David Brill talks to forestry expert Wayne Shepperd, at the Rocky Mountain Research Station at Fort Collins, Colorado, about the mysterious death of these trees in the western United States.

Aspen here in the western United States regenerate by sprouting from the root system of existing trees, rather than growing from seeds. Normally if a parent tree dies you would expect to see thousands of sprouts coming back up immediately from the roots. But what we’re seeing now is that the roots appear to be dead and we’re not getting any new sprouts…

Read the story here.