Temporal and spatial patterns at alpine treeline in the Sierra Nevada USA : implications for global change
Bunn, Andrew Godard.
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This work focuses on developing understanding of the role of climate variability in shaping montane landscapes. Due to the temperature-related stresses on treeline populations it is thought that change at treeline is an indicator of global warming. Interpretation of treeline changes has been hampered by issues of scale and the paucity of landscape-scale data. The application of remotely sensed imagery and computer-based mapping programs has filled this gap with datasets that have large extents and fine spatial grain. I used geospatial information about treeline in concert with population and paleoecological data to answer questions on ecological patterns and processes. My research focused on the treeline tree communities in the southern Sierra Nevada. There, on the eastern crest of the Sierra Nevada foxtail pine (Pinus balfouriana) forms an abrupt treeline in the vicinity of Sequoia National Park which gives way to a less well defined treeline of whitebark pine (P. albicaulis), limber pine (P. flexilis), and lodgepole pine (P. contorta) north to Yosemite National Park. By analyzing treeline growth patterns and spatial composition at the level of the region (109 m2), stand (103 m2), and organism (10-1 m2) I was able to make inference about the interactions of autecology, biophysical setting, and climate variability in shaping these subalpine forests over the last millennium. At the regional scale I improved paleoclimatic understanding derived from long chronologies of tree rings by relating species-specific differences in the climategrowth relationship over time to the realized niche space of each species in geographic space. At the level of the stand I was able to show how decadal versus centennial modes of growth and stand density vary with biophysical setting related to drought stress. At the level of the organism I was able to show that differences in tree growth and seedling patterns are related to fine scale physical variations in surrounding each tree as well as tree autecology. Over the course of my dissertation, I developed fresh insights into the complex interactions that govern the growth and structure of forests and improved the state-of-science for monitoring treeline as a critical indicator of global change.