Spruce and fir regeneration and climate in the forest-tundra ecotone of Rocky Mountain National Park, Colorado USA

As an ecological boundary zone, the forest-tundra ecotone is a valuable location to study the initial response of vegetation to global climate change. The purpose of this study is to predict the potential response of the forest-tundra ecotone in Rocky Mountain National Park to future climate change using data on historical episodes of establishment in patch forest openings of the forest-tundra ecotone. We hypothesized that recent 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 ca. A.D. 1850. At four sampling locations distributed throughout the Park, we determined dates of establishment among patch forest trees using increment cores and basal disks. We studied the relationship between establishment dates in relation to historical climate records using t-tests and logistic regression. In Rocky Mountian National Park, tree invasion in patch forest openings is episodic in nature, concentrated between 1951-1964, 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, we 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 (ca. A.D. 1350-1850). We conclude that these climatic conditions may be related to the tree invasion we observed in patch forest openings of Rocky Mountain National Park.