DURHAM, N.C. -- A team of botanists is untangling the subterranean travels of plant roots by using DNA analysis to identify samples of roots that have pushed their way into caves. The scientists' insights into this topsy-turvy "underground forest" will contribute not only to a better understanding of the strategies plants use to seek life-giving water and nutrients, but also to improved ecosystem models of how vegetation and climate might interact in a future of global change.
The researchers, led by Robert Jackson of Duke University and Randall Linder of the University of Texas, Austin, published their latest findings in the Sept. 28 issue of the Proceedings of the National Academy of Sciences.
In analyzing the DNA of roots taken from 21 caves in central Texas, they reported the rooting depths of six tree species live oak, shin oak, ash juniper, hackberry, American elm and cedar elm. While most of the species grew roots to about 15 feet, the root-depth winner proved to be the only evergreen live oak in the system, which extends its roots up to 75 feet into the ground. The live oak might grow deeper roots, said the scientists, because it needs a more reliable water supply to maintain its leaves year-round.
"Plants have a below-ground structure that's just as critical to their functioning and survival as the above-ground structure," Jackson said. "And since the lateral range of a plant's roots almost always extends beyond the above-ground canopy, many different plants usually compete for resources. We need to understand this below-ground structure and the functional consequences of roots at different depths.
"Until now, it was nearly impossible to understand such root structure. The caves give us access to the roots at different depths underground, and the DNA analysis allows us to identify them, so we can begin to address this problem."
Other co-authors on the paper were Lisa Moore of Stanford, Bill Hoffman of Texas and Will Pockman of Duke. Their research is sponsored by the National Science Foundation and the Andrew W. Mellon Foundation.
While the researchers' DNA analysis currently only identifies species, they are now developing a DNA technique to "rootprint" plants, linking underground roots with a specific above-ground plant based on its DNA signatures.
"The ability to match roots with specific plants will allow us to do manipulative experiments, in which we change some trees and not others and explore the effects on whole-plant functioning," Jackson said in an interview. "It will also help us understand the cost to the plant of roots at different depths. Plants may take up more water by having deeper roots, but there has to be a trade-off, in terms of construction and maintenance of deep roots, versus putting out surface roots or leaves. We hope to be able to understand those trade-offs."
He said such an understanding of the underground ecology of plants has important implications for basic science, as well as practical applications. "These data would help us understand such things as how deep waste must be buried to get below the rooting zone," he said. "Also, some species such as juniper are taking over wide areas of land in the southwestern U.S. as it becomes drier, as grazing increases and as fire is suppressed in the growing suburban areas. Thus, the region is changing from relatively open savannah to juniper thickets, and we need to understand the plants' water use and how it is altering ecosystem hydrology."
In developing such insights, the scientists plan to use a combination of instruments such as sap flow gauges, combined with DNA analyses, to document the details of how such plants draw water from the ground. In general, the role of deep roots in maintaining plants has been little-studied, said Jackson.
"In any plant ecosystem around the world, 90 percent of the root biomass is in the top few feet of the earth, but there is that nagging 10 percent represented by deeper roots. To really predict uptake, we need to understand where these roots grow and what their functional significance is."
The lack of knowledge of such roots could have important impacts on the accuracy of models of land surface ecology that help predict how plant transpiration - the passage of water from plants into the atmosphere - will affect global climate.
"The depth of the model is perhaps the most important aspect of creating accurate land surface models to predict transpiration rates," Jackson said. "The deeper you make the model, the more water there is for plants to use. Make the depth too shallow, the model predicts droughts more quickly than actually occurs in nature. You make the model too deep and it never predicts drought. So, fixing that depth of biological activity is very important in getting these models right."
Such root studies also have broader implications for understanding plants' ability to store carbon in their roots and the soil, perhaps affecting the atmospheric levels of carbon dioxide. The steady increase of carbon dioxide levels due to fossil-fuel burning will cause significant global warming over the next century. Vegetation changes can profoundly affect local climates, he said.
"If we remove all the woody plants from a system, it decreases that system's ability to use deep water resources," Jackson said. "For example, we have done climate simulations that indicate that the removal of woody plants from tropical savannahs around the world, such as in Brazil, has already probably started to dry them out somewhat, making them hotter and seasonally drier."
The scientists' work in Texas also has wide application, because about 10 percent of Earth's land structure has the same geologic karst structure - with its irregular limestone strata, underground streams and caverns - as does the Edward plateau in central Texas, where they did their studies, Jackson said.
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