Composition, structure, and diversity of forest understories along the climatic and edaphic gradients in the Upper Great Lakes region (Michigan)

This study examined the composition, structure, and diversity of forest herb communities along climatic and edaphic gradients in western Upper Michigan. Hypotheses regarding the ecological distribution of seasonal photosynthetic guilds (spring ephemerals, early summer species, late summer species, evergreen species, and dimorphic species) were tested based on analyses of 1650 4-m2 quadrats in 65 stands. A principal components analysis revealed that environmental variation across the sampled landscape was essentially unidimensional, from coarse infertile soils to finer, more fertile substrates. Non-metric multi-dimensional scaling supported this conclusion, showing that variation in growing-season length had far less correlation with ordination scores (based on species or guild abundances) than soil fertility, and that such variation was essentially orthogonal to the soil-fertility gradient.; Two-factor ANOVA's (3 classes of growing-season length x 3 classes of soil fertility) showed that (1) spring ephemerals had significantly higher absolute and relative coverage on sites on sites with the highest fertility or the longest growing seasons; (2) evergreen species had significantly higher coverages on the least fertile sites; (3) dimorphic species were significantly more abundant on sites of intermediate fertility; (4) absolute coverage by early summer species increased with soil fertility; and (5) late summer species were most abundant on infertile, relatively open sites. Herb species richness varied little with soil fertility, but their modified Shannon diversity (e{dollar}sp{lcub}rm Hspprime{rcub}{dollar}) increased significantly with fertility, reflecting a dramatic and regular increase in evenness.; ANOVA's restricted to morainal sites showed that (1) spring ephemerals were significantly more abundant on horizontal sites than on N- or S-facing slopes nearby; (2) dimorphic species were more abundant on N-facing slopes; and (3) graminoids were more abundant on S-facing slopes. Regression analyses suggested that a forest classification based on understory community structure would provide a much more sensitive guide to the potential productivity of sites than other present-day classifications.