Aim: There is an intense debate about whether habitat fragmentation has a negative or positive effect on biodiversity. We examined whether species richness and incidence of forest plants were negatively or positively associated with fragmented forest configuration. We also analysed whether the results support the fragmentation threshold hypothesis with fragmentation effects only in landscapes with small habitat amount.

Location: Sodermanland province, south-eastern Sweden (8,388 km(2)).

Methods: Data consisted of plant distribution maps and landscape data on forest amount and configuration in 2.5 km x 2.5 km quadrats. We carried out models including forest area together with clumpiness index (CL models) or edge density (ED models) as the measure of habitat configuration. We focused on plant taxa with positive association between incidence and forest area (163 taxa in CL models; 119 taxa in ED models).

Results: Responses to fragmented configuration were negative more often than by random (33 and 22 taxa in CL and ED models, respectively; includes only models without significant forest area x configuration interaction), whereas positive responses were rare (four taxa in both models). When forest area x configuration interaction was significant, the most common response had a negative effect of fragmented configuration when forest area was low and no effect of configuration when forest area was high, which agrees with the fragmentation threshold hypothesis. Species richness also had this type of response. In another common interactive response, the effect of fragmented configuration was negative at low forest area and positive at high forest area.

Main conclusions: Responses to fragmented forest configuration, when significant, were usually negative. When responses to fragmented configuration were modulated by forest area, they were negative when forest area was low. The findings of complex interaction between forest area and configuration have implications for selection of appropriate patch sizes in sustainable forest management.