In northern hardwood-conifer forests, alternatives to conventional forest management practices are being developed in order to maintain biodiversity and ecosystem functioning while providing for timber revenue generation. The understory layer of vegetation encompasses the majority of plant species diversity in forested ecosystems and may be sensitive to timber harvest disturbance. Thus, monitoring the response of forest understories to new silvicultural techniques may provide a means for evaluating their intensity. In this study, we hypothesize that i) uneven-aged, low-intensity silvicultural systems can maintain understory plant diversity and support latesuccessional species through harvest disturbance; ii) retaining and enhancing stand structural complexity can increase understory plant diversity in northern hardwoodconifer forests; and iii) plant responses are influenced by interactions between canopy structure, soils, and exogenous climate processes. Experimental treatments include two conventional uneven-aged prescriptions (single-tree selection and group selection) modified to increase structural retention, and a third technique designed to promote late-successional forest structure and function, termed structural complexity enhancement (SCE). Four replications of each treatment were applied to 2 ha management units at three sites in Vermont and New York, U.S.A. Understory vegetation was monitored over 2 years pre- and 4 years post-treatment. We used a linear mixed effects model to evaluate the effects of treatment, soil properties, and drought stress on understory diversity and abundance. Compositional changes among treatments were assessed with non-metric multidimensional scaling (NMS), an ordination technique. Model results show that over time, understory responses were strongly affected by overstory treatment and less influenced by soil chemistry and drought stress. All treatments were successful in maintaining overall composition and diversity. However, late-successional diversity increased significantly in SCE units compared to group selection units. These results indicate that while conventional uneven-aged systems are capable of maintaining understory plant diversity, variations that retain or enhance structural complexity may be more efficient at retaining latesuccessional species. Increased microsite heterogeneity as a result of these techniques may also increase understory plant diversity, at least during the initial post-harvest recovery period.

Whitebark pine (Pinus albicaulis) is an endangered keystone tree species growing at the highest elevations in the mountain ranges of western North America. Across its range, whitebark pine is faced with a number of threats including fire suppression, mountain pine beetle, white pine blister rust, and climate change. Climate change is perhaps the greatest threat facing the species, yet it is the least understood. Most studies rely on model predictions and only look at the impacts on whitebark pine itself, not taking into consideration the other bird, mammal, and plant communities that are associated with it. In order to assess the potential effects of climate change on whitebark pine communities in the southern Coast Mountains of British Columbia, this thesis examined the vegetation associations and climate controls currently shaping the communities. My results showed that whitebark pine is growing in the open away from other subalpine tree species. This suggests that whitebark pine is not facilitating other subalpine tree species, contrary to what has been shown in the Rocky Mountains. Evidence of a distinct suite of understory vegetation associated with whitebark pine is weak and inconclusive. Differences in understory vegetation appear to be mainly due to site differences in climate, soils, and topography. Age distributions constructed from tree cores revealed that whitebark pine decline at lower elevation sites may be due to successional advancement to subalpine fir, and subalpine fir is currently encroaching into higher elevation sites. A dendrochronological assessment revealed that winter conditions, including snowpack, temperature, and the Aleutian Low Pressure Index (ALPI) were the most limiting to whitebark pine growth at high-elevation sites, but biotic factors including disease and competition appear to be more important than climate in determining annual ring growth at lower elevation sites. Bootstrapped correlations between annual ring widths and snowpack records showed that tree responses to fluctuating snowpack have changed over time. For most of the 20th century, low snowpack periods were associated with greater annual growth. Since around 1970, when the snowpack levels dropped below anything previously recorded for the area, annual tree growth has been reduced. It appears that these high elevation tree species require a balance between too much snow (shorter growing season) and too little snow (reduced protection from harsh winter conditions). Climate change models for the area predict drastically reduced snowpack in the coming decades. If snowpack continues to drop, as it has since 1970, it will likely lead to severe impacts on whitebark pine growth in the southern Coast Mountains. / Graduate / 0329 / carlsonkim@hotmail.com