A newly released study posits a bleak future for the world's grasslands: rapidly escalating levels of carbon dioxide in the atmosphere may prompt the large-scale conversion of grasslands to a landscape of woody shrubs. To reach these findings, Jack Morgan — a plant physiologist at the U.S. Department of Agriculture and the study's lead author — and a team of researchers artificially doubled carbon dioxide levels over enclosed portions of the Colorado prairie to simulate estimated 2100 levels. This large increase led to a drastic rise in the numbers of Artemisia frigida, a woody shrub species known as fringed sage.
The scientists set up open-topped cylinders of clear plastic, 15 ft in diameter, and pumped pure carbon dioxide into one group (maintaining a concentration of 720 ppm) while leaving another at atmospheric concentrations as a control. To simulate the effect of grazing, half the biomass was removed when each group was analyzed every July over a period of 5 years. After that time, they found a 40-fold rise in the biomass of fringed sage — from 0.72 g per sq. m. in the first year to 28.7 g per sq. m. in the fifth. This suggests that, over time, fringe sage and other woody shrubs are likely to take over large swathes of seasonal grasses.Though they acknowledge the study's results, many scientists believe that rising carbon dioxide levels will play a smaller role over the long term when compared with the combined impact of overgrazing and fire suppression (which reduces the amount of woody vegetation destroyed). "The ranchers, through their own practices, have a stronger influence on the landscape than CO2 is likely to have, at least in the foreseeable future. A herd of cows can change the landscape in the course of a week. The CO2 is working over decades," said Jeffrey Dukes, a biologist at UMass, Boston.
Via ::Los Angeles Times: Study links C02 to demise of grazing lands (newspaper)