Severe and extensive wildfires occurring in the Southwest over the past decade appear to be unprecedented when compared with fire scar records for the region. These records show that fires spread as low-severity surface fires, burning in the understory of relatively open forests. Euro-American settlement near the end of the nineteenth century halted surface fires by introducing livestock grazing, which removed grasses and fine fuels necessary for fires to spread. This was followed by fire suppression policies, which further prevented fires from spreading in the forests. The past century of fire exclusion in ponderosa pine and mixed conifer forests of the Southwest has led to anomalously dense forest conditions in these ecosystems. Today, small trees in the understory act as ‘ladder’ fuels, which connect the forest floor with the canopy of the forest. When fires are started by lightning or abandoned campfires, they now burn uncontrollably through the canopy of the forests.
This study focused on a watershed burned by the Missionary Ridge Fire, which occurred in June 2002, in the San Juan Mountains of southwestern Colorado. Debris flows following the fire exposed a late-Holocene record of fire-related alluvial deposits in the walls of the incised channel. The exposed sediment record contains abundant charcoal within the deposits, providing an ideal opportunity to simultaneously evaluate fire-related sedimentation events and tree-ring records of fire history. The combination of these records yielded information specifically about high-intensity fires.
Each year, more than $1 billion is spent on wildfire suppression in the United States. In spite of this spending and much effort going into fuel treatments, millions of acres are burned each year by wildland fires. In order to meet this considerable challenge, and spurred on by the National Fire Plan and other national initiatives, managers are developing more proactive approaches to wildland fire management. Since 2000, there has been a growing acknowledgment that “a strong science foundation is key to managing the wildfire hazard and supporting management decisions in the most cost-effective and environmentally sensitive way. Supported by scientific knowledge, decision makers are better equipped to more reliably forecast or prevent damaging fires and to understand the consequences of the decisions for society and for forest and rangeland health” (USDA Forest Service, 2003). Interdisciplinary research on climate, ecology and human society shows that the interactions between these various elements shape the biological landscape of the Southwest and the occurrence of fire, a keystone process in the ecology of the Southwest. The ability of fire and land-use managers to manage wildland fire hazards depends critically on knowledge generated by scientific examination of climate-ecosystem-fire linkages. Knowledge of opportunities for introducing climate information into wildland fire decision making, as well as knowledge of impediments to introducing such information, is critical for national, regional, and local wildland fire specialists.
The CLIMAS fire initiative fosters research on the nature, causes, and consequences climate change and variability on fire in the southwestern United States. These include efforts to improve communication between climate scientists and land managers. These efforts help fire and land-use managers use climate forecasts and historical climate fire management in the region and improve the ability of climatologists and fire specialists to collaboratively predict fire potential before the fire season begins. CLIMAS fire-climate research builds upon prior and continuing work being carried out University of Arizona and other institutions in the Southwest, including Northern Arizona University and New Mexico State University. CLIMAS research draws upon the considerable expertise of researchers at The University of Arizona’s Laboratory of Tree-Ring Research historical and paleo-reconstruction of climate-fire connections, as well as the expertise the Desert Research Institute (Program for Climate, Ecosystem and Fire Applications) Scripps Institution of Oceanography (California Nevada Applications Program) with regard climate patterns and impacts over short to very long time periods. A unique avenue of CLIMAS research currently underway is comparative analysis on institutional and decision structures and processes in wildfire management.
Scientists use multiple methods to study fire history, in order to assess fire and climate relationships over a range of time scales. With tree-ring records, scientists can document the timing, severity and relative extensiveness of past fires, yet these records are limited in length. While these records have annual precision, they usually only cover the period from 1600 to the present. Evidence from lake sediments and alluvial fan deposits can provide millennial length records of fire history, which may be correlated with records of climate variability. These longer records of fire history allow scientists to evaluate fire and climate relationships throughout the Holocene period (from 10,000 years ago to present), which broadens our knowledge of the range of variability in fire regimes in southwestern forests.
This study focuses on a watershed burned by the Missionary Ridge Fire, which occurred in June 2002, in the San Juan Mountains of southwestern Colorado. Debris flows following the fire exposed a late-Holocene record of fire-related alluvial deposits in the walls of the incised channel. The exposed sediment record contains abundant charcoal within the deposits, providing an ideal opportunity to simultaneously evaluate fire-related sedimentation events and tree-ring records of fire history. The combination of these records yields information specifically about high-intensity fires.
This study was limited to a single watershed, which precluded interpreting regional fire and climate associations from the alluvial fan stratigraphy. However, the data was reviewed with respect to other regional fire history records. In addition, the tree-ring record assisted in interpreting the fine-grained sediment deposits, by defining the fire regime in the watershed over the past 300–400 years.
Tree-ring fire scar records from the watershed show evidence of frequent surface fires over most of the past 400 years, followed by virtual cessation of widespread fire since 1879. The fires appear to be more frequent and patchy during the late 1600s and 1700s, followed by a shift to more synchronous and widespread fires in the 1800s. This pattern of fire occurrence is common to southwestern fire history studies of this forest type and elevation range, including sites in southern Arizona and New Mexico.
It is difficult to link fires observed in the tree-ring record with individual alluvial deposits observed in the exposed channel. This is partly because the alluvial deposits have been dated with radiocarbon dating, which is difficult to calibrate with calendar years over the past 500 years B.P. However, general patterns of the two records show the following:
The combined data provide a context for comparing past fires with the recent Missionary Ridge Fire, which burned a large proportion of the watershed at high severity burn conditions. The geomorphic response of the Missionary Ridge Fire included several debris flow and flood events, which completely incised the older channel deposits. This geomorphic response sharply contrasts the period of fine-grained deposition, which characterizes the past 2,000 years.
Westerling, A., H. Hidalgo, D. Cayan, and T. Swetnam. 2006. Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity. Science 313(5789):940-943.
USDA Forest Service. 2003. National Fire Plan Research & Development: 2002 Business Summary. United States Department of Agriculture Forest Service Miscellaneous Publication 1588.