Patterns and Predictors of Plant Diversity and Compositional Change in a Restored Michigan Tallgrass Prairie

Abstract

Tallgrass prairies are one of the most threatened ecosystem types in Michigan and throughout North America. Dow Field is a small remnant prairie in the University of Michigan’s Nichols Arboretum in Ann Arbor, Michigan, that is being actively restored after many years of fire suppression. Starting in 1991, the prairie was divided into 10 management zones that were burned on 1 or 3 year intervals in April or November, and vegetation in 60 2m2 sample plots was monitored annually until 2007. In this study, I examined trends in the plant community over time, including diversity, species abundance, and community compositional change. I also explored the environmental and management factors that most influenced diversity and compositional change, and evaluated successional trajectory in the context of restoration goals. Over time, native species richness increased slightly, but exotic species richness and dropped dramatically after several years of burning. Andropogon gerardii (big bluestem) was the most dominant species in the prairie and reduced diversity through competitive exclusion, but there were no clear patterns in how the different fire regimes affected diversity or the abundance of A. gerardii. Instead, soil depth and soil clay were found to be the most reliable predictors of diversity, likely because increased soil moisture led to higher A. gerardii productivity and competitive ability. Year-to-year change in community composition was found to be affected by time since fire and fluctuations in growing season temperature and rainfall. Examining successional trajectory showed that the restoration has been most successful at reducing exotic species and increasing species heterogeneity, but has largely failed to increase native species richness to the level of remnant prairies, likely because of high A. gerardii abundance and low availability of native propagules.