Aspen Decline , Aspen Chemistry , and Elk Herbivory : Are They Linked ?

A spen (Populus tremuloides) provides important habitat for mammals and birds 1–3 , improves water retention in watersheds, 4 and has high aesthetic appeal in autumn. However, the amount and quality of aspen cover in the West has been declining for many years. For example, aspen cover in the Dixie and Fishlake National forests in Utah has decreased by more than 60% from historic levels, and in the Uinta National Forest aspen cover has been reduced by nearly 40% 5. The decline is so severe that Mueggler 4 suggested that aspen-dominated woodlands might shift to shrub– grasslands under current management practices. The decline is disturbing because in the western United States, aspens are second only to riparian areas in terms of biodiversity. 1 Why is aspen declining in the West? Climate change, reduced fi re intervals, and browsing by deer, elk, and cattle 1,3,5,6 all have been suggested as contributing to the dec line of aspen. Aspen is a highly palatable browse for elk 1,7,8 and many researchers suggest that herbivory by both livestock and wild ungulates are the main causes of aspen decline. Some investigators have suggested that western aspen might be " doomed " 9 because of the magnitude and historic trends of aspen decline. In the Manti–LaSal National Forest, aspen used by either cattle or wildlife showed a reduction of 30% in the number of sprouts between 1991 and 1992. 6 In areas used by both cattle and wildlife, the number of aspen sprouts decreased by 59%. Losing almost two-thirds of the population in 1 yr to ungulate herbivory does suggest that long-term aspen regeneration might indeed be doomed. After evaluating historical and current data on aspen stands in and around exclosures in Utah, Kay and Bartos 6 concluded that ungulate (wild and domestic) herbivory was the most likely explanation for aspen decline in south-central Utah. Results from other studies performed on elk–aspen interactions in the western United States 1,10–12 tend to support those conclusions, as do results from studies on willows, 2,13 but see papers by Singer and coworkers. 14,15 Finally, in speaking with managers and other stakeholders, we found that all aspen are considered to be equally susceptible to herbivory. Aspen regeneration is perceived to be at the mercy of elk and cattle, but observations by scientists working in the Fishlake National Forest in central Utah indicate that all aspen clones are not equally susceptible …