Changes at treeline within the San Juan Mountains of Colorado

Abstract

Repeat photography is a field method to study landscape change over time, yet most studies use a single pair of photographs spanning upwards of a century or more to ascertain change. In this study, I used repeat photography to study vegetation change across high-elevation environments within the San Juan Mountains of southwestern Colorado on decadal time scales. At five photo sites, I compared present conditions to both a historical photograph (ca. 1875-1910) and one from 2002 to determine if climate-induced thresholds have impacted high-elevation forests. Results from this research suggest that (1) tree establishment is increasing at 80% of photo sites and (2) spruce beetle-induced mortality is evident at 60% of sites. To increase the temporal resolution of when the spruce beetle outbreak occurred, I used remote sensing change detection analysis for the periods 2003-2011 and 2011-2019., Given the level of change detected between 2011-2019, spruce beetle-induced mortality along upper treeline likely originated within the past eight years. Overall, results from repeat photography used in conjunction with remote sensing provide multiple lines of evidence that ecological change had a resulted from the crossing of a climate threshold over the past decade. Findings from this research suggest that hotter drought is already impacting high-elevation treeline environments in parts of the San Juan Mountains.