27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

Climatic Fluctuations and Desert Vegetation Response in the Southwestern United States

Robert H. Webb¹, Raymond M. Turner², Kathryn A. Thomas³, Todd C. Esque⁴, and Kristin H. Berry^5
1U.S. Geological Survey, 1675 W. Anklam Road, Tucson, AZ 85745; 
²Desert Laboratory, 1675 W. Anklam Road, Tucson, AZ 85745; 
³U.S. Geological Survey, Colorado Plateau Research Station, Northern Arizona; 
⁴University, P.O. Box 5614, Flagstaff, AZ 86011-5614; 
⁵U.S. Geological Survey, 6770 S. Paradise Road, Las Vegas, NV 89119; 
⁶U.S. Geological Survey, 6221 Box Springs Blvd, Riverside, CA 92507-0714

The twin concepts of preservation and restoration typically require either an assumption of an unchanging undisturbed ecosystem or that changes induced by climate or human land-use practices are predictable. Long-term monitoring ("vital signs") is being instituted for many desert parcels managed by federal and state governments with the intent of detecting change and responding with alteration in management practices. Other common concerns in the Southwest are the "degradation" of desert grasslands by establishment of woody shrubs and trees (Wilson et al. 2001); the downslope movement of junipers onto rangelands (Rogers 1982); the reported loss of riparian ecosystems (Webb et al. 2001a); and whether anthropogenically-driven directional change is occurring in desert ecosystems (Turner et al. in press).

Directional changes in species composition of Southwestern rangelands have been documented throughout the 20th century. Change has occurred irrespective of land-use practices (Turner 1990; Webb 1996). In all cases, woody vegetation with C3 photosynthetic pathways have expanded, sometimes at the expense of C4 grasses (Wilson et al. 2001; Turner et al. in press) but in other cases into previously unoccupied habitat (Webb 1996). Turner et al. (in press) have found large increases in biomass depicted in repeat photography of the Sonoran Desert, particularly between the 1960s and 1990s. The conversion of rangeland to woody C3 species has continued despite overall reduction in livestock grazing intensity in the latter part of the 20th century. Despite real concerns about removal of wetlands, riparian vegetation has dramatically increased - not decreased - in most riverine habitats of the Southwest, particularly after 1960 (Webb et al. 2001; Turner et al. in press). Biomass in common Mojave Desert ecosystems (particularly ones dominated by Larrea tridentata) have increased by a factor of 2-4 on the Nevada Test Site (Webb et al. 2001b). The expansion of C3 species appears to cross ecological types in the Southwest and northern Mexico and may result from the three interrelated effects of (1) increased winter precipitation that occurred particularly after 1975 (Wilson et al. 2001; Turner et al. in press); (2) increased winter temperatures, particularly minima (Webb 1996); and (3) increased atmospheric CO2 (Bazzaz 1990). Irrespective of the reason, considerable amounts of carbon have been stored in C3 plants in the region.

No attempts have been made to estimate the amount of carbon sequestered in plant biomass over the region using anything remotely approaching an ecosystem-based technique. Several methods can be used to quantify density, cover, and (or) biomass increases, but usually these methods are applied to a few small, permanent plots (Beatley, 1980; Goldberg and Turner, 1984; Webb et al., 2001b). Permanent plots can be used to quantitatively document long-term vegetation changes, but their number, size and local conditions at the plots may reduce their regional representation. Systems of regional plots, which typically are designed to monitor single-species changes, are one way to scale plot data to larger land areas (Pierson and Turner, 1998).

Several large-scale methods have been used to assess the magnitude of long-term ecosystem changes in the Southwest. Satellite instruments are the most common tools for global and regional assessments of change from 1974 to the present (Kepner et al., 2000), but this technique can only detect very broad habitat change. Aerial photography is commonly used to assess changes in vegetation cover and is available over most of the Southwest from 1935 to the present, but like satellite-based imagery, aerial photogrammetry is limited to assessment of broad changes. In contrast to aerial images, repeat photography can be used to assess changes in plant size, density and species dating back more than a century, and historical photography is widespread in the region. Large numbers of repeat photographs, and particularly time series photography at specific locations, are required to offset the lack of spatial coverage in each image.

Fig. 1. Locations of repeat photography camera stations in the southwestern United States and northern Mexico.

