Aspen stands are unique from previously discussed shade-intolerant species in two important ways: (1) Aspen are a short-lived seral species, typically only surviving from 60-120 years. ( 2) They are really stems originating from large underground root systems. All of the "trees" connecting to the same root system are genetic clones of one another. These large root systems, often covering several acres, reproduce by sending up thousands of "seedlings," technically suckers, after the aspen overstory is disturbed.
Lack of fire in aspen communities has allowed conifer species to establish and eventually dominate these areas. Aging aspen is subject to damage from a variety of stem and other diseases. FHM data indicate that such damage is common. Only 38 percent of the trees examined were free of damage; whereas more than 80 percent of conifers were undamaged.
Aspen forests appear to be on the decline throughout the Interior West and in portions of the Inland Northwest. In Idaho, this phenomenon appears to be most pronounced in the Intermountain, Middle and Southern Rockies Ecoregions. A recent inventory of tree cover on the Targhee National Forest indicated about a 90 percent decrease in aspen since the beginning of this century. Succession to other species from the lack of fire was the primary cause of aspen decrease. Secondarily, grazing by large numbers of cows or big-game prevents successful regeneration.