May 8, 2000

"The spreading chestnut tree" - well known from 19th-century poet Henry Wadsworth Longfellow's work - was once a dominant species in the eastern United States, comprising a quarter or more of the big trees that made up the canopy in many American forests. But by the end of the 19th century, the chestnut blight, a form of fungus, had arrived in New York City, a deadly stow-away on nursery stock shipped from Asia. And in less than 50 years, nearly every chestnut in the country had been destroyed.

Changing Forests
The consequences of such a sudden and devastating change were not entirely predictable, says Zoe Cardon, assistant professor of ecology and evolutionary biology. One result was that the northern red oak, a commercially important tree that is widely prevalent in our forests today, achieved dominance at the expense of the chestnuts. And now some oaks, in turn, are showing increasing susceptibility to their own blight, oak wilt disease.

The effects of changes in forest tree species are of special interest to Cardon, who has just been awarded a four-year $350,000 grant from the Andrew W. Mellon Foundation to study the way trees, and the ever-changing composition of the forest, affect the soil.

"Microbes in the soil are the major decomposers in forests," says Cardon, who came to UConn in 1997 after one and a half years as an assistant professor of biology at Bowdoin College in Brunswick, Me., "and in terrestrial ecosystems, carbon moves from plants to the microbes in a variety of ways. The more familiar and often more easily quantified routes are above ground. They include leaf-fall from trees in autumn or annual shoot die-back from perennial herbs and grasses. A large percentage of fixed carbon moves from photosynthetic shoots to soil through plant roots, however. These fluxes are more difficult to quantify, yet it is thought they constitute the fate of a large percentage of the productivity of plants. Some estimates, in fact, place it as high as 50 percent. We need to know more about what controls these processes."

Experimental Techniques
To find out, Cardon and her assistants will conduct three kinds of experiments over the next four years. First, they will follow the growth of 100 young trees they've planted on the nearby plant science research farm - native hardwood species including birch, beech, ash and red oak - to understand how they affect the soil around their roots.

"If we are to really understand the impact that harvesting and blighting has on the changing composition of the forest and how that, in turn, affects the sustainability of the forest, it's important to understand the relationship between microbes and the trees that depend upon them for decomposition, which replenishes the forest soil," says Cardon. The leaves of red oaks, for instance, probably do not degrade as rapidly as those of the chestnuts they replaced a century ago.

In the UConn greenhouse the team is using oak and poplar saplings to track carbon transfer over shorter periods of time. By constantly monitoring the soil around the roots of the saplings, the team will be able to

measure chemical changes that occur seasonally as well as during shorter cycles, lasting two to three weeks, that are predictable aspects of plant growth. The greenhouse studies will examine how microbes react to these fluctuations, providing a clearer picture of decomposition in soils.

In a third project, Cardon will collaborate with Daniel Gage, an assistant professor of molecular and cell biology. Gage and his students have helped develop a unique tool - genetically engineered soil microbes - that will play a key role in the experiment. When plants release sugars through their roots, Gage's microbes will raise a flag for Cardon and her assistants. Subjected to blue light, they will glow green.

Cardon's grant will provide support for two graduate students as well as an undergraduate assistant each semester, including summers. It also provides funding for a technician to support specialized equipment used in the experiments.

"We don't have a good understanding of how the forest is changing from what it was," says Cardon. "We have historic literature about the kinds of trees, but not as much information about the soil. It is not just blights that change the composition of the forest. There are many other factors. Northeast forests have been harvested repeatedly. As a result of industrial- ization, we've had more than a century of nitrogen fertilization and acid rain. But we still don't know much about the changes that are going on in the soil. More important, we don't really know what the implications are for the forest as a whole. This multifaceted study is designed to shed light on these processes."

Jim Smith