Influence of environmental parameters on small-scale distribution of soil-dwelling spiders in forests: What makes the difference, tree species or microhabitat?

We surveyed the soil-dwelling spider communities of four stand types of forests (pure stands: spruce, Douglas fir; mixed stands: beech-spruce, oak-beech) along a successional gradient (four age classes: 15-112 years) to show the effects of small-scale microhabitat heterogeneity on the composition of spider assemblages. The investigation was carried out in south Germany. Spiders were collected by pitfall traps (n = 96) in 4-week intervals. To reveal key environmental factors that affect spider communities, abiotic and biotic habitat parameters (e.g., temperature, air humidity, soil characteristics, and vegetation parameters) were systematically assessed around each pitfall trap. Spider species showed a small-scale distribution pattern on the forest floor, significantly affected by litter type, degree of canopy closure, temperature and humidity parameters, as well as cover of grass, moss, herb, and natural regeneration according to discriminant analysis and ordinations. The relevance of habitat parameters changed with the growing season. While many species were correlated to litter type and canopy closure in spring, the influence of ground vegetation and humidity parameters prevailed during summer. In fall, the strength of correlations decreased besides ground vegetation and soil humidity. The species assemblages assessed in the four forest types of different age classes indicate that the covering tree species has a profound influence on the small-scale distribution of spider species. Nevertheless, the high similarity in the composition of spider assemblages from different stand types but with similar formations of ground vegetation, microclimatic parameter and canopy closure displayed a considerable impact of environmental factors not necessarily bound to the stand type itself. (C) 2007 Elsevier B.V. All rights reserved.