Scale-dependent relationships among forest structure, resource levels, and spatial patterning of understory plant diversity: Implications for forest restoration

Northern mesic forest understories in the Lake States are composed of over 250 species of vascular plants, including: tree seedlings, shrubs, spring ephemeral and summer-green herbs, graminoids and pteridophytes. While forest understory plant communities respond to a hierarchy of controls, the importance and significance of the mechanisms affecting diversity may vary with spatial extent. We examined spatial patterns of understory diversity in a second-growth northern mesic forest in the Flambeau River State Forest in northern Wisconsin, USA. This is the site of a long-term, manipulative experiment that examines the restoration of old-growth structural characteristics to younger, second-growth forests. Scale ranged from fine microsite scales, (0.11 m²), gaps (10s-100s m²) and landscape (280 ha). Spatial analysis shows that at broad scales patterns were associated with overstory structure and composition, as well as sapling density. At meso-scales spatial patterns were related to light transmittance, soil nutrients and sapling density. Fine-scale variability was associated with the characteristics of individual species, seedling and sapling density, pit and mound topography, and coarse woody debris. Broad-scale variability in the overstory interacts with meso-scale variability in soil texture and topography, as well as fine-scale variability in microsites to result in unique combinations of plant resources. These patterns of plant resources effects patterns of species composition and diversity in understory vegetation. Restoration of patterns of forest ecosystems should occur over multiple spatial scales to account for the complex interactions that structure understory plant communities.