Climatic Variability, Fire Regimes and Carbon Dynamics in Dry Forest Ecosystems of the Western US

Historical variability of fire regimes in the western Americas is associated with climatic phenomena such as ENSO. We describe the relationship between fire occurrence and interannual to decadal climatic variability (Palmer Drought Severity Index [PDSI], El Niño/Southern Oscillation [ENSO] and the Pacific Decadal Oscillation [PDO]) and explain how land use changes in the 20th century affected these relationships. Ongoing research uses these past changes to understand the influence of fire regimes on the regional carbon balance. We used 1701 fire-scarred trees collected in five study sites in central and eastern Washington to investigate current year, lagged, and low frequency relationships between composite fire histories and PDSI, PDO, and ENSO (using the Southern Oscillation Index [SOI] as a measure of ENSO variability) using superposed epoch analysis and cross-spectral analysis. Fires tended to occur during dry summers and during the positive phase of the PDO. Cross-spectral analysis indicates that percentage of trees scarred by fire and the PDO are spectrally coherent at 47 years, the approximate cycle of the PDO. Similarly, percentage scarred and ENSO are spectrally coherent at 6 years, the approximate cycle of ENSO. However other results suggest that ENSO was only a weak driver of fire occurrence in the past three centuries. While drought and fire appear to be tightly linked between 1700-1900, the relationship between drought and fire occurrence was disrupted during the 20th century as a result of land use changes. We suggest that long-term fire planning using the PDO may be possible in the PNW, potentially allowing decadal-scale management of fire regimes, prescribed fire and potentially, carbon emissions. Future work will quantify the changes in carbon emissions associated with a return to natural fire regimes and compares them with estimates of carbon emissions associated with current management. To model past, present and future emissions from fire in Washington's dry forest system we will use: (1) our network of fire-history records for the Pacific Northwest, (2) a fuelbed classification system (FCCs), (3) state-of-the-art computer models for predicting biomass consumption (CONSUME and EPM), and (4) an emission/dispersion modeling framework (BlueSky) to build an integrated model for spatially explicit estimates to guide fire management and policy.