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Landscape Changes in the Southwestern United States: Techniques, Long-term Data Sets, and Trends


Craig D. Allen
U.S. Geological Survey
Midcontinent Ecological Science Center
Jemez Mountains Field Station
Los Alamos, New Mexico 87544
505/672-3861 ext. 541

Julio L. Betancourt
U.S. Geological Survey
Desert Laboratory
Tucson, Arizona 85745
520/670-6821 ext. 112

Thomas W. Swetnam
Laboratory of Tree-Ring Research
University of Arizona
Tucson, Arizona 85721

Abstract. The great ecological diversity of landscapes in the American Southwest results from combinations of the underlying patterns of topographic complexity, climatic variability, and environmental histories. This chapter illustrates some high-resolution and long-term data sets and approaches for reconstructing landscape change in the Southwest, including the paleobotanical record, repeat photography, and fire-scar histories from tree rings. We explore the effectiveness of collecting historical data at multiple locations to build networks that allow analyses to be scaled up from localities to regions and the use of historical data to discriminate between natural and cultural causes of environmental change.


The American Southwest is a region where great ecological diversity is maintained by topographic complexity and extreme variability in climate. Despite the pervasive influence of livestock grazing and other human land uses in the Southwest, natural vegetation predominates over vast tracts of public land. Because natural processes are still very much in play, human impacts in this region are seldom "clearly" evident. In fact, the greatest challenge in assembling and interpreting a land-use history of the Southwest is disentangling cultural from natural causes of environmental change. We have employed a variety of tools, techniques, and data types to address this challenge.

Here we illustrate some historical and paleoecological perspectives on environmental change in the Southwest. Among the themes we explore are:

  1. The importance of climatic variability in driving ecological processes, as well as in modulating human land uses and their effects on southwestern landscapes.
  2. The use of historical and paleoecological data to detect and explain trends in ecological patterns and processes across southwestern landscapes.
  3. The effectiveness of network approaches in the development of historical data sets. By aggregating data spatially, observations and inferences can be scaled up from localities to landscapes and regions.
  4. The use of historical data to discriminate between natural and cultural causes of environmental change.
  5. The use of historical data to define and constrain natural ranges of variability and, in some cases, to set targets or determine templates for restoration and sustainable use of ecosystems.

This chapter illustrates some approaches for reconstructing landscape change in the Southwest from high-resolution and long-term data sets including the paleobotanical record, repeat photography, and fire-scar histories from tree rings.


The Paleobotanical Record

An important backdrop for evaluating human impacts on southwestern landscapes is the long-term dynamics of vegetation change. Glacial climatic and vegetation patterns have characterized most of the Pleistocene (the past 1.2 million years). Just 12,000 years ago, the Earth underwent major environmental changes in the transition to the current interglacial period, the Holocene. Dramatic swings in atmospheric chemistry and climate, as well as global ice volumes and sea level, caused massive shifts in biotic distributions. Vegetation change in humid areas has been reconstructed from analysis of pollen grains preserved in lake sediments, but opportunities for pollen analysis are limited in arid regions due to scarcity of persistent water bodies, low proportion of wind-pollinated plant species, and poor pollen preservation in alkaline sediments. In the arid interior of North America, a novel way of reconstructing vegetation change has been the analysis of plant and animal remains preserved in fossil packrat (Neotoma spp.) middens, deposits that are ubiquitous in rocky environments. About the size of a laboratory rat, packrats gather nearby plant materials (within 100 m at most) and accumulate them in dry caves and crevices; there, the plant and other debris (including arthropod and vertebrate remains) are cemented into large masses of crystallized urine (referred to as amberat), which can persevere for tens of thousands of years. About 2,500 of these deposits have been dated within the limit of the radiocarbon method (the last 50,000 years) and analyzed for plant and animal remains (Betancourt et al. 1990). The preservation of plant remains in packrat middens is excellent, allowing identification of species and diverse morphological, geochemical, and genetic analyses (e.g., Van de Water et al. 1994; Smith et al. 1995). The extensive archive of sorted, identified, and dated material represents the richest and best-documented source of plant remains in the world, with hundreds of species identified and available for corollary studies. Maps of modern versus Pleistocene vegetation in the Southwest imply remarkable changes during the last 12,000 years (Fig. 9-1); plant migrations initiated during the Holocene may still be ongoing and hence complicate simple cultural versus natural explanations of vegetation change (Figs. 9-2 and 9-3).

