ENVIRONMENT: 21st Century May Belong to the Shrub

By Stephen Leahy Reprint | | Print |

Stephen Leahy

BROOKLIN, Canada, Aug 31 2007 (IPS) - As atmospheric carbon dioxide levels continue to climb shrubs and other woody plants will likely dominate grasslands, altering pastoral lifestyles around the world, a U.S. study has found.

In the first experiment of its kind done on native grassland, U.S. scientists artificially doubled carbon dioxide (CO2) levels over enclosed sections of prairie in Colorado, a state in the western United States, for five years. To their surprise, one shrub species, Artemisia frigida – commonly known as fringed sage – thrived under those conditions. In fact, it grew 40 times faster than normal, dominating other plant species.

“This kind of response to higher CO2 levels is almost unprecedented,” said Jack Morgan, a plant physiologist at the U.S. Department of Agriculture, and lead author of the study, published Aug. 28 in the ‘Proceedings of the National Academy of Sciences’ (PNAS), a science journal.

“Fringed sage is a minor species on the landscape normally. We were not expecting to see this,” Morgan told IPS.

Grasslands of various types cover 40 percent of the world’s land area and provide grazing for domestic livestock. According to 2005 figures from the United Nations Food and Agriculture Organisation, pastoral areas also occupy 40 per cent of Africa’s land mass, and support most of the 235 million cattle on the continent. Cattle cannot eat woody plants, but thrive on grass.

Many parts of the world have experienced an invasion of woody shrubs over the last 100 to 200 years, Morgan added.

At about the same time atmospheric CO2 levels have risen from 280 parts per million (ppm) to 385 ppm today due to the burning of fossil fuels and deforestation. CO2 levels are expected to rise to 500 ppm before mid-century and reach 600 to 700 ppm by 2100.

To test the effects of these future levels of CO2 Morgan and his colleagues set up open-topped cylinders of clear plastic, 15 feet in diameter, on the prairie grasslands. Pure carbon dioxide was pumped into one group of cylinders, maintaining a concentration of 720 ppm.

For five years each plot was analysed at the end of the peak growing season, and then half the vegetation was removed to simulate grazing.

In the first year fringed sage only covered 0.2 percent of the plots on average, but in the fifth year it covered 4.1 percent. Given more time the sage could eventually squeeze out the grasses, Morgan said.

Although the extent of the response was surprising, the fact that shrubs do better with more CO2 is not a new finding, he added.

Shrubs and other woody plants like trees can use CO2 more efficiently than grasses: they have deeper roots than grasses, enabling them to tap into deeper water supplies. These supplies, along with sunlight and CO2, are used to produce glucose energy for growth through photosynthesis.

Many millions of years ago when CO2 levels were in the 600 to 700 ppm range the grasslands and savannahs of the world as we know them did not exist, said Guy Midgley, chief specialist scientist at the Global Change and Biodiversity Programme of the South African National Biodiversity Institute, located in the coastal city of Cape Town.

Most grasses, especially those in temperate and tropical zones, evolved later when CO2 levels had dropped below 300 ppm.

“So, the projected changes in atmospheric CO2 levels alone represent a gigantic step backwards in time, in the space of a few decades,” Midgley told IPS in an e-mail interview.

Nearly all research into the possible effects of higher CO2 levels on forest and crops has been done in the northern hemisphere.

However, some of the ecosystems that are potentially most responsive to CO2 are grasslands and savannahs in the South, Midgley said; these are also some of the biggest ecosystems globally.

The effects of elevated CO2 may already be in play, as indicated by fairly consistent evidence of “bush encroachment” or “shrub encroachment” in many grasslands of the world, he added.

Densities of trees and shrubs in the higher rainfall area of the Kruger National Park in north-eastern South Africa have more than doubled since the 1970s said William Bond, a botanist at the University of Cape Town, in an e-mail interview with IPS.

“I think CO2 is a major driver (of shrub expansion), (but) its effects are not ubiquitous since drier parts of the park showed little change in woody encroachment.”

However, overgrazing and changes in fire regime, a term that refers to the suppression of natural fires or deliberate seasonal burns, are also altering the face of savannahs – and make it difficult to tease out the consequences of higher CO2 concentrations alone for the future of grasslands.

“Bush thickening or encroachment is a major issue across Africa and Australia, and has been strongly linked to rangeland management,” William Stock, director of the Centre for Ecosystem Management at Edith Cowan University in Australia, wrote in an e-mail interview with IPS.

Still, Morgan points out that another U.S. study on grasslands that have never been grazed has also seen increases in woody plants over the past two decades, suggesting that CO2 is indeed playing a role.

“My advice to range managers is to be cautious and aware of the need to carefully conserve their lands,” he adds, warning that grasslands are transforming.

“These changes may affect not only the traditional goods and services we have come to expect from these lands, but may alter their biodiversity as well,” Morgan and colleagues write in the PNAS study.

Their views are echoed by Bond: “Conversion of grassland to thickets is one of the slow, silent disasters for the region (Africa), destroying grazing value of livestock farms and conservation value of some of our most-loved savanna parks.”

And, with environmental change come social shifts.

“Rangelands are important not only for the plant and animal products they provide, but also as regions in which distinct pastoral cultures and societies have developed,” notes the PNAS study.