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).
|Figure 1. Map of the Southwestern U.S. showing fire-scarred tree collections (filled circles, indicating approximate location of one or more fire-scar chronologies). Fire-scarred trees are typically 200 to 300 year old ponderosa pines that contain a record of presettlement surface fires as a series of fire-caused injuries (scars) within their annual rings. These injuries can be dated to the year and season of occurrence. The range of ponderosa pine forest type is shown by irregular green outlines.|
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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.
Figure 3. Correlation of tree-ring reconstructed regional fire-scar chronology and June-July-August PDSI time series from the Southwest (average of 13 grid point reconstructions in Fig. 1). The upper plot shows the first differences (year [t] - year [t-1]), which emphasize the year-to-year, high frequency changes in the two time series. Note that the PDSI is plotted inverse relative to the fire time series; negative changes in PDSI (drying condition) correspond with positive changes in fire activity (increased numbers of sites recording fire scars). The lower plot shows the 21-year moving correlation coefficients for the two time series plotted on central years.
Figure 4. Millennia-length tree-ring width index chronology from the Southwest showing an anomalous post-1976 growth surge. The plot shows the smoothed (13-weight, low pass filter) ring-width growth averaged across six sites in New Mexico and Arizona from a variety of species. Although these sites show a consistent post-1976 surge in growth, other tree-ring chronologies from the Southwest do not.
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).
|Figure 5. Innermost ring dates of old-age conifer trees sampled for climate reconstructions in the Southwest (from data archives at the Laboratory of Tree-Ring Research). The species codes shown on the plots are: PSME = Douglas-fir; PIPO = ponderosa pine; PIED = pinyon. Few trees established before the severe drought of the late 1500s survived, whereas many of the oldest trees in the Southwest recruited into forests and woodlands during the relatively wet and cool early 1600s.
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|Figure 6. Death dates (upper plot) and recruitment dates (lower plot) of pinyon trees in Sevilleta Long Term Ecological Research area, Los Pinos Mountain New Mexico. The extreme drought of the late 1940s to 50s resulted a broad scale dieoff of pinyons and junipers throughout the Southwest (Betancourt et al. 1993). Pulse-like recruitment of pinyon before 1800 appears to have been related to wet periods following droughts. Presently, we cannot ascertain that the rise in number of recruiting trees after 1976 reflects normal survivorship of younger age classes, exclusion of cattle after this area was fenced, or an unusual pulse in recruitment due to wetter conditions.|
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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.
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.
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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.
|Figure 8. Annual area burned by wildfires in Arizona and New Mexico on all lands (state, private and federal), 1916 to 1996. The increased area burned in the late-20th century has been most commonly attributed to accumulated living and dead forest fuels since fire suppression began early in the century. However, wetter conditions since the 1970s might also be involved, particularly in ecosystems where plant productivity is ordinarily low, and grasses have increased in importance. Fine fuels, such as grasses and leaves, may be a key factor in these seasonally dry ecosystems where fuel continuity is limiting to fire spread. Expansion of non-native grasses in some parts of the Southwest may also have contributed to this pattern.|
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