  Modern Vegetation map Pleistocene Vegetation map
Fig. 9-1. As this comparison of modern and Pleistocene vegetation shows, southwestern landscapes have changed dramatically since the end of the last ice age, 12,000 years ago. During the last ice age, desert vegetation was restricted to the lower elevations (<300 m) in Death Valley and the mouth of the Colorado River. Hallmarks of the Sonoran Desert, such as the giant saguaro cactus (Carnegiea gigantea) and the palo verde (Cercidium sp.), were displaced far south into Mexico. Creosotebush (Larrea tridentata), the dominant shrub of the Chihuahuan, Sonoran, and Mojave deserts, had its northernmost populations along the Arizona-Sonora border. Extensive pinyon-juniper-oak woodlands, now restricted to the highlands, covered what are now desert elevations (300-1700 m). The extensiveness of spruce-fir, mixed-conifer, or subalpine forests and woodlands during glacial times is evident in their coverage over much the same territory as modern pinyon-juniper woodlands, currently the third largest vegetation type in the United States (20 million ha). The biggest surprise from the packrat midden record is the virtual absence of ponderosa pine (Pinus ponderosa), a tree that today extends from central Mexico along the axis of the Rockies into Canada. In the United States, much of that range developed through migration during the last 10,000 years. Populations of this commercial species in the northern Rockies and western High Plains may represent arrivals within the last few millennia and perhaps the last few centuries.


Figure 9-2 Diagram of packrat midden records with pepershell pinyon and Colorado pinyon for the last 40,000 years.
Fig. 9-2. Diagram showing fossil packrat midden records with papershell pinyon (Pinus remota, south of the Hueco Mountains near El Paso), and Colorado pinyon (Pinus edulis, north of Hueco Mountains) during radiocarbon time for the last 40,000 years. The tickmarks on each vertical line represent over 350 radiocarbon-dated middens that show the presence or absence of pinyon pines along a 15 latitude (ca. 1,00 km) transect from Bermejillo, Mexico (Durango Province), to Fort Collins, Colorado. The diagram depicts the local extinction of pinyon populations growing in the Chihuahuan Desert during the last deglaciation (around 11,000 radiocarbon years ago) and the sequential migration to higher elevations and more northerly latitudes during the Holocene (the last 11,000 years). Note that Colorado pinyon's distribution in the state of Colorado may be just a few hundred years old and probably is not yet in equilibrium with modern climate. In Colorado and northern New Mexico, this recent migration makes it difficult to discriminate the last phases of Holocene migration from historical tree expansion due to fire suppression and overgrazing.

  Figure 9-3 Photo 1950 Figure 9-3 Photo 1989
Fig. 9-3. An isolated stand of Colorado pinyon (Pinus edulis) at Owl Canyon, north of Fort Collins, Colorado, represents the endpoint of its northward migration since the end of the last ice age (Betancourt et al. 1991). This 5 km 2 stand was colonized by pinyon pine less than 500 years ago, possibly from accidental plantings by Cheyenne and Arapaho, who carried pinyon nuts in their "trail mix" on treks along the Front Range. The nearest potential source populations are 250 km to the south near Colorado Springs. It is unclear what role humans played in movement of seeds and plant migration during the Holocene. Note the rapid increase in canopy cover from 1950 to 1989, characteristic of an expanding population. (Photos: 1950, J.D. Wright; 1989, R.M. Turner.)

