Repeat photography at Montgomery Pass: 40 years of change

As Colorado scientists concentrate more on the potential effects of climate change, CNHP ecologists are wondering if there is any evidence that treeline is moving up into the previously treeless alpine zone. One limiting factor for the upper elevation treeline is summer temperatures. As summer temperatures increase we can expect to see trees colonizing the alpine zone. Approximately 3% of Colorado is currently considered the alpine zone. Documenting changes to this ecosystem is an important part of understanding the impacts to the native fauna and flora under changing climate.

CNHP’s Renée Rondeau recently used repeat photography to document a change in tree density over the past 40 years, at Montgomery Pass (near Cameron Pass) in Larimer County. The 1972 photos were taken in the Montgomery Pass area by Gordon Rodda. In September 2012, Renée, with volunteers Jennifer Kathol, Maureen DeCoursey and Anne Taylor took new photos from the same location. (click to see larger versions).
 

Looking down from the alpine to treeline. The hills in the distance are the slopes of Sawmill – the dramatic increase in tree density in that area is probably due to regrowth after  logging in the early 1900s. The higher tree density in the mid-ground, where the circled area has filled in with trees, is not related to past logging.  Many of the mature trees in 2012 appear dead, probably from spruce-bud worm or mountain pine beetle kill.
 

View of Clarks Peak. New trees have appeared at edge of treeline and subalpine tree density has increased.

Although the repeat photos do not indicate that that treeline has moved upslope in a systematic way, it appears that tree density in the subalpine and near treeline has increased. It is typical to see a few scattered trees growing some distance above the actual treeline. The increasing tree density at treeline may indicate an infill mechanism by which treeline moves upslope in incremental stages rather than as a solid advancing front. We don’t yet know if the observed changes are tied to warmer temperatures it is important to keep documenting these trends.

View of Nokhu. The bolting of wind stunted trees in the alpine (krummholz) could be another indicator of warming temperatures. In the righthand photo you can spot a couple of krummholz trees that have changed since 1972.

If you have any old (40 years or older) photographs of treeline areas in Colorado, and would be interested in sharing them with us, please contact Renée Rondeau.