Divergent aspen stand conditions under two canopy cover dynamics: Increasing conifer and persistent aspen at Lassen Volcanic National Park, California

In northern California, quaking aspen (Populus tremuloides Michaux) is at the edge of its range and stands are small, isolated, and vulnerable to extirpation. Although a minor landscape element, regional aspen stands are believed to be important for biodiversity in conifer-dominated forest landscapes. We sampled 29 aspen stands in Lassen Volcanic National Park, where park managers are in the initial stages of planning for aspen conservation. Many stands are small in size, heavily browsed by deer, and experiencing competition from white fir (Abies concolor) and lodgepole pine (Pinus contorta ssp. murrayana ). The study reported in this thesis was a survey of 29 aspen stands along a gradient of conifer canopy cover within Lassen Volcanic National Park. Examining relationships between current conifer cover and stand condition indicators revealed that for each 10% increase in conifer cover, aspen recruitment decreased by 6 sprouts/100 m2, and herbaceous species richness decreased by 1.6 species. Conversely, litter depth increased by 0.5 cm. We conducted an interpretation of aerial photos from 1952 and 1998 to determine directional changes in percent conifer and aspen cover in each of the 29 stands, revealing a dynamic of increasing conifer in half the sampled stands, with a mean rate of conifer increase of 1% a year. The remaining stands demonstrated a persistent aspen dynamic, with no significant increase in percent conifer cover. Stand condition indicators were more strongly associated with stand dynamic and current percent total canopy cover (1998) than initial percent conifer cover (1952) and rate of change in percent conifer cover. Significant relationships were found for aspen cohort establishment and C:N of the woody soil litter fraction. In addition, stand dynamic and percent total canopy cover interacted to determine species richness, diversity, recruitment, and the relative masses of woody and fine soil litter. We conclude that the ecosystem values associated with aspen stands are negatively associated with percent conifer canopy cover in Lassen Volcanic National Park. Although some stands appear to be persistent through time, the remaining stands will likely require management attention if their values are to be maintained for future generations.