Land Use History of North America
This web site serves as an archive to maintain the visibility of the important work done by the LUHNA project team.
|Table of Contents LUHNA Program Project History LINKS Contact Us|
Clues from the Past about our Future
Expanding Agriculture and Population
Night Lights and Urbanization
Patterns in Plant Diversity
Great Lakes Landscape Change
Upper Mississippi River Vegetation
Greater Yellowstone Biodiversity
Southwestern US Paleoecology
Palouse Bioregion Land Use History
Northeastern Forest Dynamics
The LUHNA Book!
Craig D. Allen
Julio L. Betancourt
Thomas W. Swetnam
Craig D. Allen
Julio L. Betancourt
Thomas W. Swetnam
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:
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).
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).
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).
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).
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.
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 http://www.fs.fed.us/news/roads), 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.
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.
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).
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).
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.
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.
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.
Biodiversity and Land-use History of the Palouse Bioregion:
Pre-European to Present