| Summary: | Trembling aspen (Populus tremuloides) is failing to survive through the sapling
stage (2 to 4 m) to tree size in many national parks in western North America.
Hypotheses for aspen decline include reduced burning, climate change, high herbivory by
native ungulates (mainly elk (Cervus elaphus)), or interactions between these factors.
Historic and current aspen condition was investigated in several watersheds, inside and
outside of national parks, in the Rocky Mountains of Alberta, Canada to determine the
causes of aspen decline. Methods included repeats of historical photographs, fire history
by dendrochronology, time-series analysis (of climate, burned area, elk density, and
aspen regeneration), wildlife exclosure measurements, analysis of elk functional
herbivory response to aspen density, and effects of predation risk on elk-aspen foraging
patterns. Aspen regeneration was abundant in historic photographs, is currently abundant
in areas with low elk density (<2 elk/ km ), but has declined precipitously in national
park areas with high elk density. Historic anthropogenic burning appeared to be
important along valley-bottom corridors occupied by aspen, but declined c. 1900 due to
changes in cultural land uses. Yearly fluctuations in climatic conditions or burned area
did not appear to be factors in aspen decline. Aspen is regenerating in wildlife exclosures
on a range of site moisture conditions. Thus, climate or disturbance, either individually,
or interactively with herbivory, do not appear to be major factors in aspen decline.
However, elk herbivory was a highly significant factor. Elk-aspen herbivory followed a
Type 2 functional response (decreased rates of browsing at higher densities of aspen
regeneration) that may have occurred because predation sensitive elk avoided dense
aspen stands. Historically, valley-bottom aspen habitats were heavily used by predators
(humans and wolves) which reduced elk herbivory, and were frequently human-burned
which stimulated regeneration of dense stems. Current land uses in national parks (with
control of hunting and fire, and high human use that displaces wary predators but
habituates elk) are the opposite of these long-term processes. Successful aspen
regeneration will likely require a period of very low densities of elk that are wary of
predators and humans, followed by restoration of anthropogenic burning. === Forestry, Faculty of === Graduate
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