The roles of environmental filtering and colonization in the fine-scale spatial patterning of ground-layer plant communities in north temperate deciduous forests

P>1. The majority of plant species in northern temperate deciduous forests are restricted to the ground layer, but the importance of colonization processes relative to environmental filtering in structuring spatial variation in ground-layer plant communities is poorly understood. 2. Using multivariate analyses, structural equation modelling and geostatistics, we examined interactions among ground-layer plant communities, the live overstorey and environmental gradients across a 70- to 90-year-old northern hardwood forest in Wisconsin (USA). We hypothesized that (i) fine-scale variation is related to environmental filtering rather than dispersal limitation and colonization processes; and (ii) exogenous 'site' filters exert more control than the composition and structure of the overstorey. 3. A transition from communities of spring ephemerals to communities of evergreen-dimorphic species is related to a hierarchy of controls driven by elevation, soil texture and associated effects on soil moisture, overstorey composition, and O-horizon and soil properties. An orthogonal axis distinguished among sparse communities associated with high levels of soil moisture early in the growing season and rich communities of early summer forbs associated with increasing O-horizon N:P and %Ca, and short-distance dispersal mechanisms. Indirect effects of tree species are significant, but cumulatively less important than exogenous site filters. 4. Synthesis. The spatial patterning of ground-layer plant communities is related to both environmental filtering and colonization. These patterns were related to species' functional and dispersal characteristics, suggesting that processes structuring ground-layer plant communities are not merely neutral. Loose regulation of environmental and resource gradients resulting in a coarse-grained spatial patterning of plant communities observed in second-growth forests may therefore be related to a simplification in overstorey composition and the absence of heterogeneity accumulating through gap dynamics.