and
Julio L. Betancourt
U.S. Geological Survey, Desert Laboratory, Tucson, AZ
Ecological responses to climatic variability in the Southwest include regionally-synchronized fires and pulses in tree demography (births and deaths). Multi-century, tree-ring reconstructions of drought, disturbance history, and tree demography reveal climatic effects across scales, from annual to decadal, and from local to mesoscale (108 to 1011 m2).
Climate-disturbance relations are more variable and complex than previously assumed. For example, regional fires occur during extreme droughts (Figs. 1 and 2), but antecedent wet conditions play a secondary role by regulating accumulation of fuels. Interdecadal changes in fire-climate associations indicate shifts in the strength of ENSO-fire teleconnections during the past three centuries (Swetnam and Betancourt 1990, 1993). High interannual, fire-climate correlations (r = 0.7 to 0.9) during specific decades (i.e., ca. 1740-1780 and 1830-1860) reflect periods of high amplitude in the Southern Oscillation and rapid switching from extreme wet to dry years in the Southwest, thereby entraining fire occurrence across the region (Fig. 3). Weak correlations from 1780 to 1830 correspond with a decrease in SO amplitude or frequency inferred from independent tree-ring width, ice core, and coral isotope reconstructions (Stahle and Cleaveland 1993, Dunbar et al. 1994).
![]() Figure 2. Number of fire-scar sites (chronologies) in the Southwest recording fire dates in each year, 1700 to present, out of a total of 63 sites. Note the regionally-synchronous fire years (labeled), and the decrease in recorded fires after ca. 1900, reflecting livestock grazing affecting fine fuels, and subsequent fire suppression by government agencies. |
Century to millennia-length tree-ring width chronologies are useful for evaluating frequency and magnitude of droughts and wet periods, and for placing ecosystem changes into a long-term, historical context of climatic change (Fig. 4). For example, droughts and wet episodes have altered age structures and species composition of woodland and conifer forests. The scarcity of old, living conifers established before ca. 1600 suggests that the extreme drought of 1575-1595 had pervasive effects on tree populations (Fig. 5). The most extreme drought of the past 300 years occurred in the mid-twentieth century (1942-1957). This drought resulted in broad scale plant dieoffs in shrublands, woodlands, and forests, and accelerated shrub invasion of grasslands (Fig. 6).
Drought conditions were broken by the post-1976 shift to the negative SO-phase and wetter cool seasons in the Southwest (Fig 7). The post-1976 period shows up as an unprecedented surge in tree-ring growth within millennia-length chronologies (Fig. 4). This unusually wet episode may have produced a pulse in tree recruitment (Fig. 6), and perhaps an increase in area burned by wildfires (Fig. 8), owing to increased grass and tree leaf production during wet seasons and years. However, additional study is needed to disentangle the interacting roles of land-use and climate. The 1950s drought and the post-1976 wet period, and their aftermaths, offer natural experiments to study long-term ecosystem response to interdecadal climate variability.
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Figures 7a and 7b. Three-dimensional time series plots of monthly precipitation totals from Tucson, Arizona (a) and Las Cruces, New Mexico (b). Note the persistence of winter and summer drought in Las Cruces during the 1950s, and the post-1976 increase in cool season precipitation in both Tucson and Las Cruces.
click on figures to view larger versions.
Our examples of ecological responses to climate in the Southwest underscore the importance of reconstructing, observing, and assessing ecological processes and patterns at the appropriate scales, i.e., mesoscales and centuries. Ecological synchroneity at these scales is the hallmark of climatic effects on ecosystems and is a key to separating cultural from natural causes of environmental change. Improved understanding of changing climatic and human controls of keystone ecological processes, such as fire, will require parallel development and comparison of well-dated networks of climatic and ecological time series.
Betancourt, J. L., E. A. Pierson, K. Aasen-Rylander, J. A. Fairchild-Parks, J. S. Dean, 1993: Influence of history and climate on New Mexico pinyon-juniper woodlands. Proceedings: Managing Pinyon-Juniper Ecosystems for Sustainability and Social Needs; April 26-30, Santa Fe, New Mexico, Aldon, E. F. and D. W. Shaw, Eds., Gen. Tech. Rep. RM-236, Fort Collins, CO, USDA Forest Service, Rocky Mountain Forest & Range Exp. Station, 42-62.Cook, E. R., D. M. Meko, D. W. Stahle, and M. K. Cleaveland, 1996: Tree-ring reconstructions of past drought across the coterminous United States: Tests of a regression method and calibration/verification results. In: J. S. Dean, D. M. Meko, and T. W. Swetnam eds., Tree Rings, Environment, and Humanity, 17-21, May 1994, Tucson, Arizona. Radiocarbon, 155-169.
D'Arrigo, R. D. and G. C. Jacoby. 1991: A 1000-year record of winter precipitation from northwestern New Mexico, USA: a reconstruction from tree-rings and its relation to El NiƱo and the southern oscillation. The Holocene, 1, 95-101.
Dunbar, R., G. M. Wellington, M. W. Colgan, and P. W. Glynn, 1994: Eastern Pacific sea surface temperature since 1600 A.D.: The d18O record of climate variability in Galapagos corals. Paleoceanography, 9, 291-316.
Fritts, H. C., and T. W. Swetnam, 1989: Dendroecology: A tool for evaluating variations in past and present forest environments. Adv. Ecol. Res., 19, 111-189.
Grissino-Mayer, H. D. 1996: A 2,129-year reconstruction of precipitation for northwestern New Mexico, USA. In: J. S. Dean, D. M. Meko, and T. W. Swetnam eds., Tree Rings, Environment, and Humanity, Radiocarbon, 191-204.
Meko, D. M., C. W. Stockton, and W. R. Boggess, 1995: The tree-ring record of severe sustained drought. Water Res. Bull., 31, 789-801.
Meko, D. M., E. R. Cook, D. W. Stahle, C. W. Stockton, and M. K. Hughes, 1993: Spatial patterns of tree-growth anomalies in the United States and southeastern Canada. J. Climate, 6, 1773-1786.
Stahle, D. W., and M. K. Cleaveland, 1993: Southern oscillation extremes reconstructed from tree rings of the Sierra Madre Occidental and Southern Great Plains. J. Climate, 6, 129-140.
Swetnam, T. W. and J. L. Betancourt, 1990: Fire-Southern Oscillation relations in the southwestern United States: Science, 24, 1017-1020.
Swetnam, T. W., and J. L. Betancourt. 1992. Temporal patterns of El Nino/Southern Oscillation - wildfire patterns in the southwestern United States. pages 259-270 In Diaz H. F. and V. M. Markgraf, eds., El Nino: Historical and Paleoclimatic Aspects of the Southern Oscillation, Cambridge University Press, Cambridge.
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