Repeat Photography

Ground-Based Photography

Historical photographs of key landscapes, from hillslopes to wetlands, are available for practically any area of the western United States (Rogers et al. 1984). As a first approximation, past environmental change can be measured by finding the site of a historical photograph, reoccupying the original camera position, and making a new photograph of the same scene. Differences between then and now provide a basis for identifying and even quantifying changes, while the new photograph establishes a benchmark for future evaluation. Repeat photography is a simple, inexpensive, and elegant tool for reconstructing past environmental changes and monitoring future ones; it is particularly well suited for the relatively open landscapes of the western United States (Hastings and Turner 1965; Rogers 1982; Veblen and Lorenz 1991; Webb 1996). Repeat photography in the Southwest has focused on key ecological concerns relevant to management of public lands, including shrub and tree encroachment upon grasslands (Figs. 9-4 and 9-5), climatic effects on demographic trends in woodlands, postdisturbance histories, and geomorphic, hydrologic, and vegetation changes in riparian areas (Figs. 9-6 and 9-7).

  Figure 9-4 Photo 1915 Figure 9-4 Photo 1989
Fig. 9-4. Creosotebush (Larrea tridentata) arrived at the Sevilleta Long-Term Ecological Research site south of Albuquerque about 2,500 years ago and expanded into what was once open grassland during the twentieth century, as shown in photographs from July 1915 and August 1989. The site of the photograph was open grassland when the Spanish began grazing sheep and cattle in the 1700's, but this grassland was invaded by snakeweed (Gutierrezia sarothrae) and later by creosotebush (Larrea tridentata). Note the lone creosotebush at the right foreground and the beachball-sized snakeweed throughout the foreground of the 1915 photograph. This photograph was taken after one of the wettest years in New Mexico history. By the time of the 1989 photograph, creosotebush had expanded throughout this former grassland, with the invasion accelerated during an extended drought between 1942 and 1972 (Betancourt et al. 1993). Also note the increase in one-seed juniper (Juniperus monosperma) on the slopes in the right background. (Photos: 1915, N.H. Darton; 1989, R.M. Turner and J.L. Betancourt.)

  Figure 9-5 Photo 1899 Figure 9-5 Photo 1977
Fig. 9-5. Views from Acoma Pueblo to Enchanted Mesa, 100 km west of Albuquerque, in 1899 and 1977. Note expansion of junipers (Juniperus monosperma) between 1899 and 1977. In many parts of the west, juniper expansion has been blamed on fire suppression and livestock grazing, justifying an aggressive program of chaining and burning pinyon-juniper woodlands in the 1960's and 1970's to improve forage and water yield. Several authors have suggested that pinyon-juniper expansion may instead represent recovery from prehistoric fuel harvesting, at least in those areas that were heavily populated within the last 1,000 years (Samuels and Betancourt 1982; Kohler 1988). One such place could be Acoma Pueblo. (Photos: 1899, W.H. Jackson; 1977, H.E. Malde.)

In the Southwest, the process of desertification has involved expansion of desert shrubs and trees into former grasslands (Figs. 9-4 and 9-5). Shrub encroachment is difficult to reverse because nutrients and other resources quickly begin to accumulate underneath shrubs, creating resource islands that discourage grassland recovery (Schlesinger et al. 1990). Explanations for shrub encroachment have ranged from fire suppression and livestock grazing (Grover and Musick 1990) to interdecadal climatic variability (Neilson 1986) and, most recently, to CO2 enrichment shifting the balance from warm-season grasses to cool-season shrubs (Idso 1992). The debate is confounded by the fact that progressive range deterioration since 1870 has been inferred from historical data (Bahre and Shelton 1993), while long-term monitoring indicates substantial range improvement with wetter conditions following the drought of the 1950's (McCormick and Galt 1994).

