So why has aspen been in decline since European settlement and why have most aspen stands not successfully regenerated in 80 or more years? At first it was thought that auxins and apical dominance were the problem. Auxins are chemicals produced in aspen’s uppermost branches that are then translocated to the roots where the auxins suppress suckering. While auxins do suppress aspen root-sprouting, they do not eliminate suckering. Where aspen is protected by game-proof and/or livestock-proof fencing, termed exclosures — please see my earlier Muley Crazy article on range reference areas — aspen successfully regenerates and produces stands in which the stems (trees, saplings, ramets) are multi-aged. Thus, the even-aged stands common in the West today are NOT a biological attribute of aspen. Instead, they are the result of excessive ungulate grazing, where herbivores have repeatedly browsed the aspen suckers, and thereby prevented the growth of aspen saplings and trees.
In most places where I have conducted research, the problem has been too many elk — either no hunting in national parks or not enough hunting on national forests. I spent three summers measuring nearly 400 aspen stands on BLM lands in north-central Nevada and there the problem was improper livestock use. As soon as any new aspen suckers appeared, they were repeatedly browsed by cattle and/or domestic sheep. It was amazing how many aspen saplings you could produce with a simple, three or four strand barbed wire fence. The areas I studied in Nevada had no elk and mule deer numbers were exceedingly low.
Elk can be especially damaging to aspen, because in addition to browsing aspen suckers, elk also like to eat the bark of mature aspen trees. Elk do this by digging the front teeth of their lower jaws into the soft aspen bark and then moving their heads upward, gouging out large sections of bark. Any injury to the bark of aspen exposes that stem to increased attack by a host of pathogens. Needless to say, this hastens stem mortality and clonal decline. Moose will also strip aspen bark, but mule deer do not. Aspen responds to elk bark-stripping, or any other bark damage, by producing black scar tissue. Where elk concentrate, especially during winter, all the normally white-barked aspen are black the lower six feet. Thus, if large numbers of elk always occupied western ranges, as some would have you believe, then late 1800’s photos should show that aspen was as heavily bark-damaged in the past as it is today. Well, I and my colleagues in Canada have searched all the major photographic archives and not one aspen tree in any of the earliest images shows any sign of elk bark damage. NONE!
Charles E. Kay. 2009. Aspen: A Vanishing Resource. Mule Deer Foundation Magazine, No. 25, pp. 32-39.