The influence of spatial scale on detecting climatic controls of wildfire in subalpine forests for the last 2000 years in northern Colorado

Background/Question/Methods

The frequency and extent of wildfire in subalpine forests is primarily controlled by drought.  Historical records show that years with the largest fires across a region occur almost exclusively during years of moderate to extreme drought stress.  Most of the research examining this linkage between climate and fire in subalpine forests has examined fire records extending through the past 400-500 years. Work examining fires at millennial scales found little evidence of a climatic influence on fires but hypothesized that increased number of fire records may detect a climatic effect on fire.  We reconstructed fire records from lake sediments for the past 2000 years from >10 lakes across the Mt Zirkel Wilderness, Colorado.  This period has contrasting climate histories.  Between 2000-1000 BP this region experienced severe, mutli-century droughts; from 1000 BP – today there has been increased moisture with milder, shorter droughts. It is hypothesized that our high spatial resolution of fire records will allow us to detect climatic controls on fire across the differing climatic periods of the last two millennia. 

Results/Conclusions

Our preliminary results show a small increase of fire events from 2000-1000 BP when there was increased drought severity.  Combined records of charcoal influx suggest a slight decrease in regional burning at approximately 900 BP that persisted to the present.  While there is evidence of changes in fire frequency and burning over the past 2000 years the differences are smaller than expected given the large differences in climate history. This suggests that millennial and century scale climate processes have had little influence on burning in subalpine systems.  This also suggests that at the spatial scale of our study (approximately 800 km²) either vegetative influences or stochastic processes, such as the distribution of ignitions, have heavily influenced the spatial and temporal patterns of fire.  These results are preliminary and will be updated as more fire reconstructions are completed within the study area.