Figure 10: Conceptual scheme of the “Waldlabor” forest water cycle, accounting for the storage in the forest litter layer estimated from field & laboratory experiments and their implications for the larger forest water cycle. Roughly 38 % of annual precipitation are intercepted in the forest-floor litter layer and canopies, therefore only 62 % of annual precipitation reaches the subsurface soil and groundwater storages and is available for plant transpiration, groundwater recharge, and streamflow.
Overlooking forest floor interception losses can lead to substantial overestimates of forest transpiration rates (Coenders-Gerrits et al., 2014). A simple thought experiment illustrates the potential magnitudes. In many humid temperate forests, total evaporation and transpiration from all sources (as estimated from long-term mass balances of precipitation and stream discharge) is very roughly 60 % of precipitation (Zhang et al., 2016; Oki and Kanae, 2006; Schlesinger and Jasechko, 2014). If canopy interception and evaporation account for 20 % of precipitation ((Miralles et al., 2010) van Dijk et al., 2015; Wang et al., 2007; Zheng and Jia, 2020; Allen et al., 2017), the remaining 60 – 20 = 40 % of precipitation would be attributed to transpiration if forest floor evaporation is ignored. But if interception and evaporation from the forest floor accounts for an additional ~20 % of precipitation, as suggested by our analysis and previous studies (Miller et al., 1990; Gerrits et al., 2010; Putuhena and Cordery, 1996; Van Stan et al., 2017), then transpiration accounts for only 60 – 40 = 20 % of precipitation, or only half of the transpiration flux that would be inferred if forest floor evaporation is ignored. If these magnitudes are even approximately correct, many transpiration fluxes may be overestimated by factors of roughly 2, with potential implications for other fluxes linked to transpiration i.e., underestimates of plant water use efficiency.
Conclusion
Our measurements at a mixed beech-spruce forest site indicate that the forest-floor litter layer intercepts and temporarily stores significant amounts of total annual precipitation, and that this is an important (yet often overlooked) component of the forest water cycle. Field observations and laboratory measurements show that storage capacities of spruce needle litter, beech broadleaf litter, and deadwood average 3.1 mm, 1.9 mm, and 0.7 mm, respectively (Figure 2), with the storage capacity of deadwood varying with its degree of decay (Figure 7). Fluctuations in soil moisture suggest that the fraction of precipitation reaching the forest soil at 10 cm depth was a factor of 2.7 smaller in litter-covered plots than at plots without litter (Figure 4). Similarly, the litter layer shields the underlying soil from evaporation (2.8 times less evaporation at the plot with litter) and potentially also insulates against ground heat fluxes from the subsurface, thus decreasing snowmelt rates (Figure 10). Measurement time series following rain events show that beech broadleaf and spruce needle litter can retain water for 2 days or longer (Figure 3), and deadwood can retain water for more than 7 days (Figure 6). Water storage in deadwood fluctuated diurnally, suggesting water uptake from the night-time atmosphere (from dew and fog) when VPD was low, and evaporation of water during daytime, when VPD was high (Figure 5). Evaporation from forest-floor litter is likely to significantly increase the humidity, lower the temperature, and reduce the VPD in the subcanopy environment (Figure 8). Overall, interception by the canopy and forest-floor litter layer amounted to approximately 38 % of annual precipitation, thereby significantly reducing the water available for recharge and transpiration. Forest water balances that overlook forest floor interception and evaporation are likely to overestimate recharge and transpiration rates, potentially by large factors.