2.3. Field estimation of deadwood and spruce-cone storage and
retention
We estimated the storage and retention behavior of individual
forest-floor deadwood pieces and spruce cones in several different ways.
First, we monitored the variations in water content over time in pieces
of deadwood using four self-made pressure cushions, constructed from a
standard drinking-water bladder with a hose to which we attached a
pressure sensor (Keller DCX-22AA). On top of each pressure cushion, we
placed pieces of deadwood from 95 to 222 g dry weight at different
states of decay (inferred qualitatively), and recorded the pressure at
10-minute resolution. From the recorded absolute pressure, we subtracted
the atmospheric pressure measured on site and then normalized between 0
and 1 by the minimum and maximum of each sensor, respectively (since the
measurement is inherently relative rather than absolute, because the
relationship between pressure and deadwood weight is determined by the
contact area between the deadwood piece and the pressure cushion, which
cannot be controlled). A replicate pressure cushion with no deadwood
showed virtually no variations in pressure, confirming that the pressure
variations observed under the deadwood pieces could be attributed to
changes in deadwood weight.
In a second experiment, we selected 40 pieces of deadwood with dry
weights of 6.2 g to 88.5 g (median = 20.2 g) and 20 spruce cones with
dry weights of 15.8 g to 36.4 g (median = 24.9 g) in different states of
decay. Their weights were measured daily at the same time of day (always
between 2 and 3 PM) for >8 weeks from 20 March to 22 May
2020. A major difference from the samples for which weights were
measured continuously is that these manually measured deadwood pieces
had direct contact with the forest floor and thus could absorb water
from the soil or adjacent litter particles. After the experiments, all
deadwood samples from the experiments described above were fully
saturated and weighed (submerged for 24 hours) and then dried and
weighed (multiple days at 105°, until no weight difference was measured)
in the laboratory, to assess the maximum storage capacity of the
individual deadwood pieces. We repeated these experiments to test the
reproducibility of the saturation and drying steps; results presented
here are the mean values from both experiments. To assess the effect of
deadwood size, we additionally repeated the evaluation of maximum water
storage with 30 larger deadwood pieces which were not used in the
routine measurement experiments. The state of decay of the deadwood was
categorized qualitatively as high, intermediate, or low, assessed by
“pocket knife testing” similar to what was described by Robin & Brang
(2008): we considered decay to be low if one can superficially cut only
a few mm into the deadwood surface, intermediate if the knife can be
pushed directly into the wood easily at some locations, and high if the
deadwood is easily friable by the pocket knife and it readily
disintegrates.
Results & discussion
Maximum water storage in the forest-floor litter layer
First, we assessed the maximum storage capacity of the two dominant
litter types, collected underneath beech and spruce trees, in laboratory
saturation experiments (n = 40 for each litter type). The broadleaf
litter below beech trees (Fagus sylvatica ) could store
approximately 4.7 times its dry weight and the needle litter below
spruce trees of Picea abies species could store up to 2.4 times
its dry weight (Figure 2). Sensitivity analyses with thicker litter
layers (doubled and quadrupled, n = 4 for each condition) yielded
similar results, implying that storage capacities scaled linearly with
depth: the maximum storage, averaged over four experiments, was 3.9 and
4.2 times the dry weight, respectively, for doubled and quadrupledFagus sylvatica litter, and 1.8 times the dry weight for both
doubled and quadrupled Picea abies litter.