Forests are globally important ecosystems host to outstanding biological diversity. Widespread efforts have addressed the impacts of climate change on biodiversity in these ecosystems. We show that a metacommunity model founded on basic ecological processes offers direct linkage from large-scale forcing, such as precipitation, to tree diversity patterns of the Mississippi-Missouri River System and its subregions. We quantify changes in tree diversity patterns under various projected precipitation patterns, resulting in a range of responses. Uncertainties accompanying global climate models necessitate the use of scenarios of biodiversity. Here we present results from scenarios with the largest losses and gains in tree diversity. Our results suggest that species losses under scenarios with the most dramatic contractions tend to be greater in magnitude, spatial extent, and statistical significance than gains under alternative scenarios. These findings are expected to have important implications for conservation policy and resource management.