THE GEOLOGIC IMPACT OF WILDFIRES IN THE SAN JUAN MOUNTAINS, SOUTHWESTERN COLORADO

We propose that the geologic impact of wildfire across a landscape be considered in terms of both the history of fire in different ecological settings and the erosional response to fire in these settings. Dating of fire scars and tree ring analyses in the San Juan Mountains has defined both the character of fires and their recurrence intervals. Fire regimes prior to European settlement varied strikingly with elevation and forest type. Lower elevations, occupied primarily by Ponderosa Pine forests, experienced frequent (recurrence intervals of 10 to 20 years), low-severity fires. Higher elevations, occupied primarily by spruce-fir communities, experienced infrequent (recurrence intervals of 300 to 500 years), but high severity, stand-replacing fires. Open forest structures were maintained in the lower elevations, while higher elevations demonstrated a patch mosaic with stands of variable ages. The character of fire in the San Juans changed abruptly around 1880 with the advent of European settlement and concurrent introduction of grazing animals. In the lower and mid-elevations, grazing removed the fine herbaceous vegetation and impaired the spread of low intensity surface fires, leading to fuel accumulations. The exclusion of fire in these settings has resulted in unprecedented fuel loadings and increased stand densities, the consequences of which are just now being felt with the occurrence of extensive, high-severity fires in these settings.

The 2002 Missionary Ridge fire, which burned 29,184 hectares across a range of elevations, 31% of it at high severity, reflects these changing ecologic conditions. A broad spectrum of erosional responses, ranging from debris flow to sheetflow, has occurred in response to the first summer monsoon season. What we do not know is whether this erosional response is comparable to those in the past, or outside the historic range of variability. Deep incisions into deposits of valley-fill material throughout the burned area have exposed stratigraphies that reveal many past episodes of fire-related deposition. Dating and characterizing the processes for these fire-related events will better define the critical links between fire history and post-fire erosional response.