A profound unraveling is underway in the American Southwest, happening across a thousand-mile arc from Santa Fe, New Mexico, to the central Sierra. In an unprecedented calamity, the most widely distributed, most iconic tree of the region—the beautiful ponderosa pine—is disappearing. …It was the ponderosa pine that more than 1,100 years ago allowed the rise of the first cities in what would later become the United States, providing structural beams for the multi-storied dwellings of the Ancestral Pueblo. …Since 2000, more than 200 million ponderosa have died. More alarming still is that many of those forests won’t be coming back, likely yielding the ground to what will be grass and shrublands for centuries to come. …The loss of forest will also mean much faster melting of the spring snowpacks, since the snow will no longer be shaded by trees.