No. 49, May/June 2001
Linkages between Climate Change and Desertification
by Stanley D. Smith and Travis E. Huxman
"Clearly, global change will present us with interesting scientific and political challenges in the future, and the deserts of the southwestern U.S. may be a particularly responsive system to global change."
(Back to top)
In addition to its effect on climate, increased atmospheric CO2 concentration has direct and relatively immediate effects on two important physiological processes in plants it increases photosynthetic rate, but decreases stomatal opening and therefore the rate at which plant leaves lose water (Bowes 1993). In combination increased photosynthesis and decreased water loss plants have been shown to significantly increase water-use efficiency (WUE, the ratio of carbon gain per unit water lost).
Increased WUE at elevated CO2 is anticipated to be particularly important in water-limited environments because it will allow plants to maintain larger leaf canopies or to maintain photosynthesis and growth longer into dry seasons (Smith et al. 1997). This occurs primarily as a result of compounding leaf-level water savings through an individual plant and, within a plant community, improving water balance at a number of scales. Such changes would stimulate greater annual production and store more carbon in dryland soils. Additionally, this may result in the expansion of plants into currently non-vegetated areas.
Using this logic, several conceptual models have predicted that water-limited ecosystems such as deserts will respond more strongly to elevated CO2 than will other ecosystem types. For example, Melillo et al. (1993) predicted that deserts would increase in annual primary (plant) production by 50-70% in response to a doubling of atmospheric CO2 concentration, whereas most forests will exhibit less than a 20% increase in production. However, deserts are both water- and nutrient-limited systems (Smith et al. 1997), so it is not clear what effects increased growth at high CO2 will have on already limiting supplies of soil nutrients. Unfortunately, data on the responses of deserts to global change scenarios, with which the above predictions could be evaluated, are almost completely lacking.
Deserts account for more terrestrial surface area than any other major biome-type, and they are showing, through the process of desertification, the most rapid increase in surface area (Dregne 1991). Understanding and predicting the response of dryland ecosystems to future atmospheric and climate scenarios is a high priority given that desert regions are experiencing considerable biological change from the combined effects of urbanization, water harvesting, overgrazing by domestic livestock, invasion of alien species, and other anthropogenic disturbances. The purported beneficial effect of elevated CO2, that of increasing plant growth in water-limited habitats, may have the long-term effect of actually reversing the desertification process, particularly if land use practices that are at the root of the desertification process are relaxed. From an applied perspective, these beneficial aspects of global change could have important implications for restoration efforts aimed at reclaiming disturbed or salinized desert lands. This is especially true if the effects of increasing CO2 are such that they promote the growth and survival of plants in seedling stages, which appears to be likely (Huxman, Hamerlynck, Loik and Smith, 1998).
Until recently, most of the experimental techniques available to test these predictions have involved growing plants in containers (e.g., pots) in controlled environment facilities (e.g., growth chambers, greenhouses) at elevated CO2. In addition to the problems associated with growing plants in artificial environments, usually at lower light and higher humidity than they experience in the desert, scientists cannot accurately extrapolate those results to intact ecosystems where multiple environmental factors are constantly varying and affecting plants in complex ways. Therefore, the best way to examine how a desert will respond is to expose an intact ecosystem to elevated CO2 concentrations and observe the system over time (Mooney and Koch 1994). This is why a team of scientists in the U.S. state of Nevada created the Nevada Desert FACE Facility in 1997.
(Back to top)
The NDFF consists of nine study plots (three FACE rings at elevated CO2 concentration (550 parts per million [ppm], a level predicted to occur in the atmosphere by the year 2050), three FACE rings at ambient CO2 concentration (360 ppm), and three non-FACE apparatus control plots (each 23 m in diameter). For a full description of the NDFF, see Jordan et al. (1999). The array of study plots is located on a broad bajada (alluvial fan) in vegetation that is dominated by creosotebush (Larrea tridentata) and white bur-sage (Ambrosia dumosa), with several other co-dominant species of deciduous shrubs and perennial bunchgrasses. Because the Nevada Test Site has been free from grazing by domestic livestock for about 50 years, soils at the site have well-developed biological soil crusts made up of bacteria, algae, mosses and lichens. Such crusts are able to fix atmospheric nitrogen directly and thus are an important source of nitrogen to this nutrient-limited ecosystem. Because of delicacy of these crusts, we designed a rotating walkway system at the NDFF so that researchers can access all points in the plots without disturbing soil. This will enable us to preserve the physical structure of the plots throughout this anticipated long-term experiment.
