Application of historical rephotography for quantifying post-Little Ice Age changes in timberline forests, northern Uinta Mountains, northeastern Utah

MUNROE, Jeffrey S., Geology Department, Middlebury College, Bicentennial Hall, Middlebury, VT 05753,

Six historical photographs acquired in 1870 near upper treeline in the northern Uinta Mountains were retaken in July 2001. The vantage points for these photographs were relocated as precisely as possible (within 5 m) by matching the alignment of prominent ridgeline features and foreground boulders. All photographs were printed at 8x10”, scanned at 1000 dpi, and analyzed digitally to quantify changes in the timberline forests. Three dramatic differences were noted. First, the historical photographs document a treeline (the uppermost elevation of upright trees) 60 to 180 m (mean of ~100 m) lower than at present. Given the modern July lapse rate of 6.9ºC/km, this difference corresponds to a mean July temperature of 0.4 to 1.3ºC (mean of 0.7ºC) lower than the modern value. Treeline depression on west-facing slopes was roughly twice that on slopes with northeast aspects. Second, timberline forests in all historical photographs were significantly (P < 0.01) less dense, with tree densities approximately half those measured in the modern photos. Third, in the two photo-pairs that record the distribution of trees on valley bottoms, the area of floodplain meadows decreased ~75% from 1870 to the present. Because the original photographs were taken within a few decades of the end of the Little Ice Age (LIA) in other western ranges (ca. A.D. 1850), it is assumed that differences in treeline elevation and timberline forest density documented in the repeat photographs represent the response of vegetation to post-LIA climate warming.