North Central Research Station

Buried Treasure Reveals an Active Afterlife for Riparian Trees

"The banks themselves, along with perhaps millions of trees, are ever tumbling, falling, and washing away from the spots where they may have stood and grown for centuries past. ...We saw several trees falling in, and beautiful, though painful, was the sight. As they fell, the spray which rose along their whole length was exquisite; but alas! these magnificent trees had reached the day of oblivion." John James Audubon 1843 Missouri River Journal.

Audubon's "day of oblivion" is actually the start of a long and fruitful second career for fallen trees in midwestern waterways. Dan Dey, a research forester with NC's central hardwoods unit in Columbia, has found that dead wood can last 9,500 years or more in meandering rivers with erodible banks. Depending on the year, the log may either be entombed in oxygen-starved sediments, or exposed by a raging flood.

"The same forces that bury a log in sediments can scour a bank and expose it," Dey said. "Once excavated, this 'coarse woody debris' returns to active duty, providing a substrate for aquatic insects, quiet pools for fish spawning, or waterfalls for aeration. Given enough time, the log begins to contribute its nutrients to the stream- nutrients captured several millennia ago! Just as quickly, another flood can bury it for several more millennia."

The implications of such a life span are huge, says Dey. "It makes you wonder, for instance, how much carbon is actually sequestered in fallen, buried logs. And how unearthed logs contribute to the geomorphol-ogy and ecology of the stream. Given the lack of information about this important contributor to carbon cycling and stream ecology, we decided to do a study."

When Guyette waded into Medicine Creek, he was delighted to find logs as old as those found in the famous peat bogs of Europe. "The logs were in good shape thanks to the oxygen-free conditions, and some of them still had their bark on," Guyette said. "When I carbon dated them, I found them to be 2,000 to 8,000 years old." The site became the first of five streams that Dey and Guyette would sample. To reconstruct the dynamics of large wood in stream systems, they re-corded length, diameter, volume, mass, density, age, and species. They were interested in finding out when the tree had died, how long it had been in the system, how fast it had decayed or moved around, and what ecological functions it was providing.

"At first Rich had nightmares about people removing the ancient logs before we could study them," Dey remembered. "His worries subsided as we found more ancient logs, and finally realized that alluvial burial and excavation of logs is a characteristic component of rapidly eroding streams. It was a dream come true for a dendrochonologist."

One of the most interesting discoveries was the presence of several well-preserved forest litter layers containing leaves, twigs, tree boles, bark, hickory nuts, seeds, acorns, tubers, etc. "By studying these layers, an investigator could reconstruct what the forest looked like at various points over 10,000 years," Dey noted.

At this point, Guyette and Dey are building their database of samples. By comparing ring-width sequences of oaks and looking for overlapping patterns, they hope to compile what's called a master chronology, an index that will allow scientists to pinpoint log age with more accuracy than carbon dating allows. "Knowing the exact age of a log, in addition to its wood chemistry, tells us something about soil and atmospheric condition over time. The size and pattern of tree rings may give us insight into our own changing climate. In one of the worlds richest agricultural regions, long-term information like this has real value," Guyette said.

Data from the preserved litter studies will be used to complement existing charcoal and pollen studies to arrive at a picture of how the forest flora changed over the long march of time. Dey hopes to use this informa-tion to create management guide-lines that provide a sustainable supply of wood of the right size and species-not just for human harvest, but for recruitment into streams as well.

"For me, studying ancient logs and litter is one more way to investigate the interplay between riparian forests and aquatic systems-how the health and productivity of one affects the other," Dey said. "We may find that riparian systems, because of the burial of coarse woody debris, are the most important systems in our landscape in terms of carbon sequestration. Knowing this might encourage people to invest in programs to reforest bottomlands and to reconnect the flood plain with the forest- to let natural forces, such as meandering and scouring, continue their important work."

Source: NC News, April/May/June 2001