CLIMATIC INFLUENCES ON THE GROWTH OF SUB-ALPINE TREES IN THE COLORADO FRONT RANGE

We examined variations in tree growth responses to climatic variations among different tree species and habitat types in the subalpine zone of the Colorado Front Range. We constructed 25 tree ring site chronologies (11 of Picea engelmannii, 9 of Abies lasiocarpa, 4 of Pinus contorta var. latifolia, and 1 of Pinus flexilis) from a series of subalpine habitats ranging from xeric to wet. To establish tree growth responses to climatic variation, we used correlation and response function analyses to compare variations in ring widths with monthly temperature and precipitation records. At the driest sites, growth of Picea and Abies tracked climatic variation similarly. At mesic and wet sites, however, these species differed in their responses to climatic variation. The responses of Pinus contorta, sampled over a narrower range of habitat types, differed from those of Picea and Abies but did not differ among sites. Steep environmental gradients in the subalpine zone of the Front Range accounted for most of the observed differences in growth responses to climatic variation. Even at adjacent sites that differ only slightly in topographic position, tree growth responses to climatic variation were distinct. Interspecific differences in response to climatic variations generally were less important than site differences. Intersite differences in tree growth responses to climatic variation can be used as indicators of environmental differences among subalpine habitats.