Treeline monitoring in the San Juan Mountains

Abstract

Temperatures in the San Juan Mountain region have risen approximately 1.8°F over the last 30 years, primarily after 1990, and are projected to continue warming. As temperatures rise we expect increased rates of tree growth and tree establishment at the subalpine/alpine ecotone (treeline). We wanted to discern if upper treeline changes could already be detected through remote sensing. We compared aerial photographs from 1951 and 2011 for 8 San Juan mountain peaks. The images were georeferenced and virtual transects were created to help establish position of treeline in each sample year. We found that the treeline has not moved, but that tree density has increased. Therefore, the difference between 1951 and 2011 treeline was calculated by determining differences in tree density within the area delimited as treeline. Differences in shadows between images were corrected for by examining shadows of immutable objects and calculating a correction factor. Detected differences varied widely, from 2 - 27% increase in tree density (mean 12%) over the last 60 years. We conclude that treeline changes can be detected, although the rate of change is slow and variable. The high variability may be due to aspect, with the wetter aspects increasing faster. We also suggest that this cost-effective remote sensing technique could be a useful monitoring tool for determining landscape changes in areas that are hard to access.