This report was commissioned by the USDA Forest Service Lake Tahoe Basin Management Unit to synthesize existing information on the ecology and management of aspen (Populus tremuloides) in the Sierra Nevada of California and surrounding environs. It summarizes available information on aspen throughout North America from published literature, internal government agency reports, and experienced scientists and managers. The historic distribution, abundance, and ecologic role of aspen in the Sierra Nevada are discussed, along with the reproductive physiology of aspen. Issues that affect aspen health and vigor in the Sierra Nevada and elsewhere are covered, along with methodology for assessing the condition of aspen and monitoring the effects of management activities to restore and maintain aspen. Descriptions of the types of aspen that occur in the Sierra Nevada are presented along with alternative techniques to manage and restore aspen that are applicable wherever aspen is found.
The role of livestock grazing and big-game browsing in the decline of aspen (Populus tremuloides Michx.) in the Intermountain West has long been questioned.All known aspen exclosures (n=8) on the Dixie and Fishlake National Forests in south-central Utah were measured during late summer of 1995 and 1996 to determine aspen stem dynamics, successional status, and understory species composition. Five of the exclosures were of a 3-part design with a total-exclusion portion, a livestockexclusion portion, and a combined-use portion which permitted the effects of deer (Odocoileus hemionus) and elk (C e r v u s elaphus) herbivory to be measured separately from those of livestock. Aspen within all total-exclusion plots successfully regenerated and developed multi-aged stems without the influence of fire or other disturbance. Aspen subjected to browsing by wildlife, primarily mule deer, either failed to regenerate successfully or regenerated at stem densities significantly lower (2,498 stems ha-1) than that on total-exclusion plots (4,474 stems ha-1). On comb i n e d wildlife-livestock-use plots, most aspen failed to regenerate successfully, or did so at low stem densities (1,012 stems/ha-1). Aspen successfully regenerated on ungulate-use plots only when deer numbers were low. Similarly, ungulate herbivory had significant effects on understory species composition. In general, utilization by deer tended to reduce shrubs and tall palatable forbs while favoring the growth of native grasses. The addition of livestock grazing, however, tended to reduce native grasses while promoting introduced species and bare soil. Thus, communities dominated by old-age or single-age trees appear to be a product of ungulate browsing, not a biological attribute of aspen as has been commonly assumed. There was no evidence that climatic variation affected aspen regeneration. Observed differences are attributed to varied histories of ungulate herbivory.
Aspen clones in varying degrees of deterioration were burned in northwestern Wyoming in an attempt to regenerate the site. Large numbers of aspen suckers are necessary to perpetuate these stands under current heavy ungulate use. Sucker numbers doubled the second year after burning and by the end of the third year bad returned to near preburn levels of 15,000~20,000 suckers per bectare. This slight increase in sucker numbers is probably not sufficient to regenerate the stands under current browsing pressures. Total understory production declined the first year following tire and then increased to 3,600 kg/ha tbeseeondyear-almost double preburn conditions. Production decreased the third year to about one-third greater than before burning. Forb and grass production increased and shrubs decreased as a result of burning. Fireweed (Epilobium cnrgustifolium) was the largest post-tire contributor to total understory production.
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