(Back to top)
These results have potentially important implications for the structure and function of desert ecosystems. These ecosystems have been characterized as "pulse-reserve" systems, in which a pulse such as a large rainfall event stimulates the activity of inactive biological reserves such as seeds and dormant plants and animals (Noy-Meir 1973). These organisms then have a flurry of activity until the resources associated with the pulse are used up and the organisms return to inactive, reserve status as seeds or as a dormant stage. As a result, deserts are much more highly episodic systems than are systems such as forests and grasslands, where water is less limiting.
By enhancing plant growth most strongly when water and nutrients are most available (i.e., in a wet year), elevated CO2 appears to increase an already high level of year-to-year variability in biological activity in this desert system. This contrasts with our original expectation of how this system would respond to elevated CO2. We hypothesized that the enhanced WUE effect of elevated CO2 would ameliorate the effects of dry seasons and therefore reduce the year-to-year variation in plant production, but our early results suggest just the opposite. Perhaps the effects of drought are so pronounced in this desert that elevated CO2 has little influence on plant physiological processes during drought conditions, in which case we will see an increase in episodic behavior of this already episodic system.
The outlook, based on this understanding of year-to-year variability, suggests that the Mojave Desert will be more "desert-like" in a future high CO2 environment. In other words, the plants and animals that evolved to deal with the "pulse-reserve" nature of deserts may continue to be at an advantage relative to other vegetation types. We are hoping to continue our studies at the NDFF for several more wet-dry cycles to see if this indeed will be the case.
(Back to top)
Red brome is closely related to cheatgrass (Bromus tectorum), another exotic annual grass that has invaded much of western North America. Cheatgrass is well known to be able to out-compete seedlings of native perennial grasses, and to initiate a fire cycle that further ensures its success on disturbed rangelands. Similar to our findings with red brome at the NDFF, controlled-environment studies have found cheatgrass to be more responsive to elevated CO2 than are native grasses (Smith et al. 1987). What these studies suggest is that these exotic brome grasses may be more responsive to elevated CO2 than are native annuals and grasses, which could further enhance the success of a group of highly invasive species in western North America.
(Back to top)
However, our results from the NDFF provide evidence that contradicts that hypothesis. First, we observed even greater year-to-year variation in production cycles at elevated CO2, suggesting that this system may become even more episodic, and thus "desert-like," in a future high-CO2 world. But of even greater potential significance, we observed an exotic annual grass to be the most responsive species to elevated CO2. If this differential stimulation continues over multiple wet-dry cycles, it could expose this system to an accelerated fire cycle to which the Mojave Desert is not adapted.
We are already observing range fires in the southern Mojave and northern Sonoran Deserts that are caused primarily by red brome, and this cycle could accelerate as atmospheric CO2 concentrations continue to rise. The prospect of an accelerated fire cycle in this region, which would convert long-lived shrublands to low-diversity annual grasslands dominated by exotic species, is a form of desertification that could have profound implications for the ability of these deserts to provide the ecosystem services upon which a growing population increasingly depends.
Clearly, global change will present us with interesting scientific and political challenges in the future, and the deserts of the southwestern U.S. may be a particularly responsive system to global change. Whether or not elevated CO2 and associated climate change will ultimately have positive or negative impacts on these desert systems remains to be seen. It is clear that anthropogenic forces that influence disturbance or non-native species introductions will be an important interacting variable with rising CO2 concentrations, and may ultimately dictate ecosystem consequences. Because we remain uncertain as to how the Mojave and Sonoran Deserts may respond long-term to elevated CO2, we are hoping to continue our high-CO2 studies for many more years in order to account for the many complex feedbacks inherent in desert ecosystems.
(Back to top)
Dregne, H.E. 1991. Global status of desertification. Annals of the Arid Zone 30:179-185.
Hendrey, G.R., and B.A. Kimball. 1994. The FACE program. Agricultural and Forest Meteorology 70:3-14.
Huxman, T.E., E.P. Hamerlynck, D.N. Jordan, K.J. Salsman, and S.D. Smith. 1998. The effects of parental CO2 environment on seed quality and subsequent seedling performance in Bromus rubens. Oecologia 114:202-208.
