Since at least 2004, sudden aspen decline, or SAD for short, has killed trees in five Western states in sweeping fashion. By 2008, in Colorado alone, more than a half million acres were afflicted. But after a few wet, cool years, the fatal phenomenon is finally relenting. "We're pretty sure that the drought in 2002 was the major inciting event," says Jim Worrall, a forest pathologist with the U.S. Forest Service. That year -- one of the driest on record in Colorado -- weakened trees, leaving them vulnerable to insects and disease. Aspen stands at lower elevations and on the upper reaches of south- and southwest-facing slopes were most affected, suggesting that warm, dry conditions were the primary triggers of decline. Trees are still dying, but new research shows that SAD is no longer spreading, at least not to large new areas, and is not affecting new growth. Even so, aspen's glory days may be past. So far, regeneration has been too slight to replace dieback in the most hard-hit areas. And with hot, dry spells expected to become more frequent, the next epidemic is likely not far off. Shown here, an aspen tree with Cytospora canker, which often kills trees affected by SAD.