Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on life and ecosystems

Land Use History of North America (LUHNA):
Repeat Photography


Craig D. Allen

U.S. Geological Survey, Midcontinent Ecological Science Center

Julio L. Betancourt

U.S. Geological Survey, Desert Laboratory

Thomas W. Swetnam

University of Arizona, Laboratory of Tree-Ring Research

LUHNA - 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 (Malde 1973). 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 U.S. (Phillips 1963, Hastings and Turner 1965, Turner and Karpiscak 1980, Rogers 1982, Dutton and Bunting 1982, Gruell 1983, Humphrey 1987, McGinnies et al. 1991, Veblen and Lorenz 1991, Webb 1996). Rephotography in the southwest has focused on key ecological concerns relevant to management of public lands, including shrub and tree encroachment upon grasslands (Figs. 6-9), climatic effects on demographic trends in woodlands, post-disturbance histories (Figs. 10-11), and geomorphic, hydrologic and vegetation changes in riparian areas (Figs. 12-19, Figs. 20-21).

Figure 20 & 21 - Downstream view of the confluence of the west branch of the Santa Cruz River in Tucson, looking northeast from 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.

In the Southwest, the process of desertification has involved expansion of desert shrubs and trees into former grasslands (Figs. 6-9). 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/livestock grazing (Grover and Musik 1990) to interdecadal climatic variability (Neilson 1986, Betancourt 1996, Swetnam and Betancourt - in review) and most recently, to CO2 enrichment shifting the balance from C4 grasses to C3 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 1950s drought (McCormick and Galt 1994).

One of the most remarkable changes in Southwestern landscapes involved late 19th-early 20th century channel entrenchment (Figs. 12-17). 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, be they 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, with recent summaries furnished by Cooke and Reeves (1976) and Graf (1983). In many respects, the various explanations for why arroyos happened have reflected professional interests. Range managers have been quick to suggest removal of plant cover by livestock, whereas climatologists have naturally looked to the skies for an explanation. The geologist, accustomed to 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 streamside vegetation with groundwater withdrawal (Figs. 16-17) and urbanization (Figs. 18-19, Figs. 20-21).

Aerial Photography

Aerial photography and other remote sensing approaches (e.g., satellite imagery) provide powerful means to determine widespread changes in landscape patterns through time, especially when used in concert with geographic information systems (Sample 1994). Aerial photography was flown across most of the Southwest in the mid-1930's, providing a baseline from which modern landscape changes can be assessed. Ground-based evidence such as tree ages and soil patterns indicate that during this century conifer trees have greatly encroached upon ancient montane grasslands in the Jemez Mountains (Allen 1989).

Figure 22 - 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) determined from aerial photographs.

Aerial photographs confirm these observations and reveal the extensiveness of the tree invasion (Fig. 22), which reduced the area of open montane grasslands by 55% between 1935 and 1941 in the mapped area. The tree invasion is tied to changes in land use history, primarily livestock grazing and fire suppression (Allen 1989).

Changes in road networks through time reflect land use history, as illustrated in this Jemez Mountains example. Total road density in 1935 (Fig. 23) 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 roads provided access to ranches, mines, and some timber operations. Large portions of the Jemez area remained roadless.

Figures 23 & 24
(Left image) Map of all roads visible in 1935 aerial photographs across 187,858 hectares around Bandelier National Monument, in the Jemez Mountains, New Mexico. The current Monument boundaries are shown.

(Right image) Map of all roads visible in 1981 aerial photographs across 187,858 hectares around Bandelier National Monument, in the Jemez Mountains, New Mexico. "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 (the "Chili Line") was still in operation through the eastern edge of the map area, with 18.9 km of track. This stretch of track was completed between 1880 and 1886, and abandoned in 1941. This important connection between the Jemez Mountains and the outside world markedly altered land use patterns in this area (Allen 1989, Rothman 1992). The improved linkages to outside markets provided by railroads allowed the tremendous increase in livestock numbers that occurred throughout the Southwest in the late 1800's (Wooton 1908), which precipitated key landscape changes such as vegetation transformations and altered fire regimes.

By 1981 (Fig. 24) the length of mapped roads increased nearly 12-fold, from 719 km in 1935 to 8433 km 1981. Paved roads had not yet reached the Jemez Mountains in 1935, but by 1981 450 km of paved roads had been built. 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 early 1970's (e.g., the striking spiral patterns of dirt roads observed in the northwest quadrant of Fig. 24). The largest remaining roadless tract was the designated wilderness areas in and adjoining Bandelier. Estimated total area of road surfaces grew from 0.13% of the map area in 1935 (247 ha) to 1.67% in 1981 (3132 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). Roads can have many ecological effects, ranging from habitat fragmentation (Reed et al. 1996), reduced landscape productivity through the direct conversion of roadways into compacted and little vegetated surfaces (McGurk and Fong 1995), provision of routes for the spread of non-native weeds (Tyser and Worley 1992), accelerated erosion rates, and increased stream sediment loads (Eaglin and Hubert 1993). 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 (Reijnen et al. 1995, McLellan and Shackleton 1988). Overall, road networks often provide distinctive landscape signatures of the histories and ecological effects of human land uses.


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