Figure 7: The relation of deadwood weight and maximum water storage for 76 deadwood pieces with different levels of decay (low – intermediate – high). Note that the regression lines indicate the general direction of the relationship and not its statistical significance.
The overall timescale over which water was stored in the deadwood can only be roughly estimated from our experiments. From the daily measurements of relatively small pieces of deadwood that never were close to their potential maximum saturation, we found a water retention timescale of at least 7 days (Figure 6). However, the time series from the longer-term experiments suggest that larger pieces may store water for much longer timescales (Figure 5).
Forest floor deadwood may be important in water cycling beyond its obvious role in directly retaining precipitation. Deadwood pieces lying on the forest floor may actively contribute to water cycling from the underlying soil and litter layer to the atmosphere, if the capillary forces of the deadwood structures are larger than those of the forest top soil layer or compounds of the litter layer, such that these deadwood pieces will take up water from the underlying soil and litter. In this scenario, deadwood may increase soil and litter evaporation rates by wicking water from soil and litter upward to the atmospheric interface.
We observed daily fluctuations in deadwood water content with the pressure-cushion experiments (Figure 5), consistent with deadwood taking up atmospheric water (i.e., dew, fog, humidity) whenever the vapor pressure deficit (VPD) is smaller than the capillary forces of the deadwood (i.e., during night-time), and evaporating water when VPD is higher (i.e., during daytime). It should be noted that in these measurements, the wood was not lying directly on the forest-floor litter or soil, but on the pressure cushions. Given the water storages and flux modulation introduced by the litter layer, we hypothesize that the litter layer may function as a moisture battery, buffering (and generally increasing) subcanopy microclimatic humidity.
Effect of litter layer-evaporation on subcanopy microclimate
Forest canopies modulate the spatial and temporal input of precipitation (Levia and Frost, 2006; Staelens et al., 2006); they also create a within-forest microclimate by inhibiting exchange between subcanopy air masses and the above-canopy atmosphere. To monitor vapor transport to and from the forest floor, we recorded relative humidity and temperature with two small towers, one surrounded by spruce trees (tower 1, up to 4.7 m) and the other surrounded by beech trees (tower 2, up to 4 m). Figure 8a shows the temporal variations in absolute humidity at ~2 m height, relative to the humidity at the lowest monitored level (0.2 and 0.5 m at towers 1 and 2, respectively). The scale is inverted so that negative humidity gradients (i.e., lower absolute humidity at ~2 m height than at the near-surface, consistent with evaporation from the surface into the subcanopy atmosphere) are plotted as upward fluxes. From Figure 8a one can see that, unsurprisingly, humidity gradients above the forest floor are stronger during daytime, synchronized with the daily cycle in subcanopy air temperature. This pattern is also consistent with the daily variations in deadwood weight shown in Figure 5; humidity gradients above the forest floor are stronger during daytime, when VPD is relatively high and deadwood weights are declining, consistent with water loss to evaporation. The height profiles in Figures 8b-e show average daytime (8 AM to 7 PM) absolute humidity and VPD for April through July, expressed relative to the near-surface values for each tower. Consistent with turbulent exchange of water vapor fluxes originating from the litter layer (and potentially also the underlying soil), humidity decreases, and VPD increases, nonlinearly with height above the forest floor.