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).