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.