Figure 8: Time series of the difference in absolute humidity
measured at two small towers within the forest, below spruce (tower 1;
difference between 20 cm and 170 cm; light blue line) and beech (tower
2; difference between 50 cm and 200 cm; dark blue line) as well as
forest air temperature (red line) (a). Daytime height profiles
of absolute humidity and VPD, relative to values measured at the
lowermost sensors at tower 1 (spruce, panels b and c )
and tower 2 (beech, panels d and e ), respectively. The
profiles show monthly averages at the respective height for daytime
hours (8AM to 7PM).
Thermal heat shielding of the forest floor litter layer
The forest-floor litter not only modulates water fluxes, but also
affects energy fluxes, with potential effects on snow cover and melt
dynamics. During winter 2020-2021, we observed that snow melted more
quickly on the litter-free plots compared to the litter-covered plots
(Figure 9b). The following winter, we used infrared thermometry to
compare the surface temperatures of a litter-covered and adjacent
litter-free plot on a snow-free December day with overcast skies and
temperatures below freezing (thus minimizing the effects of radiative
heating and evaporative cooling). Surface temperatures in the
litter-free plot were approximately 1-2°C higher than in the
litter-covered plot (Figure 9c). This could reflect the insulating
effects of the litter layer; however, the differences could also result
from other measurement effects, e.g., the difference in infrared
diffusivity between the two plots.