But after about 26 years of exceptionally high heat and drought, hundreds of million of these trees in lands stretching from New Mexico and Colorado to the southern Sierra Nevada of California have died. And in many places, something even more startling is happening: The trees aren’t coming back.

Ecologists warn that in just 25 years, more than 70 percent of the Southwestern needle leaf evergreen forests, which include ponderosa pines, may be replaced by grass in what might qualify as the first significant post-climate change landscape in America.