Abstract
Rates of spruce (Picea engelmannii) and fir (Abies lasicocarpa) tree—island migration were estimated by radiocarbon dating the trails of dead wood left by three krummholz tree islands as they moved downwind across the forest—tundra ecotone on Niwot Ridge, Colorado Front Range. The leeward ends of the tree islands have expanded eastward at average rates of 1.5 to <2.6 cm/yr due to rooting of low horizontal branches; the windward ends of the tree islands have receded eastward at average rates of 0.9 to 1.9 cm/yr due to needle death by freezing or desiccation. The trees are in equilibrium with their environments, or are becoming elongated. Long—term movement rates determined by radiocarbon dating are lower than short—term rates estimated from annual measurements of leeward shoot growth. Changes in the direction of tree—island migration with time show no consistent trends that can be used to reconstruct past changes in winter wind direction. Errors in archaeological radiocarbon dates resulting from prehistoric use of old tree—island wood for fuel are unlikely to exceed 100 yr in the Front Range timberline environment.
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