One of the most remarkable changes in southwestern landscapes involved late nineteenth and early twentieth century channel entrenchment (Fig. 9-6). Between 1865 and 1915, arroyos developed in alluvial valleys of the southwestern United States across a wide variety of hydrological, ecological, and cultural settings. That they developed more or less simultaneously has encouraged the search for a common cause, some phenomenon that was equally widespread and synchronous. As with most recent environmental changes, whether global or local, efforts to understand arroyo genesis have been hindered by the inability to discriminate between natural and cultural factors. Much debate has focused on the regional and local causes for historic arroyo-cutting (Bull 1997). Range managers have been quick to point to the removal of plant cover by livestock, whereas climatologists have naturally looked to the skies for an explanation. The geologist, accustomed to studying the products of erosion over long periods of time, sees arroyos as symptomatic of inherent instability in arid landscapes, while acknowledging that geomorphic thresholds may be exceeded with changes in climate and vegetation. Following arroyo initiation, two of the more pervasive impacts on southwestern watersheds have been deterioration of wetlands and degradation of streamside vegetation, caused by groundwater withdrawal and urbanization (Fig. 9-7).

  Figure 9-6 Photo 1891 Figure 9-6 Photo 1982
Fig. 9-6. In July and August of 1890, heavy flooding cut a deep channel in the Santa Cruz River at Tucson. As arroyo cutting progressed, it ultimately destroyed nearby Silver Lake, an impoundment on the Santa Cruz River which powered the waterwheels of local flour mills and provided irrigation water for agricultural lands downstream. Compare these photographs of Silver Lake in 1891 and 1982 (Betancourt and Turner 1988). (Photos: 1891, unknown; 1982, R.M. Turner and J.L. Betancourt.)

  Figure 9-7 Photo 1904 Figure 9-7 Photo 1981
Fig.9-7. Downstream view of the confluence of the west branch of the Santa Cruz River in Tucson, looking northeast from the lower slope of Sentinel Peak. Between 1904 and 1981 deterioration of the riparian vegetation is evident due to groundwater depletion and urbanization, along with arroyo cutting. Many other southwestern floodplains have undergone similar changes, including reaches of the Rio Grande, the Salt River, and the Gila River. (Photos: 1904, unknown; 1981, R.M. Turner and J.L. Betancourt.)

Aerial Photography

Aerial photography and other remote sensing approaches (e.g., satellite imagery) provide powerful means of determining widespread changes in landscape patterns through time, especially when used in concert with geographic information systems (Sample 1994). Aerial photography was performed across most of the Southwest in the mid-1930's, providing a baseline from which modern landscape changes can be assessed (Allen and Breshears 1998).

  Figure 9-8 Map illustrating changes in montane grassland area from 1935 to 1981 in the Jemez Mountains, New Mexico
Fig. 9-8. Map of changes in montane grassland area between 1935 and 1981 in the southeastern Jemez Mountains, New Mexico. Area of open grassland (with less than 10% tree canopy cover) was determined from aerial photographs.

Groundbased evidence, such as tree ages and soil patterns, indicate that conifer trees have widely invaded ancient montane grasslands in the Jemez Mountains of northern New Mexico during this century (Allen 1989). Aerial photographs confirm these observations and reveal the extensiveness of the tree encroachment (Fig. 9-8), which reduced the area of open montane grasslands by 55% between 1935 and 1981 across the 100,000 ha mapped area. The tree invasion has been tied to changes in land-use history, primarily livestock grazing and fire suppression (Allen 1989).

Changes in road networks through time reflect and determine land use histories, as illustrated in this Jemez Mountains example. Total road density in 1935 (Fig. 9-9) was greatest on the homesteaded lands just north of Bandelier National Monument, where dirt and primitive roads provided access to agricultural fields, dwellings, and timber and fuelwood resources. West of Bandelier National Monument, roads provided access to ranches, mines, and some timber operations. Large portions of the Jemez area remained roadless.

