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.