A century of vegetation change in the San Juan Mountains, Colorado: An analysis using repeat photography

Repeat photography samples were used to analyze how the structure and site-specific distribution of forests may or may not have changed during the past century in the San Juan Mountains, Colorado. Systematic evaluation of 146 photosets was combined with field observations to identify trends in vegetation change. Both conifers and deciduous trees (e.g., aspen) have increased in extent. Forest recovery from large disturbances that occurred during Euro-American settlement contributed substantially to this increase. Trees also encroached into grass/shrublands, but less than half of photosets show tree invasion, and invasion is more common in small grass/shrubland openings interspersed with forest than in large openings. Establishment of conifers into willow wetlands was common. Photosets show aspen regenerating along boundary edges of both self-replacing stands and seral stands, and persistence in many stands even in the presence of conifers. A visible trend towards the loss of native bunchgrasses and increases in less palatable species is documented in the photosets, and is a testimony to the ecological impacts of livestock grazing.