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Rangelands cover about 40% of the earth's land surface, including vast areas in the western and southwestern United States. These ecosystems are managed primarily to produce forage for wild and domesticated animals. A prevailing mission of the Grassland Protection Unit at GSWRL is to improve understanding of mechanisms that affect the productivity and composition of plants on rangelands. In addressing this mission, research has been directed at understanding why grasses and other herbaceous species are being replaced by woody plants on rangelands in the southwestern U.S. and, indeed, throughout the world. Effects of this change in plant composition on hydrology, carbon storage, and other processes on rangelands are also being studied. Recent research has focused on the suspected role of rising atmospheric carbon dioxide (CO2) concentration in vegetation change, and on the consequences of rising CO2 for plant production and other processes on rangelands.
During the last 200 years, fossil fuel burning and deforestation and other changes in land use have released more CO2 into the atmosphere than has been removed by diffusion into oceans or net uptake by plants. As a result, atmospheric CO2 concentration has risen about 30%, from 275 parts per million (ppm) to about 360 ppm. Continued release of CO2 may swell the atmospheric concentration to twice its current level during the next century. The impact of atmospheric CO2 concentration on vegetation derives largely from its influence on plant carbon balance and water relations. Plants grow by converting CO2 that diffuses into leaves through small openings called stomata into carbon-based compounds, the process of photosynthesis. Inevitably associated with CO2 uptake is loss of water vapor through open stomata, a process called transpiration. In many plants, an increase in CO2 concentration increases photosynthesis and growth, while reducing transpiration, thus allowing plants to produce more growth for a given amount of water. Carbon loss to processes involved in the growth and maintenance of plant tissues (respiration) is reduced by higher CO2 in some species, potentially further benefiting growth. These "primary" responses to CO2 can lead to higher-level responses, including changes in the nutritional value of plants for cattle and other herbivores and shifts in hydrology, carbon storage, and element cycling on rangelands. Because plants sometimes differ in their response to CO2, rising CO2 concentration can also change the species and genetic composition of rangeland vegetation. Invasive shrubs usually respond to higher CO2 with greater growth than the warm-season grasses that once dominated rangelands in the southwestern U.S. These and other differences between shrubs and grasses, together with the near synchrony of woody invasion and the historical increase in atmospheric CO2 concentration, prompted the hypothesis that rising CO2 concentration is contributing to the increase in woody abundance on former grasslands.
At the GSWRL, various approaches are being used to learn more about effects of rising atmospheric CO2 concentration on rangelands in the southwestern U.S. These include:
-controlled environment greenhouses in which CO2 is maintained at the current concentration and elevated levels expected in the future,
-a unique elongated chamber located in a greenhouse in which plants are grown along a continuous gradient in CO2 from the very low concentrations that persisted during the last Ice Age to the present 360 ppm,
-a pair of novel elongated chambers situated on grassland at the GSWRL (Prairie CO2 Gradient, PCG)
