The hemlock-white pine-northern hardwood forest region of North America is a transition between the deciduous forests to the south and the boreal forests to the north. In a survey of vascular plant communties in northern lower Michigan we examined species diversity and landscape pattern. Surveying 47 sites we found 483 vascular plant species: 37 tree species, 67 shrub species, 4 vine species, and 375 herbaceous species; there was a mean of 74.3 +-4.2 species per site. Mean Jaccard similarity among sites was 0.22 +-0.01 and mosaic diversity, a measure of landscape complexity, was 3.01 +-0.03. Using nonmetric multidimensional scaling and cluster analysis, two distinct groups of communities were differentiated: group I consisted of communities on mesic upland to wet lowland sites dominated by sugar maple, beech, hemlock, cedar, tamarack, and spruce; group II consisted of communities on xeric to dry-mesic upland moraines and level plains dominated by pines, red oak, birch, and aspen. Each group encompassed a separate south to north transition. Contrary to continental trends, species richness was greatest in sites with the greatest percentage of northern species. The region is more species-rich than adjacent regions to the south and north. We present four hypotheses to explain this species enrichment.