IMPLICATIONS OF RISING ATMOSPHERIC CARBON-DIOXIDE CONCENTRATION FOR RANGELANDS

Extensive rangelands and other vegetation types that we know today for med while atmospheric carbon dioxide (CO2) concentration was low (50 t o 75% of today's concentration). Fossil fuel burning and deforestation and other land me changes during the last 200 years have increased CO 2 concentration by about 30%, to the present 360 parts per million (pp m). Atmospheric CO2 will continue to rise during the next century, pos sibly to concentrations that are unprecedented for the last several mi llion years. Much of the potential importance of CO2 concentration to vegetation derives from its influence on plant carbon balance and wate r relations. Plants grow by assimilating CO2 that diffuses into leaves through stomatal pores. Inevitably associated with CO2 uptake is tran spirational loss of water vapor through stomata. Transpiration rates u sually decline as CO2 increases, while, in many plants, photosynthesis and growth increase. These ''primary'' responses to CO2 can lead to a multitude of changes at the plant and ecosystem levels, ranging from alteration of the chemical composition of plant tissues to changes in ecosystem function and the species composition of plant communities. T he direct physiological responses of plants to CO2 and expression of t hese responses at higher scales differ among species and growing condi tions. Growth response to CO2 is usually highest in rapidly-growing pl ants that quickly export the carbohydrates formed in leaves and use th em for storage or new growth and allocate a high proportion of fixed c arbon to produce leaves. Growth is also more responsive to CO2 in plan ts with the C-3 (most woody plants and 'cool-season' grasses) than C-4 photosynthetic pathway (most 'warm-season' grasses), These and other differences among species could lead to changes in the composition of rangeland vegetation, but generalizations are difficult. On many range lands, species abundances are determined more by morphological and phe nological attributes that influence plant access to essential resource s like nitrogen and light and reaction to fire, grazing, and other dis turbances than by physiological traits that are sensitive to CO2 conce ntration. Species composition probably will be most responsive to CO2 on moderately water-limited and disturbed rangelands where multiple po sitive effects of CO2 on plant water relations can be expressed and co mpetition for light is minimized. Greatest initial changes in species composition likely will occur on C-3/C-4 grasslands and at the transit ion between grasslands and woodlands. Plant production should also inc rease on water-limited rangelands, but CO2 may have little influence o n production when nutrient elements like nitrogen are severely Limitin g.