  Figure 9-9 Maps of roads in 1935 and 1981
Fig. 9-9. Map of all roads visible in 1935 and 1981 aerial photographs across 187,858 ha around Bandelier National Monument, in the Jemez Mountains, New Mexico. The current national monument boundaries are shown. "Dirt" roads have a bulldozed surface, while "primitive" is a variable category that includes logging skid trails, informal woodcutting tracks, some powerline corridors, and off-road vehicle paths.

In 1935 the Denver and Rio Grande Railroad was still in operation through the eastern edge of the map area. Completed between 1880 and 1886, this important connection between the Jemez Mountains and the outside world markedly altered land use patterns in this area (Rothman 1992). The improved linkages to outside markets provided by railroads throughout the Southwest in the late 1800's allowed the concurrent, region-wide buildup of extreme numbers of livestock (Wooton 1908), which precipitated key landscape changes such as vegetation transformations and altered fire regimes.

By 1981 (Fig. 9-9) the length of mapped roads increased nearly twelvefold, from 719 km in 1935 to 8,433 km (Allen 1989). The pattern of paved roads north of Bandelier reflects intensive human development activities, as the agricultural homesteads turned into the industrialized technical areas of Los Alamos National Laboratory, with its associated townsites of Los Alamos and White Rock. The dense networks of dirt and primitive roads to the west of Bandelier were created by a variety of logging activities on public and private lands during the 1960's and 1970's (e.g., the striking spiral patterns of dirt roads observed in the northwest quadrant of Fig. 9-9). The largest remaining roadless tract was the designated wilderness areas in and adjoining Bandelier National Monument. Estimated total area of road surfaces grew from 0.13% of the map area in 1935 (247 ha) to 1.67% in 1981 (3,132 ha). These estimates of road surface areas do not include shoulders, cut and fill slopes, or ditches, and thus are conservative estimates of landscape area directly altered by roads.

The great increase in road networks observed since 1935 in the Jemez Mountains suggests the possibility of significant, landscape-wide ecological impacts (Allen 1989). The U.S. Forest Service has recently recognized the existence of over 690,000 km of national-forest roads on its lands across the United States (see details at, highlighting the magnitude of wildland road networks in this country. Roads can have many ecological effects, ranging from habitat fragmentation and reduced landscape productivity to the direct conversion of roadways into compacted and sparsely vegetated surfaces. They can also provide routes for the spread of nonnative weeds, accelerate erosion rates, and increase stream sediment loads. Roads act as fire breaks and facilitate extensive access to formerly remote areas for fire suppression. Roads also allow increased human access for recreational and consumptive purposes, resulting in widespread habitat modifications (e.g., cutting of snags for fuelwood) and disturbances to wildlife (e.g., through vehicle traffic and hunting) that alter biotic communities. Overall, road networks often provide distinctive landscape signatures of the histories and ecological effects of human land uses.

Fire-Scar Histories

  Figure 9-10 Photo of tree rings and fire scars Well-dated fire-scar chronologies aggregated over space and time provide powerful, multiscale perspectives of the variability of past fire regimes (Figs. 9-10 and 9-11). These fire-scar chronologies document a history of frequent, widespread surface fires in many southwestern forest types (Swetnam and Baisan 1996; Fig. 9-12). Fire is a "keystone process" (see Holling 1992) in the Southwest, and patterns of change in the fire-scar record are interpretable in the context of climatic variation and changes in land use and forest stand structures (including fuel conditions). Thus, fire histories record the ecological "pulse" of southwestern forests, integrating both natural and cultural histories.

Fig. 9-10. Repeated surface fires cause a sequence of overlapping wounds. The heat-killed wood tissues extend into the annual rings, which can be dated to the calendar year.

  Figure 9-11 Map of fire history study sites

Fig. 9-11. Map of fire history study sites in the southwestern United States. The red dots show locations of tree-ring and fire-scar collections in 27 mountain ranges. Most collections are in ponderosa pine and mixed conifer forests.

