Treeline dynamics on Pikes Peak, Colorado: Is the treeline moving and what is controlling the rate of movement

Background/Question/Methods

Past studies have shown that alpine treeline location is dependent on growing season temperatures. Warmer growing season temperatures should shift the treeline further up in elevation. The goal of this study is to first confirm that the treeline is moving up in elevation on Pikes Peak, Colorado, and second to examine what factors are affecting the rate of movement. Factors analyzed as possibly affecting treeline movement are aspect, land use change, such as logging and forest fires, slope, physiography, and original treeline location. Engelmann Spruce (Picea engelmannii) trees located on the western slope were cored in six 5-meter wide transects beginning above the treeline, in the transition zone, and in the closed-in forest. Age and growth rates were calculated using standard dendrochronological procedure. Historical aerial photographs for seven western slope sites and two eastern slope sites from 1938, 1954, 1978, 1983, 1990, and 1999 were georeferenced to a 2009 aerial using GIS. Areas of interest were classified using neighborhood statistics into “tree” and “tundra”, creating a “treeline” between the two categories. Stationary transects were drawn over the treeline in each photograph and the elevation where the treeline intersected the stationary transect was recorded.

Results/Conclusions

The Pikes Peak treeline is migrating up in elevation at all 9 aerial sites. The treeline moved up in elevation fastest from 1999-2009 (F=9.919 df=5,365 p<0.0005), during a decade of high temperatures. Western slopes had faster movement in treeline from 1954-2009 (t=-2.689 df=64 p=0.009), possibly because of higher temperatures and rainfall. Differences in physiographical features on the western slope resulted in different movement rates (F=6.688 df=2,65 p=0.002) – treelines on convex landscapes moved upwards faster than concave and flat landscapes, perhaps due to faster snowmelt. Treelines located at a lower elevation in 1938 moved upwards slower (Rsq=0.068 n=61 p=0.043), possibly because the factors limiting tree growth in these areas in 1938 are still limiting growth today. Land use changes and slope were found to not have a significant impact on movement rates. Dendrochronological results showed younger trees located around treeline, approximately 5-20 years old, and trees of mixed ages lower in elevation. Trees within the 1938-2009 treeline and above the 2009 treeline showed an increase in standardized growth from 1990-present while trees below the 1938 treeline did not show the same increase in growth, possibly because competition in the closed-in forest is a stronger limiting factor than temperature.