"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."

• Introduction
• The Nevada Desert FACE Facility
• Responses of a desert ecosystem to elevated CO₂
• Response of an invasive grass
• Implications for desertification
• References
• Author information
• Additional web resources

Introduction

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The human species has had a tremendous impact on the earth's environment. One of the most significant effects of anthropogenic activities is that on the composition of the earth's atmosphere through increases in atmospheric content of carbon dioxide (CO₂) and other trace gases (Vitousek et al. 1997). Except for human land use, no type of global change has been documented to be more substantial and rapid than the increase in atmospheric CO₂ content. From the beginning of the industrial age (ca. 1850) until today, atmospheric CO₂ concentration has risen from approximately 280 to 360 ppm (Keeling and Whorf 1990), a 30% rise in just the last 140 years. This rise is continuing, with a doubling of current-day CO₂ concentration projected to occur by the middle of this century. Increased CO₂ represents the most important human enhancement to the greenhouse effect. The consensus of the climate research community is that it has already had a detectable influence on the earth's climate (Stott et al. 2000; Levitus et al. 2001), and will further drive substantial climate change during the 21st century (Schneider 1992).

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.

The Nevada Desert FACE Facility

Thumbnail link to photo of stand pipes

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The Nevada Desert FACE Facility (NDFF) is a state-of-the-art research facility at the Nevada Test Site, located within the Mojave Desert of southern Nevada. FACE stands for Free Air CO₂ Enrichment, and is a technology that allows us to increase the CO₂ content within plots of an intact ecosystem while keeping all other environmental conditions natural (Hendrey and Kimball 1994). It does this through a ring structure of "stand pipes" from which air mixed with elevated CO₂ is continuously released on the upwind side of a plot of intact desert vegetation. FACE technology avoids the many problems that are associated with elevated CO₂ chambers and greenhouses and allows direct extrapolation of potential ecosystem responses to future global environmental conditions. The NDFF is the only FACE facility currently operating in a natural ecosystem: other FACE sites occur in agricultural fields, meadows, and managed forests.

Thumbnail link to photo of elevated walkway at FACE facility

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.

Responses of a desert ecosystem to elevated CO₂

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We have now been continuously exposing intact plots of a Mojave Desert ecosystem to elevated CO₂ since April 1997. During the 1997-1998 hydrologic year (October to September in the Mojave Desert), the NDFF experienced a pronounced El Niño cycle during which total precipitation was more than twice as great as the long-term average. In contrast, the 1998-1999 and 1999-2000 hydrologic years were significantly drier than the long-term average. This resulted in three contrasting growth cycles: a wet year in 1998, a drought year in 1999, and a moderately dry year in 2000. We found that plants responded to elevated CO₂ in very different ways in those three years, particularly the first two (Smith et al. 2000). In 1998, shrub growth and production was about twice as great at elevated CO₂ as it was at ambient CO₂ in both the evergreen and deciduous shrubs, whereas there was little difference in shrub growth between CO₂ treatments in the drought year of 1999. Similarly, there was an explosion of annual plants in the El Niño year, and total productivity and seed rain of annuals was more than two-fold greater at elevated CO₂. In 1999, there was no germination of annuals at either CO₂ treatment concentration. In 2000, there was again reasonably good germination of annuals, but the lack of spring rains resulted in a shortened growing season and a much less pronounced CO₂ stimulation of growth and seed production.

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.

Response of an invasive grass

Thumbnail link to photo of red brome growth

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In the El Niño year of 1998, a large number of annual plants germinated at the NDFF during the winter months. Most of these species were native annuals, but one of the most abundant annuals on the site was red brome (Bromus madritensis ssp. rubens), an exotic annual grass that has invaded many areas of the Mojave and Sonoran Deserts. Consistent with the stimulatory effects of elevated CO₂, both red brome and the native annuals showed higher individual plant size. However, of interest was the fact that red brome also showed higher plant density at elevated CO₂, whereas the native species had lower density. Combining these two responses, we observed red brome to have substantially higher total plant biomass and seed production at elevated CO₂, and this increase was much more pronounced than what we observed for the native species (Smith et al. 2000). However, individual seed mass and nitrogen content decreased at elevated CO₂ in red brome (Huxman et al. 1999), a phenomenon that did not occur in the native species. This reduction in seed quality for red brome at elevated CO₂ results in a pronounced decline in growth rate of subsequent seedlings (Huxman, Hamerlynck, Jordan, Salsman, and Smith 1998). Therefore, red brome appears to respond to elevated CO₂ with the production of many more seeds, but those seeds are of lower quality. How this will influence the future success of red brome is uncertain, and is the focus of ongoing studies by our research group.

Thumbnail link to photo of burn scar from red brome fire

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.

Implications for desertification

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At the outset of these studies, we hypothesized that elevated CO₂ would act to increase the efficiency with which desert plants use water, which should result in greater water retention in soils and greater total plant growth and production. Because deserts are water-limited systems with low annual primary production, and the desertification process has been defined as one in which formerly productive lands are converted into low-productivity deserts, it seems reasonable to assume that elevated CO₂ may alleviate or even reverse the desertification process by enhancing plant production in arid and semiarid lands.

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.

References

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Bowes, G. 1993. Facing the inevitable: Plants and increasing atmospheric CO₂. Annual Review of Plant Physiology and Plant Molecular Biology 44:309-332.

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.