Wildfires are a fundamental ecological process. Sophisticated models have been developed to evaluate the influence of daily weather on fire behavior but the role of seasonal or longer term climate is less certain. In ecological terms, a close linkage between fire and climate could diminish the importance of local processes, such as competition and predation, in the long-term dynamics of fire-prone ecosystems. The structure and diversity of such communities, which are regulated by fire frequency, extent, and intensity, may have nonequilibrial properties associated with variations in global climate. Successful prediction of vegetation change hinges on a better understanding of climatically driven disturbance regimes and the relative contributions of regional versus local processes to community dynamics.
The southwestern United States is an ideal area for assessment of regional fire-climate patterns. Detailed meteorological records and fire statistics are available for extensive areas, and centuries-long climate and fire history proxies have been obtained from tree rings at many sites. Forests in this region lead the nation in average number of lightning fires and area burned by these fires each year. This vigorous fire regime ensues from an annual cycle of a variably wet cool season, a normally arid foresummer, and isolated lightning storms ushering the onset of the summer monsoonal rains. Lightning fires begin in the spring and peak in late June to early July and decrease significantly as the summer rainy season progresses. Interannual variations in fire activity probably derive from the influence of winter-spring precipitation on the accumulation and moisture content of the fuels. Annual ring growth in southwestern conifers is primarily a function of cool season moisture. Local surface burns are also recorded as fire scars in tree rings. Thus, tree-ring analysis allows simultaneous evaluation of the linkage between fire and climate.
During the 1982-1983 El Nino episode, arguably the most severe of this century, National Forests in the United States sustained little fire activity while millions of hectares burned in Indonesia and Australia. Subsequently, a nationwide survey suggested that the relation between wildland fires and the El Nino--Southern Oscillation (ENSO) phenomenon is statistically significant only in the southeastern United States. However, this analysis relied on only 57 years of fire statistics and focused entirely on warm episodes in the tropical Pacific. In this report both warm (El Nino) and cold (La Nina) episodes in a 300-year record of fire activity for the southwestern United States are evaluated.
Teleconnections with the tropical Pacific are indicated by correlations between the Southern Oscillation index (SOI) and rainfall over the Line Islands (LIRI) against precipitation, streamflow, and tree growth in the American Southwest. During the high-SO phase (La Nina), when sea surface pressure is higher than normal in the Southeast Pacific, the central Pacific cools anomalously and the Intertropical convergence zone (ITCZ) and South Pacific convergence zone (SPCZ) diverge on either side of the equator, the latter bringing abundant rains to Indonesia and eastern Australia. During the low-SO phase, when sea surface pressure is lower than normal over Tahiti, the central Pacific warms, the ITCZ and SPCZ converge on the equator, and the zone of deep convection shifts eastward to the Line Islands in the central Pacific, where tropospheric disturbances then propagate to extratropical regions. Northern winter (December to February, DJF) values of SOI are preferred for studying teleconnections because this is the season when the maximum pressure anomalies occur; precipitation surges or deficiencies over the Line Islands are most persistent from August through February. During the low-SO phase (abundant rainfall over the Line Islands), warm waters in the eastern Pacific provide the necessary energy for development of west coast troughs and weaken the tradewind inversion. This situation enhances interaction between tropical and temperate weather systems, and thus more moist air penetrates into the southwestern United States during fall and spring.