Figure 9: Plots with and without litter (a) transmit different amounts of ground heat flux to the forest floor surface from the underlying soil. Snowmelt was quicker (b) and surface temperatures on a cold day without snow cover were higher (c)in the litter-free plot.
Our observations suggest that the litter layer affects forest-floor temperature dynamics and associated heat fluxes. In previous research (Floriancic et al., 2018) we showed that snowmelt from geothermal heat fluxes might account for up to 2% of winter baseflow, but the winter heat fluxes from energy stored during warmer seasons are potentially much larger. The porous matrix of the litter layer should reduce energy exchange in both directions. That is, it should shield the snow cover from the ground heat flux in winter and reduce heat fluxes to the underlying soil during summer, with the net result that during winter, there should be less subsurface heat available to melt snow where the litter layer is thicker. We acknowledge that these are qualitative interpretations that require more empirical support and theoretical exploration, but they suggest that these thermal effects could be important enough to deserve further study.
Limitations and uncertainties in our single-site setup
Our study has obvious limitations. The findings presented above are derived from plot-scale observations within a single small forest site. Most of our measurements span only a single growing season (or less), and the degree of replication (among plots and precipitation events, for example) is limited. Many of our experiments were labor-intensive, and thus could only be carried out for short time spans and for few replications. For example, the sampling campaign for litter grab samples occurred during a rather dry period with only three (rather low-intensity) rain events. Because the major purpose of that experiment was to confirm the maximum storage capacity of the litter, rather than to estimate litter retention timescales, the data here are limited and so are the potential conclusions we can draw from them. Many of our time series were also cut short due to technical issues. For example, the time series of soil moisture in Figure 4 show the only six weeks where all four sensors at both plots were working reliably. Similar sensor problems limited the pressure cushion data in Figure 5 to only one month. Nevertheless, these limited observations reveal processes that are potentially important but widely neglected in studies of the forest water cycle. Thus, although many of our results are suggestive rather than definitive, they point to the need for further work to more rigorously quantify the water dynamics of the forest-floor litter layer.
Upscaling the impact of litter on water fluxes to the overall forest water cycle
Using the procedures described below, we quantified how interception by, and evaporation from, the forest-floor litter layer may contribute to the overall forest water cycle.