Dendrochronology and Dendroclimatology

Adapted from: Swetnam, Thomas W. and Julio L. Betancourt. 1999. "Mesoscale ecological responses to climatic variability in the American Southwest." [http://geochange.er.usgs.gov/sw/impacts/biology/fires_SOI/]. 2/16/99.

Dendrochronology (dendron = tree, chronos = time) is the science that uses tree rings dated to their exact year of formation to analyze temporal and spatial patterns of processes in the physical and cultural sciences. Dendroclimatology is the use of tree rings to study and reconstruct the past and present climate. Dendroarchaeology is the use of tree rings to date when timber has been felled, transported, processed, and used for construction (e.g. dating the tree rings of a beam from a ruin in the American Southwest to determine when it was built). Dendroecology is the use of tree rings to study factors that affect the earth's ecosystems. For example, tree-ring data from the Southwest shows high interannual fire-climate correlations reflecting periods of high amplitude in the Southern Oscillation (SO) and rapid switching from extreme wet to dry years, thereby influencing fire occurrence across the region.

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. 1). For example, droughts and wet episodes have altered age structures and species composition of pinyon-juniper woodlands 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. 2). 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. 3).
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Figure 1. 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.

Figure 2. 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.

Figure 3. 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.

Drought conditions were broken by the post-1976 shift to the negative SO-phase and wetter cool seasons in the Southwest (Fig. 4) The post-1976 period shows up as an unprecedented surge in tree-ring growth within millennia-length chronologies (Fig. 1). This unusually wet episode may have produced a pulse in tree recruitment (Fig. 3), and perhaps an increase in area burned by wildfires, 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 4a and 4b. 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.

Fire-Southern Oscillation Relations in the Southwestern United States. A close linkage between fire and climate could diminish the importance of local processes in the long-term dynamics of fire-prone ecosystems. The structure and diversity of communities regulated by fire 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. Adapted from a journal article by Thomas W. Swetnam and Julio L. Betancourt.

Changed Southwestern Forests: Resource effects and management remedies. Over 150 years of occupancy by northern Europeans has markedly changed vegetative conditions in the Southwest. Less fire due to grazing and fire suppression triggered a shift to forests with very high tree densities, which in turn contributed to destructive forest fires. Options to deal with these changes include prescribed fire, thinning and timber harvest to mimic natural disturbances and conditions. However, there are barriers to implementing these activities on a scale large enough to have a significant benefit. Adapted from a published journal article by Marlin Johnson.

Where have all the grasslands gone? Numerous ecological studies across the Southwest have documented the decline in herbaceous vegetation (grasses and non-woody flowering plants) while forests thicken and brush invades. Documenting the changes in the Jemez Mountains of northern New Mexico, ecologist Craig Allen considers the evidence that these patterns are tied to changes in land use history, primarily livestock grazing and fire suppression.

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