The principal objective of the current four-year study was to evaluate the current and possible long-term effects of atmospheric deposition on northern hardwood forests in the Great Lakes region. The research was designed to address selected hypotheses about how regional air pollution alters nutrient cycling, growth efficiency, carbon allocation, species diversity, seedling recruitment, and genetic diversity of tree species populations. This summary report emphasizes results and interpretations of research data that have been analyzed and summarized by the Michigan Gradient Team as of March 31, 1991. Additional research analysis is underway and further results of the Michigan Gradient Study will be submitted as completed.