Embargo Release: 08/13/96 17:00 EDT (US)
Contact: Monte Basgall, email@example.com
'Islands' in the Desert Yield Surprises for Botanists
PROVIDENCE, R.I. -- A four-year study of how rainfall affects desert plants has brought botanists a humbling surprise, emphasizing the lack of understanding about the impact of climate change on the arid landscapes.
In a report delivered at the annual meeting of the Ecological Society of America, the researchers said that winter rainfall in the desert, once believed to be a minor resource for plant growth, may be far more critical than the heavier summer rains. Such summer rains constitute about 75 percent of annual rainfall in the desert.
The research was conducted by James Reynolds, Paul Kemp and David Tremmel of Duke University; Ross Virginia of Dartmouth College, and Amrita DeSoyza of the U.S. Department of Agriculture Experimental Range in Las Cruces, N.M.
The projects are part of a long-term study supported by the National Science Foundation of the Jornada Basin in southern New Mexico. In the Jornada Long-Term Ecological Research Program, researchers are trying to understand the complex factors that caused the basin to change from grassland to desert a century ago. Such studies are critical to understanding the desertification process, which could affect the arid and semi-arid grasslands that cover half of the Earth's land surface.
In other Jornada-related papers delivered at the society meeting, researchers reported studies of the drought-sensitivity of grasses and a comparison of carbon-content of desert versus grassland.
In the new rainfall study, the scientists compared the effects of seasonal rainfall by artificially inducing winter or summer drought conditions in "islands of resources" in the barren desert. Such islands that exist beneath shrubs such as creosote and mesquite consist of higher concentrations of moisture, nutrients, organic matter and microflora that the shrubs create as they grow.
To induce the artificial drought, the scientists installed shelters with clear plastic tops, averaging six feet by nine feet, over young and old plants, either during summers or winters.
Measurements of spring growth in the shrubs revealed that their growth was far more affected by loss of winter rainfall, said Reynolds, a professor of botany.
"We had originally dismissed the importance of winter rainfall in a plant's growth cycle," he said. "Now we suspect that winter rainfall, which is very important in recharging soil moisture deep in the soil, is important to plants. If we excluded rainfall from those shrubs in the winter, the activity of growth in the springtime was seriously affected. However, the summer-droughted shrubs were not that greatly impacted by exclusion of rainfall."
The new insights will be incorporated into computer models of how deserts form, Reynolds said, remedying a clear weakness in the computer programs. But in a broader sense, he said, the discovery emphasizes the lack of knowledge about desert ecology.
"These findings make me somewhat skeptical of our ability to predict the year-to-year variation in how these ecosystems are going to respond to future climate change," Reynolds said. "The global modelers are telling us how increased carbon dioxide is going to affect climate in the next century. But I can't tell you what the Jornada is going to look like next year. Yet we have armies of researchers out there collecting data."
In another paper, Dartmouth Professor of Environmental Studies Ross Virginia, Duke research scientist Mengchi Ho and Reynolds reported studies of the stability of resource islands, even when the shrubs that produced them disappeared. In the study, they analyzed resource islands beneath shrubs that had been killed with herbicide in 1972. By measuring carbon and nitrogen concentrations in the islands, they found that the islands persisted for a surprisingly long time.
In a third paper, Duke graduate student Roberto Fernandez and Reynolds reported results of the first study comparing the drought resistance of grass species that had once covered the Jornada desert. The report represents part of an effort to understand how cattle grazing and drought-sensitivity interacted in the loss of grassland to desert.
"Our idea is that grass species that cattle find most palatable are also the species with the highest potential growth rates and lowest drought resistance," Fernandez.said. "So, the introduction of large herds of cattle a century ago would have resulted in the decline of the most palatable grasses under any weather sequence."
The scientists are growing the eight species of grass in the greenhouses and controlled-environment chambers of Duke's Phytotron. Their preliminary data do indicate that the favorite grasses of cattle do seem to be the most drought-sensitive. According to Fernandez, such studies could offer a look back in time to the era of grassland.
"A lot of work has been done on why desert shrubs are increasing and the consequences of their spread, but surprisingly little work done on grasses in the Jornada area," he said. "We think that comparing how the different grasses do under varied conditions could be a way of going back and reconstructing how past ecosystems were different than current ones."
In another paper, Duke Professor of Botany William Schlesinger and his colleagues used data from the Jornada to cast doubt on proposed schemes -- called "afforestation" -- to lower atmospheric carbon dioxide by planting trees in the desert to take up and store carbon as biomass. Besides Schlesinger, co-authors of the paper were Sean Connin and Ross Virginia of Dartmouth, Laura Huenneke of New Mexico State University and Kevin Harrison of Duke.
Working with previously gathered data, the researchers compared the total carbon -- as biomass and soil organic matter -- found in grasslands and desert shrublands. The idea, said Schlesinger, was to determine whether the growth of woody plants would, indeed, cause the storage of carbon, compared to grassland.
"We found out that as grassland, which has a lot of soil carbon, is replaced with some of these shrub lands that have a lot of biomass carbon in their roots and limbs, you lose soil organic matter and gain biomass carbon, and it's pretty much of a wash," said Schlesinger. Thus, he said, the analysis does not support the fundamental tenet of afforestation. Nor does the afforestation concept hold up in the face of the realities of desertification, he said.
"Most desertification worldwide is driven by human population growth at the edges of deserts, and the last thing those humans are going to be able to do is plant trees," he said. What's more, Schlesinger said, afforestation could add to the carbon dioxide load, given the need to burn fossil fuel to irrigate land and plant trees, and the fact that groundwater for irrigation would release dissolved carbon dioxide when brought to the surface.