Huxman, T.E., E.P. Hamerlynck, M.E. Loik, and S.D. Smith. 1998. Gas exchange and chlorophyll fluorescence responses of three southwestern Yucca species to elevated CO2 and high temperature. Plant, Cell and Environment 21:1275-1283.
Huxman, T.E., E. P. Hamerlynck, and S.D. Smith. 1999. Reproductive allocation and seed production in Bromus madritensis ssp. rubens at elevated atmospheric CO2. Functional Ecology 13:769-777.
Jordan, D.N., S.F. Zitzer, G.R. Hendrey, K.F. Lewin, J. Nagy, R.S. Nowak, S.D. Smith, J.S. Coleman, and J.R. Seemann. 1999. Biotic, abiotic and performance aspects of the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility. Global Change Biology 5:659-668.
Keeling, C.D., and T.P. Whorf. 1990. Atmospheric CO2 concentrations, Mauna Loa. In Trends '90: A compendium of data on global change, ed. T.A. Bodem, P. Kanciruk, and M.P. Farrell, 8-9. Oak Ridge, Tenn.: Oak Ridge National Lab, Carbon Dioxide Information Analysis Center.
Levitus, S., J.I. Antonov, J. Wang, T.L. Delworth, K.W. Dixon, and A.J. Broccoli. 2001. Anthropogenic warming of earth's climate system. Science 292:267-270.
Melillo, J.M., A.D. McGuire, D.W. Kicklighter, B. Moore III, C.J. Vorosmarty, and A.L. Schloss. 1993. Global climatic change and terrestrial net primary production. Nature 363: 234-240.
Mooney, H.A. and G.W. Koch. 1994. The impact of rising CO2 concentrations on the terrestrial biosphere. Ambio 23:74-76.
Noy-Meir, I. 1973. Desert ecosystems: environment and producers. Annual Review of Ecology and Systematics 4:51-58.
Schneider, S.H. 1992. The climatic response to greenhouse gases. Advances in Ecological Research 22:1-32.
Smith, S.D., T.E. Huxman, S.F. Zitzer, T.N. Charlet, D.C. Housman, J.S. Coleman, L.K. Fenstermaker, J.R. Seemann. and R.S. Nowak. 2000. Elevated CO2 increases productivity and invasive species success in an arid ecosystem. Nature 408:79-82.
Smith, S.D., R.K. Monson, and J.E. Anderson. 1997. Physiological ecology of North American desert plants. Berlin: Springer-Verlag.
Stott, P.A., S.F.B. Tett, G.S. Jones, M.R. Allen, J.F.B. Mitchell, and G.J. Jenkins. 2000. External control of 20th century temperature by natural and anthropogenic forcings. Science 290: 2133-2137.
Vitousek, P.M., H.A. Mooney, J. Lubchenko, and J.M. Melillo.1997. Human domination of earth's ecosystems. Science 277:494-499.
(Back to top)
Dr. Stanley D. Smith
Department of Biological Sciences
University of Nevada, Las Vegas
Las Vegas, NV 89154-4004
Dr. Travis E. Huxman
Dept. of Ecology & Evolutionary Biology
University of Arizona
Tucson, AZ 85721
(Back to top)
Nevada Desert FACE Facility
While the following web sites do not specifically deal with FACE technology, they may be of interest to readers who would like to know more about global change research in the US Southwest:
Preparing for climate change: The potential consequences of climate variability
and change http://www.ispe.arizona.edu/research/swassess/
This web site presents a Report of the Southwest Regional Assessment Group for the U.S. Global Change Research Program.
Semi-Arid Land-Surface-Atmosphere (SALSA) Program
SALSA is a multi-agency, multi-national global-change research effort that seeks to evaluate the consequences of natural and human-induced changes in semi-arid environments. Current SALSA research is focused on the upper San Pedro River basin in southeastern Arizona, USA, and northeastern Sonora, Mexico.
Global Change and the Dynamics of Plant Communities in the Chihuahuan Desert
of Southern New Mexico
This project is intended to construct a framework for predicting and detecting early responses to various predicted climate change scenarios from investigations on regional vegetation dynamics in the Chihuahuan Desert of southern New Mexico.
About the Arid Lands Newsletter