The mutualistic relationship between plants and mycorrhizal root fungi has been long-studied in evolutionary and population ecology, but is gaining attention in light of ecosystem responses to global change. We have been studying such biotic interactions in sites from Harvard Forest to Georgia and into the tropics. At Harvard Forest we have found that a particular suite of mycorrhizal associate – namely, the arbuscular (AM) “endomycorrhizal†fungi – and their host trees are particularly well suited to use the additional inorganic soil nitrogen made available from a 5 degree C increase in soil temperature. These results suggest with a warming world, Harvard Forest and the Northeast may see a shift in dominant tree species towards maple, ash and cherry that associate with AM fungal species. With dominance of AM fungi and trees, Harvard Forest may more resemble the AM-dominance of current tropical forests.