Alpine Treeline Changes in the Central Rocky Mountains: A Progress Report

Changes in the elevation, composition, and structure of alpine treeline can reflect climate variability and change, as well as patterns of disturbance which themselves may be mediated by climate variability. Information about past treelines may provide insight into the likely character of future changes. We have recently begun a new project to investigate changes in past treeline and determine the status of the current treeline in the central Rocky Mountains of Colorado, documenting the past and present treeline and their relationships to climate and fire using dendrochronological techniques. In summer 2007, we sampled three sites near Monarch Pass, Cottonwood Pass, and on Sheep Mountain, west of Fairplay, CO. The Monarch site shows evidence of a relict treeline of mixed Pinus flexilis and Pinus aristata above the current treeline, while the current treeline composition is predominantly Picea engelmannii. Dating, when completed, will reveal whether this relict treeline is evidence of a Medieval-era warm period in Colorado or an older period of warmth. At Sheep Mountain, remnant material above a stand of living Pinus flexilis and Pinus aristata trees of 800 years old or more was sampled. The remnant collections include samples with up to 700-1000 rings, and if they overlap in time with the living trees, will provide an extended chronology of climate variability, as well as information on the timing of tree establishment. At both of these sites, the presence of seedlings above the current tree line may be evidence of a rising treeline and warming temperatures, although this study may not be sufficient to confirm this. Evidence of fire at two of the three study sites may also shed light on the role of fire in shaping treeline in this region.