From above, the Southwestern U.S. is a sea of caramel and brown desert. Yet, a closer look uncovers small ribbons of green—desert forests that flourish along valuable springs and rivers. Wildlife thrives at these oases, drawn to the shade of trees that offer some respite from extreme temperatures. But in recent years, vast swathes of these forests have turned brown as trees died in high numbers. Scientists are puzzled by the die-offs and looking for answers. In 2019, the Nature Conservancy invited researchers to a workshop focused on this question: what is killing screwbean mesquite trees?