Coarse woody debris (CWD), i.e. standing dead trees (snags) and fallen logs are rare elements in managed forests of the Swiss lowlands. Mean CWD abundance is 5 m³/ha, which is similar to many central European managed forests but 10–20 times lower compared to unused, virgin-like forests mainly known from eastern Europe. North American forests also have on average higher abundances of CWD than their European counterparts, apparently as a result of different utilization schemes by forestry. CWD strongly shapes forest floor microhabitat and is important for the habitat quality of many small mammals (e.g. voles Clethrionomys, mice Apodemus, shrews Soricidae, and others), but also smaller carnivores such as several species of marten (Martes). This paper briefly reviews the pertinent literature and reports findings of an own study.
Using live-traps in a beech-dominated forest (Sihlwald, near Zurich, Switzerland) from June to August 1996, we tested the hypothesis that abundance of CWD would influence densities and community composition of small mammals. We caught 904 bank voles (Clethrionomys glareolus) and yellow-necked/wood mice (Apodemus flavicolis/sylvaticus) represented by 380 individuals. Numbers of trapped individuals did not differ between 3 study plots (900 m²) rich in CWD and 3 plots with little CWD, but retraps were significantly more common on the richer plots, and the proportion of individuals whose minimum stay on the plots was >30 days was higher on the richer plots. The effects of CWD were also scale-dependent and were manifest more strongly on small scales: traps having more CWD nearby (models using 100 m² and 1 m², respectively, around the traps) caught significantly more voles and mice, although mice seemed to prefer thick CWD while voles preferred thin CWD. Depending on scale, young beech growth and low shrubs also had an influence. The results are discussed with respect to cover requirements and type of locomotion of the two rodent species.