Hessl synthesized and compared current hypotheses regarding aspen decline by focusing on studies of aspen in the elk winter ranges of Rocky Mountain National Park (RMNP), Jackson Hole (JH), and northern Yellowstone National Park (YNP). Hessl used tree-ring studies, performed in the last decade from the three elk winter ranges, to date aspen ramet recruitment episodes. Using age structures, he synthesized data on aspen recruitment by comparing major episodes of recruitment at decadal time scales with data on drought, fire history, and elk population history. The history of aspen recruitment in the three winter ranges varies but appears related to the history of fire, elk management, and climatic variability. JH has experienced two episodes of recent recruitment (1915-1940 and 1955-1990) that appear to be related to declines in elk populations. Market hunting of elk continued well into the 20th century in RMNP, suggesting that the early periods of aspen recruitment may have been associated with low elk populations. Drought variability appears to be only a weak driver of aspen regeneration in RMNP, JH, and YNP. The role of mortality in creating the current structure of aspen stands has been largely ignored in studies of aspen age structure. Results of a study conducted by Romme et al. (1995) in YNP, suggest that the 1988 fires will not produce a flush of new aspen ramet recruitment, but Hessl notes that their study represents only one fire event and a single period of postfire climatic conditions. Future fires with different intensities, patterns, and postfire climatic conditions may have different effects on aspen recruitment. The results of studies reviewed suggest that heavy browsing by elk populations has had a strong influence on episodes of aspen regeneration in the past 150 years. Fire may have been an important driver of aspen regeneration before 1900, as demonstrated by the extensive regeneration in YNP and JH following the fires of the late 1800s, but heavy elk browsing is limited to winter range, calving areas, and specific locations with high elk densities and does not appear to threaten aspen regeneration where elk are less common. There is an incomplete knowledge of aspen's ability to persist in its current environment. To preserve and protect complex ecological systems, ecosystem management of large protected areas must consider past and present human influences on ecosystem variability and must match the scale of management to key ecological processes and drivers.
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