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 valuabl e location to study the initial response of vegetation to global clima te change. The purpose of this study is to predict the potential respo nse of the forest-tundra ecotone in Rocky Mountain National Park to fu ture climate change using data on historical episodes of establishment in patch forest openings of the forest-tundra ecotone. We hypothesize d that recent seedling establishment in patch forest openings of the f orest-tundra ecotone, not balanced by mortality, was triggered by a wa rm, but wet period following the end of the Little Ice Age ca. A.D. 18 50. At four sampling locations distributed throughout the Park, we det ermined dates of establishment among patch forest trees using incremen t cores and basal disks. We studied the relationship between establish ment dates in relation to historical climate records using t-tests and logistic regression. In Rocky Mountian National Park, tree invasion i n patch forest openings is episodic in nature, concentrated between 19 51-1964, and is not balanced by mortality, suggesting more than a shor t-term change in the ecotone. On the basis of the climate record, t-te sts, 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 occu rred during the 1950s and 1960s, but climate data are unavailable befo re 1880. According to proxy climate records, it appears that the regio nal 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 Nation al Park.