Rosemary Sherriff - Mapping evidence of historical & potential wildfire
A driving factor for fuels reduction is the belief that increases in woody biomass have resulted in a greater risk of severe wildfire. In this research, we compared spatial models of present-day wildfire potential under extreme (99th percentile) weather scenarios and historical fire severity in a >562,000 ha study area in the Colorado Front Range characterized by ponderosa pine and mixed conifer cover types. We then verified these models using fire severity from four large recent fires, and then compared the wildfire potential models to a spatial model of historical fire severity. The model of historical fire severity was based on data from 232 fire history-age structure sites in which topographic variables were used to predict the spatial pattern of historical fire severity across the study area. Under extreme weather conditions, approximately one-third of the study area exhibited the potential for low-severity (surface) fire and two-thirds showed the potential for mixed-severity (torching to crown) fire. The results indicate strong spatial overlap of reconstructed fire regimes and present-day wildfire potential: crown fire potential today is associated with higher-severity fire historically (>80% overlap). Our results indicate <20% of the area has shifted from an historical low-severity fire regime to present-day risk of higher severity under extreme weather conditions. The areas of greatest change are at low elevation, and along the plains-grassland ecotone, where there have been substantial changes related to grazing and fire-exclusion policies. As the temperatures under extreme conditions from ca. 1964-2007 are similar to 39th percentile conditions predicted by the IPCC for 2100, the extreme fuel conditions described in this study are likely to become average conditions in the future.