The purpose of this study is to predict the potential response of the forest-tundra ecotone in Rocky Mountain National Park, Colorado, to future climate change using data on historical episodes of establishment in patch forest openings of the forest-tundra ecotone. Forest vegetation in this ecotone consists primarily of coniferous forest dominated by Picea engelmannii and Abies lasiocarpa. It was hypothesized that recent tree seedling establishment in patch forest openings of the forest-tundra ecotone, not balanced by mortality, was triggered by a warm, but wet period following the end of the Little Ice Age about AD 1850. At four sampling locations distributed throughout the Park, increment cores and basal discs were used to determine dates of establishment among patch forest trees. Logistic regression and t-tests were used to study establishment dates in relation to historical climate records. Tree invasion in patch forest openings is episodic in nature, concentrated between 1951-64, and is not balanced by mortality, suggesting more than a short-term change in the ecotone. On the basis of the climate record, t-tests, and logistic models, it was concluded that both high temperatures and high snow depths must occur simultaneously for several years in order to generate climatic conditions suitable for tree establishment. The historic climate record indicates that a warmer and wetter period occurred during the 1950s and 1960s, but climate data are unavailable before 1880. According to proxy climate records, it appears that the regional climate of the southern Rocky Mountain region has been both warmer and wetter since the end of the Little Ice Age. These climatic conditions may be related to the tree invasion observed in patch forest openings of Rocky Mountain National Park.
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