The amount of deadwood water storage was estimated similarly, from randomly selecting three 4 m² plots and weighing the (oven-dried) deadwood. These measurements yielded a deadwood volume of about 32 m³ ha^-1. This is consistent with values reported across Switzerland of 34 m³ ha^‑1 (Lachat et al., 2019) and from other temperate mixed forests in previous studies including 47 m³ ha⁻¹ in Romania (Öder et al., 2021) and up to 50.5 m³ ha⁻¹ in Poland (Bujoczek et al., 2021). We estimated the overall storage capacity of deadwood at our site by using a mean dry deadwood density of 240 kg m^-3 (Přívětivý and Šamonil, 2021) and assuming that deadwood pieces usually can take up at least 50 % of the median saturation estimated during our laboratory saturation experiments (0.85 times the dry weight), resulting in a storage potential of 0.4 mm for our forest site.

Assuming that our mixed beech/spruce forest is underlain by half beech litter and half spruce litter, and adding 0.4 mm for deadwood storage, yields a total value of 2.2 mm of water storage in the forest-floor litter layer, including deadwood. This is consistent with previous estimates of 1.8 to 2.8 mm reported by Gerrits et al. (2010) for a beech forest in Luxemburg, where they also reported that forest-floor interception was nearly constant throughout the year, with no significant seasonal variation. Zagyvai-Kiss et al. (2019) estimated an average water holding capacity of forest-floor litter in an oak, beech and spruce mixed forest site in Hungary of around 2.6 mm, and Sato et al. (2004) found litter storage capacities between 1.7 and 3.0 mm for pine and oak plots, respectively. Van Stan et al. (2017) reported 5.3 mm of total storage in all litter components of a pine forest in the Southeastern US. Although the absolute magnitudes of water storage of the forest-floor broadleaf litter, needle litter, and deadwood seem to be rather small, they rapidly fill and empty (either by drainage or by evaporation). Thus, fluxes from litter storages can be a significant fraction of the total annual precipitation, because they can retain this small absolute amount of water following many individual precipitation events.

We can estimate the cumulative effect of interception by forest-floor litter (including deadwood) as follows. For each precipitation event during our study period (15 March 2020 - 08 August 2022), we assume that 20% of incoming precipitation is lost to canopy interception, and the remaining 80% reaches the forest floor as throughfall. Literature values for canopy interception range from 9% to 29% for beech forests (Minďaš et al., 2018; Rowe, 1983), and from 23% to 37% for spruce forests (Holko et al., 2009; Xiao et al., 2000; Kofroňová et al., 2021; Dohnal et al., 2014; Ringgaard et al., 2014). For each precipitation event, we assume that interception by the litter layer equals the total throughfall amount, or the available litter-layer water storage (the storage capacity of 1.75 mm for litter and 0.4 for deadwood, minus the actual storage in litter and deadwood), whichever is smaller. Between precipitation events, the actual litter storage is assumed to decline linearly to zero over 48 hours (consistent with Figure 3), and deadwood storage is assumed to decrease exponentially with a characteristic time constant of 4 days (corresponding to a half-life of 2.8 days), consistent with the observations shown in Figure 6. In this way, we account for the litter-layer storage that is already filled at the onset of each precipitation event. Summing the water taken up by the litter layer across all precipitation events yields a total of about 23 % of annual throughfall or 18 % of total annual precipitation (Table 2, Figure 10). This is not an unrealistic estimate, as previous studies found that interception by leaf litter alone accounts for up to 18 % of total annual precipitation in a spruce forest in Scotland (Miller et al., 1990) and up to 22 % in a beech forest in Luxembourg (Gerrits et al. 2010). When all forest-floor litter components are included, forest-floor interception has been calculated at up to 47 % of precipitation in a pine catchment in Australia (Putuhena and Cordery, 1996) and up to 32 % in a pine catchment in the Southwestern US (Van Stan et al., 2017).

Table 2: Storage in leaf litter, deadwood, and the sum of all forest-floor litter components, and the resulting soil water recharge as annual totals and fractions of annual precipitation and throughfall, respectively.