Spatial Variability of Radial Growth in the Forest-Tundra Ecotone of Rocky Mountain National Park, Colorado

Abstract

HESSL, A. E., P. J. WEISBERG3 AND W L. BAKER (Department of Geography and Recreation, University of Wyoming, Laramie, WY 82071). Spatial variability of radial growth in the forest-tundra ecotone of Rocky Mountain National Park, Colorado. Bull. Torrey ecotone of Rocky Mountain National Park, Colorado. Bull. Torrey Bot. Club 123: 206-212. 1996.-Radial growth in trees has often been used to measure the relationship between climate and tree growth. Variation in radial growth with respect to environmental variables may indicate how trees in diverse landscapes may respond to climate change. The purpose of this study is to determine whether radial growth is spatially heterogeneous throughout the forest-tundra ecotone of Rocky Mountain National Park and whether this spatial variability can be related to environmental variables. Increment cores were taken from 5-10 of the largest trees in 59 sampling locations distributed across the forest-tundra ecotone of Rocky Mountain National Park. Mean annual ring width for 1978-1987 was measured in the laboratory. Stepwise weighted least squares linear regression was used to relate mean annual growth to 25 environmental variables measured at each sampling location. Mean radial growth is higher at sampling locations with higher soil pH values and where there is small rock material but varies with zone and associated shrub species. When all statistically significant variables are considered, radial growth is slowest in the krummholz zone, intermediate in the patch forest zone, and fastest in the closed forest zone. Within each zone, the presence of Juniperus communis indicates slow radial growth, Vaccinium spp. indicate intermediate radial growth, and Salix spp. indicate rapid radial growth. These results differ from previous studies of vertical seedling leader growth in the FTE, which found higher rates of growth in the krummholz zone. Differing rates of radial and vertical growth may reflect different settings or different responses, though both may indicate tree success.