Heat Lamps Strung Above Mountain Meadows for a Global Warming
Experiment Tell the Tale: Flowers, Grasses Lose Out
by Robert Sanders
One likely effect of global warming is the loss of lush high-elevation meadows to the encroachment of dry-adapted shrubs such as sagebrush, according to a team of Berkeley scientists.
The prediction comes from the first realistic test of the effects of increased global temperatures expected next century as a result of rising levels of carbon dioxide and greenhouse gases in the atmosphere.
In the four-year experiment, the Berkeley team simulated a temperature increase by suspending infrared heating lamps above a flowered, grassy Colorado meadow at 9,580 feet elevation on the western slope of the Rocky Mountains.
The findings could apply equally to many high-elevation grasslands, from Europe's Alpine meadows and the altiplano of South America to the Tibetan plateau, said study leader John Harte, professor of energy and resources. In such habitats dry-adapted shrubs could squeeze out grasses and non-woody flowering plants (forbs) in less than a century.
"To get an idea of the effect global warming could have on mountain meadows, imagine the opening scenes of 'The Sound of Music' filmed outside Elko, Nevada," Harte said. "The beautiful flowering forbs that make the Rockies famous will be replaced with a sagebrush cover."
Harte and graduate student Rebecca Shaw reported the first results of their elaborate experiment in the Feb. 9 issue of Science magazine.
While the study confirmed some predictions of current global warming models, it highlighted several of the models' inadequacies. In particular, current models do not account for the fact that some plants can respond to warming in ways that tend to accelerate warming.
"This study is an indication that the climate models we rely on could be greatly underestimating the actual magnitude of the warming we can expect, because they neglect this positive feedback," Harte said. "In the long run we need to know how to put the biology into the climate models, not just the plants but the microorganisms, the beetles, everything, because they all are coupled."
The team set out to simulate the temperature increases expected by a doubling of the amount of carbon dioxide in the atmosphere from pre-industrial times. This level of greenhouse gases, which could come about by the year 2040 if today's upward trends in fossil fuel use and forest burning remain unchecked, corresponds to an increase in global averaged air temperature of from 3 to 8 degrees Fahrenheit (1.4 to 4.5 degrees C), depending on the model, Harte said.
To reproduce this increase in as natural a setting as possible, the team strung up infrared heat lamps over a combined area of 150 square meters (1/25 acre) at a remote research site that is part of the Rocky Mountain Biological Laboratory in Gunnison County, Colo. Other warming experiments have relied on heated wires imbedded in the soil, experiments in greenhouses or laboratory tests, none of which truly mimic actual conditions, Harte said.
The lamps have been on since the summer of 1991, through lightning storm and blizzard, and in winter conditions of as much as six feet of snow. This first report summarizes the effects on vegetation through the summer of 1994.
The study site--a ridge line and a downhill slope ending in a boggy flat--is a patchwork of vegetation typical of the area, including about a hundred species of grasses and forbs. The site also is at the upper elevational range for sagebrush (Artemesia tridentata), which dominates 2,000 feet lower.
The most dramatic effect during four years of heating was a switch from forb dominance to sagebrush dominance in the drier ridge and slope of the plot.
"We saw indications of an invasion by sagebrush, which suggests that with global warming we could see widespread change in the high mountain habitat within 50 to 100 years," Harte said.
Ranchers who graze cattle in the lush parks of the Rockies would likely be one group to feel the impact, since sagebrush is avoided by steers. The implications are global, however, since meadows and grasslands make up a large portion of the earth.
The sagebrush invasion comes about through increased survival of sagebrush seedlings in the warmer earth and a faster rate of growth, Harte said. This may be because sagebrush is more adapted to drier conditions caused by the increased temperature or because sagebrush emerges from the snow in spring before other plants, and thus gets a head start on the growing season. In the wetter areas of the plot, another shrub, a cinquefoil, pushed out the forbs.
"One question that fascinates me is how changes in vegetation will feed back and affect the climate," Harte said. "One thing that determines the climate of a region is how reflective the environment is. Sagebrush absorbs more sunlight than most forbs it replaces, so that sets in motion a positive feedback, causing more warming. This could be a significant effect.
"It seems that when we look at the response of plants to global warming, we see more positive feedback than negative."
The study confirmed several predictions of current warming models, for example that a temperature increase would delay snow coverage one to two weeks in the fall and accelerate melting by one to two weeks in the spring.
The high elevation meadow study is funded by the National Science Foundation, NASA and the Department of Energy.