  Figure 9-12 Graphic illustrating fire scar chronologies
Fig.9-12. Fifty-five fire-scar chronologies for different forest sites in 27 mountain ranges of the southwestern United States. The yellow and red tick marks on each time series are fire dates recorded by fire-scarred trees. At least 10 fire-scarred trees were sampled in each site, and the tree rings and fire scars were dated by dendrochronology methods. Each time series is a composite of the fires recorded by at least two trees in each site. The red tick marks show regional fire years defined by 10 or more of the 55 chronologies (sites) recording the fire date. The yellow tick marks show the other fire dates recorded in nine or fewer sites. The step-line graph at the bottom is a summation of the number of sites recording the fire dates; the regional fire years are in red and are labeled.

Regional fire years (Fig. 9-12) were an episodic phenomena in southwestern forests, and the synchronized nature of these events demonstrates the importance of interannual climate in controlling local to regional-scale fire occurrence. The El Niņo-Southern Oscillation (ENSO), a global climatic pattern, is associated with these fire patterns, both in the past and in current southwestern fire regimes (Swetnam and Betancourt 1990, 1998). Regional fire years tend to occur during La Niņa events and droughts, while reduced fire activity corresponds to El Niņos and wet years. Moreover, regional fire years tend to occur during average or dry years that follow one to three wet years, indicating the important role of fine fuel production (i.e., grasses and tree needles) in fire dynamics, especially in ponderosa pine forests and lower elevation forests. Hence, when the ENSO has high variation and amplitude, with extreme dry years following extreme wet years, fire activity is entrained across regional scales.

Long-term changes in fire frequency over the past four centuries (Fig. 9-12) were related to both climate and human activities. Native Americans probably set many of the fires recorded by fire scars before 1900, but lightning was (and is) so frequent in the Southwest that, in most places and times, fire frequencies were probably controlled primarily by climate and fuel dynamics, rather than by ignition source. The decrease in fire frequency after the late 1800's (Fig. 9-12) was due mainly to the rise of intensive livestock grazing, when fine fuels (e.g., grasses) that carried surface fires were consumed by millions of sheep, goats, cattle, and horses (Wooton 1908; Swetnam and Baisan 1996). Disruption of fuel continuity by trailing and herding large numbers of animals was probably also involved.

Disentangling climatic factors (regional scale) from cultural factors (local scale) as causes of observed variations can proceed from comparative analyses within a regional network of paleoecological study sites. Interpretations can be based on the degree of synchronism among events across spatial scales and the degree of correspondence among multiple, independently derived time series of disturbance, climate, and land-use chronologies. For example, the importance of intense livestock grazing as a cause of the disruption of natural fire regimes is confirmed by the comparison of different case studies. A few sites in northern New Mexico and Arizona that were grazed by sheep and goats owned by Spanish colonists and Navajos (DinJ) show fire frequencies declining in the early nineteenth century, or earlier, and corresponding to the documented timing of pastoral activities in these areas (Savage and Swetnam 1990; Touchan et al. 1996; Baisan and Swetnam 1997). In contrast, remote sites with no evidence of early, intensive grazing sustained some surface fires into the middle of the twentieth century, when aerial firefighting resources began to be most effective in suppressing fires (Grissino-Mayer 1995). Finally, a remote mountain in northern Sonora, Mexico (lowermost fire-scar chronology in Fig. 9-12), where neither intensive livestock grazing nor effective fire suppression has occurred, shows episodic surface fires burning throughout the twentieth century.

One of the strengths of spatial networks of well-dated fire chronologies (or other disturbance chronologies) is that they can be aggregated across spatial scales, providing multiscale spatial and temporal perspectives. Analyzing patterns in such spatio-temporal data networks may reveal scaling rules and underlying mechanisms and controls of disturbance processes (e.g., see Holling 1992). The 1748 fire year in the Southwest (Fig. 9-13) was an example of a cross-scale disturbance event; extensive fires burned at all spatial scales within the region. This extensiveness is indicated by the high synchrony of fires for this date recorded in most sampled trees within stands, in most sampled stands within watersheds, in most watersheds within mountain ranges, and in most mountain ranges within the region. The importance of extreme interannual climate changes in triggering this regional event is indicated by dendroclimatic and Spanish archival sources confirming that 1748 was an extreme drought year following an extremely wet year (1747).

