Running head : CLIMATE AND ECOPROVINCE FIRE AREA BURNED 1 2 CLIMATE AND WILDFIRE AREA BURNED IN WESTERN U

23 We demonstrate that wildfire area burned (WFAB) in the American West was significantly 24 controlled by climate (combinations of precipitation, temperature, and/or drought) during most 25 of the 20 century (1916-2003). Persistent, ecosystem-specific correlations between climate 26 variables and area burned are grouped by vegetation type for 16 ecoprovinces across the West. 27 Most mountainous ecoprovinces exhibit strong year-of-fire relationships with anomalously low 28 precipitation, low Palmer Drought Severity Index (PDSI), and high temperature. Many 29 grass/shrub dominated ecoprovinces have stronger positive relationships with antecedent 30 precipitation/PDSI. Some ecoprovinces appear sensitive to different climate variables between 31 1977-2003 than 1916-2003, potentially reflecting changes in the reporting areas included in 32 wildfire statistics. For 1977-2003, a few climate variables explain a substantial fraction of the 33 area burned (from 33 to 87 percent, mean 64 percent), indicating strong linkages between climate 34 and area burned. For 1916-2003, the relationships are weaker, but between 25 percent and 57 35 percent (mean 39 percent) of the total variability could be accounted for with climate. At 36 ecoprovince scales, seasonal climate is sufficient to explain a significant fraction of the variance 37 in WFAB, although the specific climate mechanisms vary with ecosystem vegetation in the 38 ecoprovinces. The relationship between the mean and the variance for WFAB repeatedly exhibits 39 a gamma distribution for independent data sets and spatial scales of fire data. We hypothesize 40 that the variance is commonly proportional to the mean squared for WFAB, and that the 41 mechanism leading to the gamma distribution is connected to the climate-vegetation interactions 42 that lead to fuel drying and production. The importance of antecedent climate (summer drought 43 in forested ecosystems and antecedent winter precipitation in shrub and grassland ecosystems) 44 Littell et al. Climate and Ecoprovince Fire Area Burned 3 indicates that the mechanism behind the observed fire-climate relationships is climatic 45 preconditioning of large areas of low fuel moisture. Despite the possible influence of fire 46 suppression, exclusion, and fuel treatment, WFAB is still substantially controlled by climate. The 47 implications for future planning and management in natural resource and ecosystem management 48 are that future WFAB will likely depend on ecosystem-specific, seasonal variation in climate. 49 The impacts of climate change on fire regimes will vary with the relative energy or water 50 limitations of ecosystems. 51 52