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.