USDA scientists fenced 1 hectare of aspen against elk but allowed deer, bears and other animals through; within 1 year, protected trees gained 14.5 cm more height

In areas where elk browse heavily, repeated feeding can stunt young shoots and make it difficult for aspen stands to regenerate. According to the USDA, scientists designed a special fence that could help address this problem without excluding other wildlife. The researchers enclosed a one-hectare stand of quaking aspen with a fence that kept elk out while allowing deer, bears and other animals to pass through. Within a year, the protected aspen stems had gained 14.5 cm more height than those outside the enclosure. The findings demonstrated how a targeted approach to managing browsing pressure could help young trees grow while allowing other wildlife to continue using the habitat.

A fence with a difference

The research notes that the experiment ran from December 2003 to May 2006 on a working cattle ranch near Rocky Mountain National Park in Colorado. The researchers wanted to solve a practical problem. Conventional woven-wire fences could keep elk away from recovering vegetation, but they could also prevent deer and other animals from entering. Complete exclusion was not necessarily desirable if the aim was to restore vegetation while allowing wildlife to continue using the habitat.


Led by Kurt C. VerCauteren, the USDA team designed a shorter fence intended to keep out elk while allowing smaller animals through. This fence used 1.2-metre-high woven wire, with the bottom raised to 0.5 metres above the ground. Posts were used to hold up the fence, leaving a gap underneath where deer and other wild creatures could cross through.

To monitor the enclosures, the researchers used track plots, camera traps triggered by animals, and measurements of aspen stem height and density. The study showed that the fence was effective at reducing elk browsing. During two years, only one elk managed to enter the enclosure, and the presence of elk tracks beyond the fence was observed 24% of the time.

Letting wildlife back in

The fence did not turn the enclosure into an animal-free area, as deer and a wide variety of other mammals were still found there. Beaver, black bear, bobcat, coyote, mountain lion, raccoon, red fox, and lagomorphs were recorded within the enclosed fence. Deer and coyote were found both inside and outside the fence without any differences in using these two places. It was important since not only elk, but also other animals were grazing on the vegetation. However, the exclusion of elk from this ecosystem made some difference among the young aspens.

After one year, mean stem height had increased more inside the enclosure than outside. In other words, the protected stems had a clear growth advantage over the same period. However, the researchers did not find a comparable change in stem density. The number of stems inside the enclosure changed little relative to the area outside it. The finding fitted into a wider body of research showing how strongly browsing can influence aspen regeneration. A study in Journal of Range Management, for example, found that aspen subjected to wildlife browsing often regenerated less successfully than trees protected from ungulates. In that long-term assessment, total-exclusion plots developed substantially higher stem densities than areas exposed to deer and elk browsing.

Why a few centimetres mattered

It is evident that aspen stands are more than individual trees; their sprouts and suckers can become the next generation of a forest. However, constant browsing may prevent the growth of young aspens and make them weak and stunted for many years. For a young aspen to survive and grow into a mature tree, it must grow beyond the reach of herbivores. The results of the USDA experiment indicate that this objective does not necessarily involve exclusion of all the large mammals. Since it is possible to target the elk by fencing them out of the protected area, the study revealed that the fence may serve as a valuable tool for habitat managers.

The method also suggests a broader restoration principle: sometimes it is more useful to relieve pressure on an ecosystem than to rebuild it from scratch. In this case, a gap in a relatively low fence was enough to keep elk out while allowing other animals in the surrounding habitat to continue using the area.