  Figure 9-13 Graphic illustrating hierarchical spatial views of the 1748 fire year.
Fig. 9-13. A cross-scale comparison of the largest regional fire year in the Southwest during the past 400 years: 1748. The synchrony of the 1748 fire year among fire-scarred trees at the smallest spatial scale (a forest stand) is shown in the bottom panel. Patterns of synchrony, which are a measure of relative areal extent, are then illustrated at higher levels of aggregation (larger scales, coarser grain size) up through the watershed, mountain range, and finally the regional level (uppermost panel).

Ecological changes are often best evaluated by comparing multiple lines of historical evidence. Twentieth-century changes in southwestern ponderosa pine forests have been well documented by several generations of ecologists and foresters, ranging from Aldo Leopold (1924) and Gus Pearson (1933) to Weaver (1951) and Covington and Moore (1994). Numerous comparisons of early versus recent photographs and forest stand descriptions have demonstrated that stand densities have increased while grass cover has decreased. These changes were caused by a combination of intensive livestock grazing and, subsequently, organized fire suppression by government agencies. Tree-ring reconstructions of forest age structure and fire history, however, can identify new elements in this story. For example, while many pine forests today are dominated by the post-grazing/fire suppression "tree irruption" of the early 1900's, another pulse of tree recruitment apparently took place during the early 1800's. This pulse is evident in the Monument Canyon Research Natural Area (Fig. 9-14) and other southwestern sites. This pulse corresponds to the longest intervals between widespread fires in numerous sites in the Southwest, changes in fire frequencies and seasonality, and shifts in climate (Grissino-Mayer 1995; Swetnam and Betancourt 1998).

  Figure 9-14 Photos and graph of ponderosa pine forest changes.
Fig. 9-14. Ponderosa pine forest changes from repeat photography, tree demographic data, and fire history. The upper left photograph is of an open ponderosa pine stand around 1930 with a few clumps of 10- to 20-year-old saplings and the upper right a recent photo of a typical ponderosa pine stand today in Monument Canyon Research Natural Area (RNA), Jemez Mountains. The current stand is choked with dense "dog-hair thickets." The bar graph below the photographs shows the age structure (tree-recruitment dates) of more than 400 trees sampled in the Monument Canyon RNA. The horizontal line with vertical tick marks below the bar graph shows the fire dates recorded by widespread fires within the same stand.

These patterns may indicate that the historical variability 1in age structures of southwestern ponderosa pine were characterized by pulses of heavy tree recruitment in particularly favorable years embedded in a background of a more continuous but lower level of tree recruitment. Recent studies have confirmed the importance of the famous "1919 seed year" first identified by Pearson (1933) in the Southwest and have demonstrated the role of warm, wet summers in good ponderosa pine seed germination and seedling survival (Savage et al. 1996). Hence, ponderosa generational groups were a contingent product of climatic variability and fire regime responses in both the presettlement and postsettlement eras. An implication for new forest restoration initiatives in the Southwest is that current ponderosa forests, characterized by trees that germinated in the 1919 seed year, may not be entirely an artifact of grazing and fire suppression, and therefore thinning programs should not necessarily seek to eliminate this cohort as a distinct demographic pulse.

While climate is often a key driver of plant regeneration in the semiarid Southwest, ultimately it is the linked influences of climate, fire regimes (and other disturbances), and land-use histories that determine the demography of plant populations and southwestern vegetation patterns. These interactive effects are demonstrated by the extensive mortality of ponderosa pines and pinyon during the 1950's drought in the Southwest (Betancourt et al. 1993; Allen and Breshears 1998), as the drought effects (climate) were likely exacerbated by competition for scarce water among unusually dense stands of woody plants (a result of modern changes in land use and fire regimes). Also, while a pulse of tree seedlings has established since about 1976 in southwestern forests and woodlands (Swetnam and Betancourt 1998) in conjunction with a recent wet period (associated with an unusual string of El Niņo events), the survivorship and ultimate recruitment of these trees partly depends upon patterns of land use and fire. Monitoring of these current demographic processes and reconstruction of past patterns are needed to fully understand ongoing changes and their historical context.


