Aspen are by far the showiest of Rocky Mountain tree species, and their flame-like fall foliage makes them easy to spot in a forest otherwise dominated by conifers. This is especially true in areas where the surrounding Lodgepole pine trees have been killed by the mountain pine beetle, as in the picture above.
Aspen is a clonal species, which means that it can reproduce by sending up new shoots from its roots. These new shoots are called “suckers”, and they can grow incredibly fast when a disturbance removes competing trees- sometimes more than a meter in a single summer.
In 2008, Kellen Nelson established 48 research plots in mixed aspen-lodgepole stands within Rocky Mountain National Park, and found that high mountain pine beetle damage was associated with higher numbers of aspen suckers. Three years later I re-sampled the same plots, expecting to see even greater sucker densities as well as an increase in the number of young trees. What I found was the exact opposite. By 2011, sucker density in these plots had dropped by an average of 33%, and very few suckers had grown more than a meter tall. So why aren’t we seeing more young aspen trees in the park?
The problem appears to be elk, which browse on young aspen suckers throughout their winter feeding grounds. Browse rates ranged from 58-94% of suckers, and most showed evidence of browsing in each of the past 6 years. When aspen suckers are browsed repeatedly, they are unable to grow tall enough to escape the elk and instead develop a clump-like growth form, as shown below. As a result, we will probably not see an increase in the number of aspen trees in areas with high elk populations, despite more favorable growing conditions created by the mountain pine beetle outbreak.
Photo: Browsed aspen suckers, June 2012, in Rocky Mountain National Park
Katie Renwick is a PhD candidate in the Rocca Lab at NREL. She thinks trees are nifty, and has measured over 10,000 of them while researching climate change and disturbance impacts in Rocky Mountain forests.