Climatic Variability and Tree Response within the Forest-Alpine Tundra Ecotone

Abstract

Abstract Relationships between climatic variables and tree-ring widths (1954–79) were determined for three sites in the forest-alpine tundra ecotone on Niwot Ridge of the Colorado Front Range. Correlations were tested using regression techniques and results are displayed by response surfaces. Climatic data from within the ecotone (1978–84) indicate the occurrence of large temperature gradients over short distances and frequent inversions. This finding demonstrates that frost-free seasons are shorter in the lower ecotone, although critical winter minima may be experienced in the upper ecotone. Regression techniques indicate that tree-ring widths are significantly and negatively correlated with autumn precipitation of the previous calendar year and significantly and positively correlated with the previous calendar year's autumn temperature and the actual year's late spring/early summer temperature. High winter temperatures as well as large amounts of snow in the early spring show negative correlations with tree-ring width. The temperature correlations were used to reconstruct past climate variability (mean previous-year autumn and actual-year late spring/early summer air temperatures) to the mid-1500s (the limit of the tree-ring chronology). From this reconstruction we conclude that the relevant air temperatures were lower than average in the mid-to-late 1500s, higher during the seventeenth century, variable around the average from 1690–1830, lower from 1830–95, higher from 1925–65, and lower from 1965–79. These results, indicating a close relationship between climatic change and tree-growth response, demand that upper timberline management strategies be developed to take into account natural fluctuations as well as possible human impacts on the forest-alpine tundra ecotone.