Fine-scale environmental variation and structure of understorey plant communities in two old-growth pine forests

1. Although it is well established that nitrogen and light play major roles in structuring plant communities across the landscape, it is not as clear how they structure communities within forest stands. Virtually nothing is known about within-stand structure of understorey communities of herbs and small shrubs in near-boreal forests. 2. We tested the hypothesis that fine-scale (5-20 m) variability in N and light structure forest-floor plant communities in two old-growth mixed Pinus resinosa and Pinus strobus forests in north-eastern Minnesota, USA. 3. In each forest, all trees > 1.4 m tall were mapped on a 0.75-1.0 ha area. A grid of subplots 5-10 m apart was established (total n = 147), and N mineralization (μg g^-1 soil day^-1), soil depth (cm), light (% canopy openness), and percentage cover of all herbs and small shrubs were measured on each subplot. 4. Cluster analysis showed that the dominant understorey species fall into three groups. Group 1 is unrelated to N and light, and is negatively associated with a midstorey of the small tree Acer rubrum and the most abundant tall shrub Corylus cornuta. Group 2 reaches maximum abundance in places (mostly gaps) with relatively high light, but is unrelated to within-stand variation in N availability. Group 3 consists of a single species, Aster macrophyllus, and reaches maximum abundance in areas with low N availability and low abundance of Corylus, but higher than average abundance of P. strobus and Betula papyrifera overstorey trees. 5. N and light have a moderate influence on understorey plant community structure. The plant species do arrange themselves along N and light gradients, but the gradients are likely to be too narrow to allow the degree of differentiation seen at the landscape level. Spatial patterning of the species groups is probably influenced by other factors, including disturbance history, chance and neighbourhood effects such as clonal reproduction.