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.