Spatial patterns of litter-dwelling taxa in relation to the amounts of coarse woody debris in European temperate deciduous forests
Introduction
Coarse woody debris (CWD) provides a patchy and ephemeral structure on the forest floor that is a subject of decay over time. Its decomposition locally broadens the resource spectrum (e.g. Kappes, 2005, Lonsdale et al., 2008) and provides a habitat that contrasts that of the forest floor (e.g. Evans et al., 2003, Varady-Szabo and Buddle, 2006).
Deadwood has been acknowledged to be vital for populations of many forest-associated taxa, and is well known for its role in sustaining species richness (e.g. Maguire, 2002, Penttilä et al., 2004, Lonsdale et al., 2008, Roberge et al., 2008). Downed woody debris is not only associated with many xylophageous specialists; it also affects the more generalist biota of the adjacent forest floor. The litter-dwelling fauna shows a strong positive reaction to coarse woody debris in different temperate regions irrespective of the regional species pool (e.g. Kappes, 2005, Kappes et al., 2006, Kappes et al., 2007).
Three alternative scenarios that have different implications for biodiversity conservation may underlie high species richness: (1) spatial and temporal heterogeneity, that is, the species are not all in the same place at the same time and are subjected to different environmental conditions, (2) resource partitioning, i.e. the species are not using exactly the same resource in the same biotope, or (3) weak competition at ample resources, i.e. niches are not fully exploited (Hartley and Shorrocks, 2002). In the case of CWD, (1) implies that the species are patchily distributed and each species utilizes different logs, or that species use the logs at different times of the year or at different stages of decay, as CWD itself is part of the temporal heterogeneity. (2) Implies that the species co-occur at a given log, but differ in the use of its parts such as the bark, the soil contact zone, or the leaf litter accumulating at this structure. Finally, (3) implies that the resources provided by CWD cannot be fully exploited by a given taxonomic group.
Aggregation behavior of single species adds to the heterogeneity. Intra-specific aggregation stabilizes the coexistence of potential competitors of patchy and ephemeral resources, as was shown for mycophagous (e.g. Wertheim et al., 2000), carpophagous (e.g. Krijger and Sevenster, 2001) and necrophagous insects (e.g. Kouki and Hanski, 1995). This intra-specific aggregation is associated with low alpha-diversity and high beta-diversity. In contrast, inter-specific aggregation results in high alpha-diversity at a comparatively low beta-diversity (Wertheim et al., 2000).
We performed an analysis on the litter-dwelling fauna of broad-leaved forests (dominated by fagaceans, Fagus sylvaticus and Quercus sp.) in Western and Central Europe. As a model for different microhabitats, we compared samples taken close to and distant from CWD. Species assemblages from the leaf litter have so far not been used to assess potential thresholds in the amount of downed deadwood needed for biodiversity conservation. Thus, by comparing managed and unmanaged forests, we addressed the following questions:
- (1)
Does intra-specific or inter-specific aggregation occur close to CWD?
- (2)
Does the aggregation pattern of the litter-dwelling fauna change with the amount of deadwood on the forest floor?
- (3)
If so, what is the minimum volume of CWD required to sustain or enhance the diversity of litter-dwelling taxa in managed forests?
Section snippets
Study sites and sampling
The reference data originate from four broad-leaved old-growth forest reserves in the West Carpathians, Slovak Republic (Fig. 1, R1–R8 in Table 1). All of them are located inside a large, mostly unmanaged or extensively managed forest area. Trees were uneven-aged, resulting in a maximal crown closure of 2.0 or above (Table 1). Several trees exceeded 140 years of age in each stand. The amounts of deadwood were ≥20 m3 ha−1 in Boky and Rohy, and ≥100 m3 ha−1 in Badin and Pol’ana. The forests grow on
Effects on heterogeneity among samples
In a first step, we assessed the effect of CWD on the heterogeneity among samples in the reference forests. The faunal heterogeneity was significantly lower close to CWD than in the controls for the shelled Gastropoda (paired t-tests; mean difference: −0.37 ± 0.27; t = −3.86, P = 0.006), the Diplopoda/Isopoda (mean difference: −0.44 ± 0.31; t = −4.07, P = 0.005) and the Chilopoda (mean difference: −0.38 ± 0.33; t = −3.25, P = 0.014). The effect was marginally significant for the Coleoptera (mean difference: −0.28
Discussions and conclusions
Environmental gradients are likely to be more complex close to coarse woody debris, but the assumption of increased heterogeneity between samples close to coarse woody debris did not hold true. This may have been an effect of a higher carrying capacity close to CWD. Close to coarse woody debris, inter-specific aggregation occurred rather than intra-specific aggregation, indicating that nested habitat quality underlies the distribution pattern of the forest floor fauna, as demonstrated for
Acknowledgements
Peter Tucek, Katarina Bucinova (Institute of Forest Ecology SAS) and Susanne Miethaner assisted during the field collections in the Slovak Republic. The Institute of Forest Ecology SAS holds the study permit for forest reserves in the West Carpathians, Slovakia. For permissions to sample in the managed forests in Western Germany, we would like to thank N. Bischof (FO Jünkerath), F. Ebi (FO Adenau), Herrn Graf von Spee (Engelskirchen), K. Hecker (FO Hürthgenwald), F. Herhaus (BSO Nümbrecht), G.
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