The identity and structure of Southwest forests may change substantially under climate change.

Historical and projected climate change makes two-fifths (40%) of the region vulnerable to these shifts of major vegetation types or biomes; notably threatened are the conifer forests of southern California and sky islands of Arizona. Douglas-fir, ponderosa pine, and pinyon pine may be most negatively affected in Arizona, Colorado, and New Mexico. Following more frequent fires, forest systems may shift to more early-successional species, open canopy forests, and possibly invasive species.
In California, mixed evergreen forest may replace evergreen conifer forest, as Douglas-fir and white fir forests are replaced by Douglas fir and tanoak forests. White fir–ponderosa pine forest may be replaced by ponderosa pine–California black oak forest in the Sierra Nevada. Tanoak–Pacific madrone –canyon live oak woodland may replace blue oak woodlands, chaparral, and perennial grassland. In general, shrubland will replace oak woodland, and grassland may replace shrubland throughout California.