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Title: IMPLICATIONS OF RISING ATMOSPHERIC CARBON DIOXIDE CONCENTRATION FOR RANGELANDS

Author
item Polley, Wayne

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: The extensive rangelands that we know today were formed while the concentration of carbon dioxide in Earth's atmosphere was low. Carbon dioxide concentration has increased by about 30% during the last 200 years and will continue to rise during the next century, possibly to twice the current level. Plants grow by converting the gaseous carbon dioxide that diffuses into leaves through stomatal pores into compounds used to make plant tissues. Many plants, therefore, will grow faster as carbon dioxide concentration rises. Others will grow at the same rate, but require less water to do so. These and other effects of rising carbon dioxide concentration could affect rangeland vegetation. Growth increases more in shrubs and most cool-season grasses than in warm- season grasses when carbon dioxide concentration is increased, implying that shrubs may increase at the expense of warm-season grasses during the next century. Greatest initial changes in abundances of different plant species likely will occur on rangelands with both warm-season and cool-season grasses and at the transition between grasslands and woodlands. Plant growth and, hence, forage production for livestock should also increase, especially on moderately dry and disturbed rangelands where benefits of higher carbon dioxide are greatest. In contrast, atmospheric change will have little or no influence plant growth when temperatures are low or nutrient elements like nitrogen are severely limiting.

Technical Abstract: The extensive grasslands that we know today formed while atmospheric carbon dioxide (CO2) concentration was low (50 to 75% of today¿s concentration). Fossil fuel burning, deforestation and other land use changes during the last 200 years have increased CO2 concentration by about 30%. Atmospheric CO2 will continue to rise during the next century, possibly to concentrations that are unprecedented for the last several million years. Growth response to CO2 is usually highest in rapidly-growing plants that quickly export the carbohydrates formed in leaves and use them for storage or new growth and allocate a high proportion of fixed C to produce leaves. Growth is also more responsive to CO2 in plants with the C3 (most shrubs and ¿cool-season¿ grasses) than C4 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. Species composition probably will be most responsive to CO2 on moderately water- limited and disturbed rangelands where multiple positive effects of CO2 on plant water relations can be expressed and competition for light is minimized. Greatest initial changes in species composition likely will occur on C3/C4 grasslands and at the transition between grasslands and woodlands.