New Report: Treeline Monitoring in the San Juan Basin Tundra

Mountain ecosystems, particularly alpine, are among the most vulnerable to climate change. Warming conditions are likely to cause trees and shrubs to encroach into alpine tundra habitats, which are home to many species found nowhere else.

CHNP, in cooperation with the Bureau of Land Management, U.S. Forest Service and Colorado College, began a pilot project to monitor treeline expansion in the San Juan mountains. A combination of fieldwork and spatial analysis was used to determine if the treeline on the southwest slope of Kendall Mountain had changed in the last 50 years.

Aerial imagery from 1951 and 2011 were used to establish transects perpendicular to the treeline to detect growth of new trees upslope, which would indicate an upward advancement of trees into alpine areas. The location of trees along these transects were then field verified, and a subset of trees were cored to determine their age. Additionally, the density of trees below the treeline was calculated for both time periods.

Recruitment above treeline

Results show that the treeline has not advanced significantly in the last 50 years. However, there are signs of upslope recruitment (trees between 20-50 years old above the current treeline, but not in sufficient density to form a new treeline). Should this area of new recruitment continue to fill in and become the new treeline, it will be 160 feet above the existing line. Additionally, tree density below the current treeline has increased by approximately 12% in the last 50 years. All tree cores taken demonstrated an increase in growth rate since 1996.

For more information, see the complete report. We hope to expand this pilot project to study more treelines throughout the San Juan mountains, to determine if these increases in growth rate and tree density are consistent trends that can be correlated with measured increases in average summer temperatures.