Ecological diversity (the product of ecosystem, species, and genetic diversity) will change significantly in the 21st Century in response to the combined influence of climate, human activities, the movement of indigenous and non-indigenous species, and natural disturbances like fire (also modified by climate). Many species will acclimate (phenotypic variation) and/or adapt (genotypic variation) to changing conditions. Many will not. Species with a high rate of reproduction that are able to move long distances, rapidly colonize new habitats, tolerate humans, and survive within a broad range of biophysical conditions will be most successful in finding new niches. Large changes in ecosystem composition, structure, and function are expected to occur at northern latitudes and higher altitudes. In some areas novel ecosystems likely will replace existing subalpine, alpine, boreal forest, and tundra ecosystems. Key words: climate change, ecodiversity, forest, ecosystem diversity, species diversity, genetic diversity