Edge effects created by wildfire and clear-cutting on boreal forest ground-dwelling spiders

The response of ground-dwelling spider assemblages to edges created by wildfire was compared to their response to clear-cut edges in black spruce forests in eastern Canada. For each disturbance six edge transects 100 m long were established. Spiders were collected with pitfall traps 10 m apart from 50 m inside the disturbances to 50 m in the forest interior. Measurements of the forest floor structure and the habitat around the traps were also taken. Most habitat variables varied similarly across both wildfire and clear-cut edges but two variables, coarse woody debris and shrub cover changed more abruptly at edges of clear-cuts than at wildfire edges. Two separate CA analyses of the burned and clear-cut edge transects showed that changes in spider community species composition were more abrupt at clear-cut edges than at burn edges. A species indicator analysis (INDVAL) was used to identify species that were significantly associated with specific edge zones. Eight species were significantly associated with open (disturbed) habitats, one species was significantly associated with both burned and clear-cut edges, and four species were significantly associated with the forest interior. T-tests of slope coefficients from separate simple regressions of abundance and richness over distance showed that open habitat specialist's abundance and richness changed significantly more abruptly at clear-cut edges than at wildfire edges. Locally weighted smoothed regressions showed that the extent of edge influence on ground-dwelling spider guilds (open habitat and forest interior specialist) penetrated 20–30 m into the disturbances, 30 m into the forest interior at clear-cut edges, and at least 50 m into the forest interior for wildfire edge transects. Our results suggest that a minimum width of 100 m is necessary to maintain characteristics of forest interior spider assemblages in forest remnants, riparian or road buffers, and forest strips between cut-blocks.