Figure 4: Plot experiments with and without a broadleaf litter
layer (a) showing soil-moisture measurements at 10 cm depth in
two litter-covered plots (black & grey lines) and two plots without
litter (red lines) (b) . Water retention in the forest-floor
litter layer reduces the amount of precipitation that reaches the soil
at 10 cm depth, as indicated by the much smaller increases in soil
moisture following precipitation events. The litter layer also appears
to reduce the rate of soil evaporation, as indicated by the smaller
decreases in soil moisture between precipitation events.
The storage and retention capacity of forest-floor broadleaf
versus needle litter
The median storage capacity (Figure 2) of broadleaf litter was roughly a
factor of two higher than that of needle litter, when expressed as a
multiple of dry weight (4.8 ± 0.7 g per g dry weight and 2.5 ± 0.4 g per
g dry weight, respectively). Similarly, several previous studies have
also found that broadleaf litter had a larger storage capacity compared
to needle litter. Walsh & Voigt (1977), Sato et al., (2004) Li et al.
(2013), and Ilek et al. (2021) found that the storage-capacity per unit
dry weight of broadleaf litter was higher than that of needle litter by
factors of up to 1.6, 1.8, 2.65, and 1.4 respectively. In general, the
larger storage capacity in broadleaf litter compared to needle litter
can be related to structural differences between the two litter types
(Klamerus-Iwan et al., 2020; Li et al., 2013). The broadleaf litter
layer tends to have a higher storage capacity due to sink effects (water
drops retained in depressions on the leaf surfaces) occurring on
individual leaves. The broadleaf litter also has a higher surface area
to weight ratio compared to the needle litter (Walsh and Voigt, 1977).
The differences between broadleaf and needle litter storage capacity
revealed by our laboratory experiments are further supported by the
event-scale grab sampling (Figure 3), where for most experiments we
observed more water stored in the broadleaf litter compared to the
needle litter (when expressed as a multiple of dry weight). The maximum
storage capacity of different litter types from several studies has been
reviewed by Gerrits & Savenije (2011). It should be noted that some of
these studies found no large differences between broadleaf and needle
litter storage capacity, or found needle storage capacity to exceed
broadleaf storage capacity. For example, Putuhena & Cordery (1996)
reported a broadleaf and needle litter storage capacity of 1.7 and 2.8 g
per g dry weight, respectively, and Zhou et al. (2018) reported that
coniferous litter storage capacity was 2.1 times larger than broadleaf
litter storage capacity. However, these studies used a different
experimental protocol based on sprinkling experiments instead of
saturating the litter by submerging it. Zagyvai-Kiss et al. (2019) found
a similar water holding capacity in beech and spruce litter (2.02 g per
g dry weight and 2.09 g per g dry weight, respectively). We also
speculate that there may be differences in broadleaf and needle litter
storage capacity, beyond those attributable to methodology, that relate
to species, age, morphology, and decay state, which all contribute to
the variability observed in nature and in previous data.
The approximate timescales of water storage following the three rainfall
events, ~2 days, were similar for both litter types.
However, retention timescales for saturated litter may be even longer,
because in all cases except one (spruce in Figure 3a), the peak storage
in our grab samples was less than half of the maximum storage capacity
as estimated by the laboratory experiments (dashed lines in Figure 3).
Even these less-than-half-saturated grab samples retained moisture
longer than the typical retention timescale of canopy interception of
<2 days (Gerrits et al., 2010). The laboratory experiments for
the assessment of maximum storage capacity revealed that 4 hours after
maximum saturation, the broadleaf litter had already lost 20 % of its
stored water, compared to only 9 % for the needle litter.
We also tested the effect of layer thickness on the litter storage
capacity and found that the thickness of the litter layer does not
affect the unit storage capacity (per mass of litter), implying that
storage capacity scales with litter mass independent of depth. A similar
linear relation between mass of litter and storage capacity has also
been reported in previous studies (Pitman, 1989; Putuhena & Cordery,
1996; Sato et al. 2004; Li et al., 2013). Sato et al. (2004) even tested
the impact of layer thickness by experimentally compacting the litter
layer and found no differences in storage capacity.
Water retention in the litter layer is also reflected in our
measurements of soil moisture below plots with and without litter.
Fluctuations in soil moisture suggest that soils at 10 cm depth below
the litter-free plots received approximately 2.7 times more infiltrated
water from precipitation over a period of 6 weeks with 180 mm of
precipitation. We hypothesize that this difference reflects interception
and evaporation of throughfall by the litter layer. However, the overall
impact on the soil water balance may be small, because the differences
in recharge appear to be offset by differences in soil evaporation, with
fluctuations in soil moisture suggesting 2.8 times greater losses to
soil evaporation (and/or percolation) in the litter-free plots.
Daily cycling of water in forest-floor deadwood
We used four self-made pressure cushions to observe temporal variations
of water content in pieces of deadwood. Daily fluctuations in deadwood
weight (Figure 5) were inversely related to vapor pressure deficit
(VPD), indicating that the deadwood pieces gained moisture from the air
during the night (when VPD was relatively low) by condensation or
absorption, and lost water to evaporation during the day (when VPD was
relatively high). These daily cycles were superimposed on increases in
deadwood moisture following rain events, and longer-term declines in
deadwood moisture during dry spells between rain events (Figure 5). The
lowest relative weights were generally measured between 2 PM and 4 PM,
and the highest weights (on non-rainy days) were generally measured
around 6 AM. VPD varied approximately inversely to deadwood weight,
reaching its maximum around 2 PM. The temporal patterns of water content
were consistent across all four pieces of deadwood (Figure 5).