Several important themes emerge from the illustrations of southwestern environmental change discussed here.

  1. High-resolution, long-term, (and in some cases unique) historical and paleoecological data sets, coupled with diverse, specialized approaches for reconstructing landscape change, are available to detect and explain trends in ecological patterns and processes across southwestern landscapes.

  2. Network approaches are very useful in the development of regional historical data sets which can be utilized to construct land use histories. By aggregating data spatially, observations and inferences can be scaled up from localities to landscapes and entire regions. Development of regional time-series networks provides opportunities to quantify both spatial and temporal variability as a function of scale.

  3. Climatic variability is a key driver of ecological processes; it also modulates human land uses and their effects on southwestern landscapes. Regional climatic signals must be extracted before landscape changes can be attributed to other causes, such as human activities.

  4. All landscapes are historically contingent systems whose structure and dynamics reflect continuous modification of preexisting systems (Brown 1995). Historical data can be used to discriminate between natural and cultural causes of environmental change. Environmental variability and trends have regional and local components. One effective approach to determining causation is to identify synchronous regional responses of biotic systems (which are often climate-driven) and asynchronous, disparate responses observed at local scales (which are often attributable to human land uses and other local disturbances). Additionally, comparison of multiple lines of evidence from different types of ecological reconstructions (e.g., photographs, tree ages, fire scars, cultural histories, climate records) can be the key to identifying causal factors.

  5. Historical data can be used to define and constrain natural ranges of variability, providing important information for management of ecosystems and landscapes (Allen 1994). In the case of wilderness areas and parks, these perspectives may be directly relevant to setting management goals or targets. In other cases, improved knowledge of the origin of existing ecosystem conditions will be the primary value of historical data. Ecosystem management efforts to sustain valued wildland resources (from endangered species to surface water) will benefit from improved knowledge of the patterns and causes of past environmental change.

Much unrealized potential exists to develop detailed land use histories and associated causal narratives in the Southwest. Valuable initiatives would include further regionalization of localized paleoecological data sets (e.g., tree-ring collections), systematic programs to assemble and use repeat photographs (including the extensive aerial photography of the mid-1930's), regional-scale applications of the extraordinary wealth of archeological data present in the Southwest to environmental histories, and the development of regional approaches to monitoring ongoing changes in landscape patterns.

Additional Figures

These figures where created for the original version of this paper, however they were not used in the official publication "Perspectives on the Land Use History of North America: A Context for Understanding Our Changing Environment". They are included here as additional references.


We appreciate the assistance and support of Chris Baisan, Kay Beeley, Hal Malde, Will Moir, Esteban Muldavin, Steve Tharnstrom, Tom Van Devender, the Global Change Program and Biological Resources Division of the U.S. Geological Survey, Bandelier National Monument, and the U.S. Forest Service (Rocky Mountain Forest and Range Experiment Station, Southwest Regional Office, and Santa Fe National Forest). This chapter benefited from review comments by R. Scott Anderson, Tom Sisk, and two anonymous reviewers.

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Drought induced shift of a forest-woodland ecotone: rapid landscape response to climate variation. Proceedings of the National Academy of Sciences USA 95: 14839-14842.

Bahre, C., and M.L. Shelton. 1993.
Historic vegetation change, mesquite increases and climate in southeastern Arizona. Journal of Biogeography 20: 489-504.

Baisan, C.H., and T.W. Swetnam. 1997.
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Biodiversity and Land-use History of the Palouse Bioregion:
Pre-European to Present

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