Fire-Southern Oscillation relations in the Southwestern United States (page 2 of 2)
Adapted from: Thomas W. Swetnam and Julio L. Betancourt. 1990. Fire-Southern Oscillation relations in the Southwestern United States. Science 249: 1017-1021.
To examine long-term relations between climate and fire in the southwestern United States, regional values for tree-ring growth from Douglas-fir, ponderosa pine, and pinyon pine growing at 28 sites in Arizona and New Mexico were compiled. These data explain at least 50% of the variance in water-year precipitation with the largest response in autumn and spring, the two seasons with the strongest teleconnections to the tropical Pacific. An independent fire scar index from ponderosa pine, southwestern white pine, and Douglas-fir growing at 15 sites throughout Arizona, New Mexico, west Texas, and northern Mexico was compiled. A regional fire scar index was computed by averaging percentages of trees scarred per year at each site. The record was terminated in 1905 because of the lack of fire scars in the 20th century. Episodic surface burns that injure but do not kill mature trees have dwindled with cessation of aboriginal fires, removal of fine fuels by livestock grazing, and a vigorous program of fire suppression.
An SO signal should be expected in the tree-growth chronologies because precipitation in the fall and spring before the growing season exerts the strongest influence on cambial growth in Douglas-fir and ponderosa pine. Negative correlations have been reported between tree growth in the southwestern United States and the SOI, that is, tree growth is enhanced during El Nino conditions. A SO signal should also be obtainable in the fire scar record if moisture conditions during spring are a primary factor in the synchroneity of regional fires. Archival evidence from Peru indicates that 8 of the 10 years that failed to produce any fire scars were El Nino events of strong or moderate intensity.
Similar results were obtained in a comparison of total area burned in National Forests of Arizona and New Mexico since 1905 with regional precipitation, DJF-SOI, and LIRI. A low signal-to-noise ratio was anticipated because the fire statistics include both lightning and person-caused fires across a wide range of vegetation types from grassland to boreal forests, each subject to different land use and management practices. However, total area burned closely tracks DJF-SOI and LIRI until the 1960s when area burned increased and became less variable, possibly because there was an increased number of person-caused fires or because fire suppression resulted in unusual accumulation of fuels. Spring (March through May) precipitation yielded the highest correlations of any season against both area burned and the Southern Oscillation.
Regional climate effects are implicit in the extreme variability of fire occurrence measured by both the fire scar record and fire statistics. In general, area burned was greatest during years with highly positive values of SOI, reduced rainfall in the Line Islands, and severe winter-spring droughts (1934, 1946, 1956, 1971, and 1974). Area burned was reduced after exceptionally wet springs of low-SO phases or El Nino years (1926, 1941, and 1958). Climatic effects are also evident in the general occurrence of narrow rings during years when more than one-fourth of the trees were scarred by fire(1716, 1748, 1785, 1837, 1847, 1851, and 1879). We have no basis for calibrating the fire scar index to area burned because the fire scar record is unreliable for the 20th century. Fire magnitudes in 1748, when more than 40% of the trees registered fire scars, happened under a different set of ecological circumstances than now exist. Such widespread fires, but of greater intensity, may become more probable as fuel accumulates with continued fire suppression, increasing the chances for rapid and pervasive ecological changes.
Fire suppression has been partly responsible for rapid conversion of grasslands to shrublands in Arizona and New Mexico. Heavy ecological impacts also can be expected in ponderosa pine forests, a widespread vegetation type in the Colorado River basin. Fire suppression has lead and could continue to lead to large-scale changes in forest structure and composition, as already evident in successional changes to more shade-tolerant trees since the turn of the century.
Even with changing vegetation dynamics due to human land use and management, the fire-SO linkage could have forecasting value and thus important implications for fire management. Both statistical and dynamical models are now being developed to predict the behavior of the SO, which leads Arizona and New Mexico weather by one or more seasons. Extensive fires in early summer of 1989 followed a dry winter and spring associated with an unusually cold episode in the tropical Pacific (La Nina), which might have been predicted from zonal wind and sea surface temperature anomalies in the tropical Pacific in fall 1988 or earlier. The fire-SO relation appears to be strongest during extreme phases of the SOI. Any skill in forecasting fire hazard, however, will be constrained to the 30 to 35% of the annual fire variance explained by indices of the Southern Oscillation.
Synchronous large fires in the Southwest over three centuries, and their association with the high-SO phase, deficient spring precipitation, and reduced tree growth, imply that seasonal climate, and not just fire weather, determines burning of vegetation on a subcontinental scale. If southwestern landscapes are to be regarded as a mosaic of patches recovering from disturbance, as specified in the current paradigm [29], then our analyses of these patches must match the spatial and temporal scales of fires as an ecological process.
Resources:
D'Arrigo, R. D. and Jacoby, G. C. 1991. A 1000-year record of winter precipitation from northwestern New Mexico, USA: A reconstruction from tree-rings and its relation to El Nino and the southern oscillation. The Holocene 1: 95-101.
Fritts, H. C. 1976. Tree rings and climate. Academic Press, New York, NY, 567 pp.
Meko, D., C., Stockton, W. and Boggess, W. R. 1995. The tree-ring record of severe sustained drought. Water Resources Bulletin 31.
Ropelewski, C. F. and Halpert, M. S. 1986. North American precipitation and temperature patterns associated with the El Niņo/Southern Oscillation (ENSO). Monthly Weather Review 114: 2352-2362.
Stahle, D. W. and Cleaveland, M. K. 1993. Southern oscillation extremes reconstructed from tree rings of the Sierra Madre Occidental and Southern Great Plains. Journal of Climate 6: 129-140.
Swetnam, T. W. 1990. Fire history and climate in the Southwestern United States. Pp. 6-17 In: Krammes, S. J., editor. Proceedings of Symposium on Effects on Fire in Management of Southwestern Natural Resources. General Technical Report RM-191. U.S. Forest Service.
Swetnam, T. W. and Baisan, C. H. 1996. Historical fire regime patterns in the Southwestern United States since AD 1700. Pp. 11-32 In: Allen, C. D., editor. Proceedings of the Second La Mesa Fire Symposium. General Technical Report RM-GTR-286. USDA Forest Service, Los Alamos, NM.
Swetnam, T. W. and Betancourt, J. L. 1990. Fire-Southern Oscillation relations in the southwestern United States. Science 249: 1017-1021.
Swetnam, T. W. and Betancourt, J. L. 1992. Temporal patterns of El Niņo/Southern Oscillation - wildfire teleconnections in the southwestern United States. Pp. 259-270 In: Diaz, H. F. and Markgraf, V., editors. Historical and Paleoclimatic Aspects of the Southern Oscillation. Cambridge University Press, New York, NY.