Sustainable forest management advocates the retention or creation of open space within plantation forests to enhance biodiversity. However, the biodiversity value of these open spaces will depend on the habitat type chosen, as well as open-space size and shape. The present study investigated ground-dwelling spider assemblages in glades, rides and forest roads of various sizes in 12 mature Sitka spruce (Picea sitchensis) plantations across Ireland. Spiders were sampled along a transect from the open space into the forest using pitfall traps. Species richness and abundance declined along the open-forest transect with the open-space supporting a unique spider fauna, absent within the forest. Total species richness and richness of species associated with open habitats was significantly greater in the glades. There were few significant linear relationships between species variables and open-space width or area, however roads and rides < 15 in wide did not support an open spider fauna due to the influence of the canopy. No such 'threshold' area was found for glades, probably because the glades investigated did not cover a low range of areas. Open-space habitat type is an important determinant of spider assemblage structure, although open spaces' with high shrub cover or implanted broadleaves did not differ in assemblage structure from those within the plantation. At a large scale the total amount of open space within 200 in of sampling plots was positively correlated with species richness and abundance. Forest management plans should encourage the retention of a range of habitat types in non-linear open space (glades), whereas the biodiversity value of linear open space (rides and roads) will be enhanced if wider than 15 m. (c) 2006 Elsevier B.V. All rights reserved.