The Desert Laboratory Repeat Photography Collection, combined with related efforts in the region (Fig. 1), provides an ideal technique for assessing carbon sequestration in arid and semiarid ecosystems. At present, the aggregate collection contains 5,795 views in most of the region's ecosystems (Table 1), excluding higher-elevation forests. The motivation for building this collection, which was done over a 40-year period by USGS and Mexican-government scientists, was to document long-term landscape changes in the desert regions, including changes in plant biomass. We have documented that the amount of carbon sequestered in arid and semiarid ecosystems has increased in the 20th century in most of the ecosystems in the southwestern United States, particularly since 1960. This biomass increase has occurred despite development and land-use pressures on ecosystems such as grazing and water developments. Increases in winter precipitation and temperatures, as well as increases in atmospheric carbon dioxide, are the probable reasons why biomass has increased in nearly all the vegetation alliances of the region.

The predicted shift to a warmer and drier future climate is based on an analogy to conditions between about 1942 and 1975 in the southwestern United States (Schmidt and Webb 2001). Because our photographs closely span this period, we have some basis for speculation on how the predicted climate shift may affect carbon sequestration. Increased winter temperatures might increase winter growth and even benefit C3 seedling establishment. However, pressures on surface-water and ground-water systems will largely drive overall changes in riparian ecosystems, and lack of winter floods and decreased winter precipitation will likely minimize recruitment along natural watercourses and across the desert in general. The recent increases in C3 plants, particularly in desert grasslands, may slow. Drought pruning will probably increase, but carbon storage in the ecosystem may not change much because decreases in soil moisture may decrease the rate of litter decomposition. If severe droughts occur, populations of certain species - particularly members of the Chenopodiaceae - may be deleteriously affected, which could change the nature of some vegetation alliances in the region.

Figure 2. Long-term precipitation changes in the upper Sonoran Desert (from Turner et al. in press).

References Cited

Bazzaz, F. A. 1990. The response of natural ecosystems to the rising global CO2 levels. Annual Review of Ecology and Systematics 21:167-196. 

Beatley, J. C. 1980. Fluctuations and stability in climax shrub and woodland vegetation of the Mojave, Great Basin and transition deserts of southern Nevada. Israel Journal of Botany 28: 149-168. 

Belnap, J., Webb, R. H., and Weisheit, J. 2003. We call this Cataract Canyon. University of California Press, Berkeley, in press. 

Goldberg, D. E., and Turner, R. M. 1984. Vegetation change and plant demography in permanent plots in the Sonoran Desert. Ecology 67: 695-712. 

Hastings, J. R., and Turner, R. M., 1965. The changing mile. University of Arizona Press, Tucson. 317 pp. 

Kepner, W. G., Watts, C. J., Edmonds, C. M., Maingi, J. K., Marsh, S.E., and Luna, G., 2000. A landscape approach for detecting and evaluating change in a semi-arid environment. Environmental Monitoring and Assessment 64: 179-195. Pierson, E. A., and Turner, R.M.. 1998. An 85-year study of saguaro (Carnegiea gigantea) demography. Ecology 79: 2676-2693. 

Rogers, G. 1982. Then and now, a photographic history of vegetation change in the central Great Basin. University of Utah Press, Salt Lake City. 152 pp. 

Schmidt, K. M., and Webb, R. H. 2001. Researchers consider U.S. Southwest's response to warmer, drier conditions. EOS 82: 475-478. 

Turner, R. M. 1990. Long-term vegetation change at a fully protected Sonoran Desert site. Ecology 71: 464-477. 

Turner, R. M., Webb, R. H., Bowers, J. E., and Hastings, J. R. 2003. The Changing Mile Revisited: University of Arizona Press, Tucson, in press. Webb, R. H. 1996. Grand Canyon, a century of change: Tucson, University of Arizona Press. 290 pp. 

Webb, R. H., Boyer, D. E., and Berry, K. H. 2001a. Changes in riparian vegetation in the southwestern United States: Historical changes along the Mojave River, California. U.S. Geological Survey Open-File Report OF 01-245, 1 sheet. 

Webb, R. H., Esque, T. C., Medica, P.A., DeFalco, L. A., and Murov, M.A. 2001b. Monitoring of ecosystem dynamics in the Mojave Desert: the Beatley permanent data. U.S. Geological Survey Fact Sheet FS-040-01, 4 p. 

Wilson, T. B., Webb, R. H., and Thompson, T. L. 2001. Mechanisms of range expansion and removal of mesquite (Prosopis spp.) in desert grasslands in the southwestern United States. U.S. Forest Service, Tempe, Arizona. General Technical Report 01-37. 23 pp.