Figure 1:Location of the “Waldlabor Zürich” in Zurich (a) and a schematic of our experimental site at its eastern border (b) , indicating the locations of trees (spruce, beech and other species shown in green, orange, and gray, respectively), two litter plots (orange squares EX I and EX II), the monitored deadwood pieces and spruce cones (blue ovals), and the small temperature/relative humidity towers (yellow crosses). The weather station is located outside the forest, approximately 150 m from our experimental site.
Laboratory estimation of litter storage capacity
We estimated the storage and retention behavior of the forest‑floor litter layer both in the laboratory and in situ. In laboratory saturation experiments, we filled 63-µm-mesh laboratory sieves (area of 0.0308 m2) with a 5 cm layer of broadleaf litter or a 2.5 cm layer of needle litter (both collected from the site), dried them at 70 °C for 24 hours and measured the dry weight, as suggested in Carnol and Bazgir (2013). The thickness of the litter layer corresponds to the mean thickness across our field site, as estimated from collected grab samples. We then submerged the sieves with litter into water for 24 hours, let the water drain gravitationally, and then weighed the sieves after 30 minutes and again after 4 hours to calculate the litter’s water retention capacity (Sato et al., 2004; Li et al., 2013). During these steps, the litter was not removed from the sieves, to preserve the litter structure and the water captured within it. We then removed the litter from the sieve and weighed the sieve to account for water that was captured in the sieve itself (Helvey and Patric, 1965). Afterwards the litter was again dried at 70 °C for 24 hours to verify the conservation of the original dry mass of the leaf litter. This procedure was repeated for 40 samples each of broadleaf and needle litter. To evaluate the impact of litter-layer thickness and structure, we also measured the retention capacity of doubled and quadrupled litter thicknesses (10 cm and 20 cm for broadleaf litter layer and 5 cm and 10 cm for needle litter), with four replicates each.