Stand Replacing Fire History and Aspen Ecology in the Upper Rio Grande Basin

Dendroecological techniques were applied to reconstruct stand-replacing fire history in mixed conifer and spruce-fir forests in northern New Mexico and southern Colorado. Stand-replacing fire dates with annual accuracy and precision were determined using four lines of evidence for each of twelve sites within a 75,000 square kilometer area. The four lines of evidence were: (1) aspen inner-ring dates, (2) conifer death dates, (3) tree-ring width changes, or other morphological indicators of injury, and (4) fire scars. The annual precision of dating allowed the identification of significant synchrony of stand replacing fires among the 12 sites and regional surface fire events previously reconstructed from the large network of fire scar collections in the Southwest. Nearly all of these synchronous stand-replacing and surface fire years coincided with extreme droughts. This suggests that stand-replacing fire activity occurred primarily when drought conditions allowed fires to ignite and spread within these high elevation forests and/or for the spread of surface fires between lower and upper elevations. Fifty percent of reconstructed stand-replacing fires pre-dated large-scale Euro-American settlement in this region. This may suggest that land use practices (such as logging and mining) were not as important in promoting stand-replacing fires in these study sites, as compared with other areas in Colorado.