Magnificence in motion: climate and forest composition in Glacier National Park
Date
2012
Authors
Urza, Alexandra, author
Sibold, Jason, advisor
Rocca, Monique, committee member
Binkley, Daniel, committee member
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Abstract
Worldwide climate is changing, resulting in inevitable shifts in ecosystems. Climate directly influences forest composition through effects on mortality, recruitment, and growth. Furthermore, disturbance can short-circuit the process of gradual species overturn in response to climate change. After a disturbance, current climate conditions may favor the establishment of species that were not historically dominant, accelerating ecosystem shifts. Whereas mature trees are generally resistant to minor environmental fluctuations, regeneration niches are often extremely sensitive to climate. While several studies have modeled climate envelopes associated with current species distributions, the climate conditions required for seedling establishment and growth may be more valuable indicators of potential species range shifts in response to climate change in disturbance-prone ecosystems. In this study, I examine the influence of post-fire climate on competition between two tree species in Glacier National Park (GNP). Western larch (Larix occidentalis) and lodgepole pine (Pinus contorta var. latifolia) are fire-adapted species that cohabit the subalpine zone of western GNP. Both species are shade intolerant, and extensive density-dependent mortality during canopy recruitment means that the seedlings that grow the fastest will be most likely to survive to the forest canopy. In Chapter One, I evaluate the use of terminal bud-scar counts for aging seedlings and measuring annual growth increments for four conifer species in GNP, including western larch and lodgepole pine. This analysis shows that terminal bud scars are reliably identifiable indicators of the location on the stem at which one year of growth ends and the next begins. Chapter Two examines the role of differential responses to site and climate in the coexistence of lodgepole pine and western larch in GNP. I used vertical growth measurements (distance between successive terminal bud scars) to develop Bayesian hierarchical models of seedling growth for each species. The results show that differential responses to environmental variability have historically promoted coexistence between these species. However, climate changes that increase disturbance frequency and move outside of the regeneration niches for these species may lead to a destabilization of their coexistence.
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Subject
climate change
disturbance
ecosystem adaptation
forest ecology
plant community
species distribution