Ectomycorrhiza (ECM) and arbuscular mycorrhiza (AM) are symbiotic mycorrhiza that help plants acquire nutrients. ECM and AM differ in their nitrogen and phosphorous uptake rates as well as in the associated carbon costs for plants. Despite the ubiquity of ECM and AM we know little about their environmental niches. In this study we attempt to predict the environmental niches of ECM versus AM. We analyzed data from forest inventory plots from the United States and calculated the proportion of trees with ectomycorrhiza (ECM) or arbuscular mycorrhiza (AM). We built a statistical model to predict the probability of each mycorrhizal type based on climatic and edaphic conditions. We found that at extreme climatic or edaphic conditions one type dominated, but that at moderate conditions both types could occur. We speculate that AM and ECM may represent alternative stable states under moderate environmental conditions. We hypothesize that under such moderate environmental conditions initial conditions, stochastic effects and species composition determine which mycorrhizal association dominates.