In Nature:

There have long been stories about declining aspen numbers, but you've now realised things have been much worse over the past few years. What is happening?

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.

This hasn't been reported before in the literature. It's a very quick thing — I've never seen anything quite like it.

More at the link.