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.