Aspen Trees Swiftly Dying Off in Western US

Colorado has rapidly lost an unusually large number of aspen trees in the last few years to insects and diseases.

Concentrated patches of dead trees extended over an area of 200 square miles (56,000 hectares) as of 2006.

The die-off has spread quickly, with the number of trees killed in some stands more than tripling since 2002. Indeed, some aspen groves now have no live trees left. San Juan National Forest in the Rocky Mountains of southwestern Colorado had a particularly sharp spike in aspen tree mortality during 2005 and 2006.

Large-stemmed trees are failing fastest, especially those growing in widely-spaced stands on low-elevation, south-facing slopes. These mature trees are not succumbing to the normal processes of ageing forests.

Instead, pests that normally don't have a huge impact on trembling aspen have become quite lethal. The perpetrators include aspen bark beetles and poplar borers, as well as a stem fungus, Cytospora canker.

Scientists at the USDA Forest Service believe the epidemic is driven by climate. A recent bout of severe drought along with high temperatures has stressed the aspens enough to reduce their resilience to pathogens.