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The vascular plant diversity of the volcanic South San Juan Mountains of southern Colorado (Archuleta and Conejos counties) was inventoried through collection of 1151 voucher specimens during portions of the summers of 2013, 2014 and 2016. A total of 744 species in 84 families were documented in the region across these and historical collections, indicating that nearly one-third of the Colorado flora can be found in these mountains. The most speciose plant families of the study area are the Asteraceae, Poaceae, and Cyperaceae, as is typical of temperate northern hemisphere climates. The most dominant floristic component of the area comprises species with a Western North American distribution, though circumboreal, widespread North American, and Southern Rocky Mountains endemic species also comprise large portions of the flora. The South San Juan Wilderness itself harbors a nearly pristine pre-Columbian flora, though wilderness boundaries currently exclude 170 native species living in the South San Juan Mountains. Ninety-five collections are novel to the study area, and 39 of these represent new county vouchers for either Archuleta or Conejos. [ABSTRACT FROM AUTHOR]