TREELINE CHANGES ALONG THE CONTINENTAL DIVIDE IN COLORADO DURING THE LATE HOLOCENE

Recent investigations along the Continental Divide in Colorado have indentified several sites containing evidence of higher treeline during the late Holocene. This evidence consists of bristlecone pine remnants as much as 30 m above the present-day treeline. Most of the sampled remnants are lying on the ground surface, although several are still standing. All remnants are highly eroded, such that bark and much of the wood, on the western (windward) side of the tree was removed by high winds carrying ice particles during winter storms or sand particles at other times of the year while the tree was standing. In almost all cases erosion is so extensive that the pith area has been removed.

Near the town of Alma, in the Mosquito Range, remnants of bristlecone pines were found at altitudes as high as 3,675 m. Radiocarbon ages of the innermost remaining annual rings from six of these remnants ranged from 2,225±25 to 1,255±30 yr BP. A radiocarbon age of 2,015±30 yr BP was obtained from the pith of a standing remnant.

In the Boreas Pass area, in the Colorado Front Range, remnants of bristlecone pines were also found lying on the ground at altitudes approximately 30 m above present-day treeline. Again, radiocarbon ages of the innermost remaining annual rings from six of these remnants ranged from 2,430±35 to 1,165±25 yr BP. A radiocarbon age of 2,245±30 yr BP was obtained from the innermost remaining annual rings of the highest remnant at an altitude of 3,665 m.

The radiocarbon ages combined with a count of the remaining annual rings indicate these remnants were growing above the present-day treeline prior to 2,400 cal yrs BP to as late as 800 cal yrs BP and indicate a period of a slightly warmer growing season that enabled treeline to reach a higher elevation than that indicated by present-day full sized trees. In addition, it was noted that at the site near Alma bristlecone pine saplings are colonizing the areas above the present-day timberline as defined by older, mature trees and have reached as high as the highest remnants.