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.