Controls on Geyser Periodicity

(Carnegie Institution of Washington, Washington, DC, 1919); P. M. Brown, M. K. Hughes, C. H. Baisan, T. W. Swetnam, A. C. Caprio, Tree-Ring Bull., in press]. 11. R. J. Hartesveldt, Nat. Hist. 73, 12 (1964); N. L. Stephenson, D. J. Parsons, T. W. Swetnam, in Proceedings-17th Tall Timbers Fire Ecology Conference, Tallahassee, FL, 18 to 21 May 1989 (Tall Timbers Research Station, Tallahassee, FL, 1991), pp. 321-327; L. Mutch and T. W. Swetnam, in Proceedings of Symposium on Fire in Wilderness and Park Management, Missoula, Montana, J. Brown, Tech. Coord., Missoula, MT, 30 March to 1 April 1993 (U.S. Forest Service, Ogden, UT, in press). 12. J. L. Vankat, Assoc. Am. Geogr. Ann. 67, 17 (1977); B. M. Kilgore and D. Taylor, Ecology 60, 129 (1979); H. T. Lewis, "Patterns of Indian Burning in California: Ecology and Ethnohistory," Ballena Press Anthropol. Pap. No. 1 (Ballena Press, Ramona, CA, 1973). 13. The joint probabilities were the products of the individual probabilities of fire occurrence within each grove, estimated from the observed centennial fire frequencies (annual fire probability equals the number of fires per century divided by 100). This approach is a simplification, because the true fire probabilities were not stationary through time. In addition to the influence of climate change on fire ignition and spread, the accumulation of fuels tends to increase the probability of fire occurrence as a function of time since the last fire. Nevertheless, this test provided a useful measure of how different the observed levels of fire synchrony were among groves, relative to fire synchrony that might have been observed by chance if fire occurrence were random and independent within and among the groves. 14. The observed frequency dependency is not always consistent. Both precipitation and temperature interact to produce droughts and fire responses lasting years to decades. For example, an extreme drought lasting several decades coincided with the largest difference between the regional fire and temperature records during the mid-1 200s (Fig. 3). This was one of the driest periods in the tree-ring reconstructions (6, 8), and past lake levels in this region also indicate that the mid-1200s were extremely dry [S. Stine, Palaeogeogr. Palaeoclimatol. Palaeoecol. 78, 333 (1990)]. 15. R. A. Minnich, Science 219, 1287 (1983). In this case, the shift to lower fire frequency on the U.S. side of the border, and consequent larger chaparral patches and fires, was primarily a result of fire suppression rather than climate. 16. T. W. Swetnam, in "Proceedings of Symposium on Effects of Fire in Management of Southwestern Natural Resources," S. Krammes, Tech. Coord., Gen. Tech. Rep. RM-191 (U.S. Forest Service, Fort Collins, CO, 1990), pp. 6-17. 17. N. L. Christensen et al., BioScience 39, 678 (1989). 18. R. Abugov, Ecology 63, 2 (1982); T. E. Miller, Am. Nat. 120, 533 (1982). 19. N. L. Christensen, in Ecosystem Management for Parks and Wilderness, J. Agee and D. Johnson, Eds. (Univ. of Washington Press, Seattle, 1989), pp. 62-86; N. L. Christensen, Forum Appl. Res. Public Policy 4, 46 (1989). 20. F. H. Bormann and G. E. Likens, Pattern and Process in a Forested Ecosystem (Springer-Verlag, New York, 1979). 21. W. H. Romme, Ecol. Monogr. 52,199 (1982); D. H. Knight, in Landscape Heterogeneity and Disturbance, M. G. Turner, Ed. (Springer-Verlag, New York, 1987), pp. 59-83. 22. thank the many assistants, students, National Park Service personnel, California Department of Forestry personnel, and volunteers who contributed by carrying out field, shop, and laboratory tasks, foremost among whom were C. H. Baisan, P. M. Brown, A. C. Caprio, and R. Touchan of the Laboratory of Tree-Ring Research, University of Arizona; also thank J. L. Betancourt, L. J. Graumlich, M. K. Hughes, D. J. Parsons, N. L. Stephenson, and two anonymous reviewers for commenting on and editing the manuscript. Supported by Sequoia, Kings Canyon, and Yosemite National Parks and National Park Service Global Change Program Cooperative Agreements, Department of the Interior 8018-1-0002 and 8032-1-0002.