the forest received 180 mm of total precipitation (measured at a weather station outside the forest boundary). The measured fluctuations in soil water content suggest that infiltration to 10 cm depth in the litter-free and litter-covered plots totalled approximately 69 and 26 mm, respectively, implying that roughly 43 mm (or roughly 24% of ambient precipitation) was intercepted and evaporated from the litter layer (assuming that throughfall fluxes to the adjaced pairs of plots were similar). The fluctuations in soil water content also suggest that roughly 73 and 26 mm of water infiltrated or evaporated from the soil in the litter-free and litter-covered plots, respectively (not counting evaporation from the litter layer itself). These observations suggest that the litter layer inhibits both recharge to, and evaporation from, the underlying soil.