Extending the duration and dendroclimatic potential of mountain hemlock (Tsuga mertensiana) tree-ring chronologies in the southern British Columbia Coast Mountains
Date
2011-12-22
Authors
Pitman, Kara Jane
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Abstract
Tree-ring records collected from living mountain hemlock trees in the southern British Columbia Coast Mountains have been used to provide insights into the character of historical climatic fluctuations and the behaviour of individual climate forcing mechanisms. The relatively short-duration of these records limits, however, their ability to describe climate variability and atmospheric processes that change gradually or undergo long-term regime shifts. The objectives of this research were to extend the duration and quality of proxy climate information extracted from mountain hemlock tree-ring chronologies.
In coastal British Columbia most existing mountain hemlock tree-ring chronologies extend from ca. AD 1600 to present. To extend the duration of these chronologies, coarse woody debris recovered from the bottom of M Gurr Lake, a high-elevation lake in the vicinity of Bella Coola, British Columbia, was cross-dated to nearby living chronologies surrounding M Gurr lake and increment core samples of ancient trees at Mt Cain on northern Vancouver Island. From this, a regional continuous 917-year long record of radial growth was constructed. The resulting regional chronology was used to construct a 785 year-long proxy record of gridded air temperature anomalies displaying periods of cooler and warmer than average regional air temperatures that contained century-long low frequency trends. Cross-dating and tree morphological evidence of snow avalanche activity displayed within living trees surrounding the lake, and within the coarse woody debris, revealed that low-magnitude avalanches occurred in the winter months of AD 1713-1714, 1764-1765, 1792-1793, 1914-1915, 1925-1926, and 1940-1941. High magnitude avalanche events occurred in the winter months of AD 1502-1502 and 1868-1869.
A second objective of the thesis was to investigate the radial growth response of mountain hemlock trees to subseasonal climate variables using standardized ring-width and densitometric analyses. Mountain hemlock chronologies from M Gurr Lake, Cyprus Provincial Park, and Mount Arrowsmith were used to describe the inherent climate-growth trends. Maximum annual tree-ring density values provided a robust data series for constructing site-specific proxy records of late-summer temperature. Annual tree-ring width measurements provided independent proxies of spring snowpack trends. Regionally-derived proxy models indicated that intervals of cooler-than-average and higher-than-average air temperatures correspond to years of higher-than-average average and cooler-than-average snowpacks, respectively. Of note were the significant decreases in air temperature and increases in snowpack depths during the early-1700s and early-1800s coinciding with documented glacier advances in the Coast Mountains. Identification of these subseasonal climate signals within the tree-rings of mountain hemlock trees demonstrates the value of incorporating investigations of multiple tree-ring parameters.
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Keywords
Dendrochronology, Dendroclimatology, British Columbia Coast Mountains