FROM THE FEBRUARY 1, 1996 ISSUE
© 1996, The Daily Beacon. All rights reserved.
However, Oak Ridge National Laboratory researchers are finding that climatic changes are having a significant effect on the smaller, less glamorous vegetative members of the forest floor community -- the understory.
Based upon climate change research that has been conducted for the past two and a half years, Paul Hanson, a plant physiology researcher at ORNL, has determined that changes such as decreased levels of precipitation may reduce the level of productivity in the understory of southeastern forests.
The understory is commonly known as the layer of vegetation that grows in the shadow of the trees that receive the most sunlight in the forest.
Tree such as dogwoods, and a variety of saplings and seedlings are among the vegetation that is found living in the shade of the overstory. This vegetation is being monitored and levels of stress based upon fluctuations in rainfall amounts are being recorded.
Hanson stressed the fact that the results of this research are highly preliminary, and any attempt to accurately predict the results of climate change is a highly hit-or-miss proposition.
"We're looking at the potential for change," said Hanson. "Species composition may change, but there is a bigger global change issue," in the event of reduced or increased levels of precipitation.
One of the methods being used at ORNL to predict the potential for change in forest composition is to monitor a football field-sized plot of land, and adjust the amount of precipitation that flows through the forest. Sometimes the amount of water allowed to reach the vegetation is reduced by 30 percent, and other times it is increased by 30 percent, said Hanson.
This variability mirrors in theory potential fluctuations in precipitation levels caused by climate change, said Hanson. The plot being studied, which is approximately two acres in size, consists mainly of secondary, upland hardwoods, such as oak, maple, and poplar. The majority of understory plants in the plot consist of dogwoods, and seedlings and saplings of the dominant canopy varieties.
"The understory is being affected. Red maple and sugar maple saplings as well as the dogwood," said Stan Wullschleger, a plant physiologist at ORNL.
The seedlings and saplings appear to be more resilient than their dogwood cousins, said Wullschleger, for reasons that are not presently clear.
"Some seedlings are ... better able to cope with stresses. Dogwood, for whatever reason, can't cope with drought conditions. But the maples definitely show stressed physiology. There's still germination, but stunted growth, and decreased productivity," said Wullschleger.
"If there is a general reduction in regional precipitation then there is the potential for reduced growth in the understory, such as dogwoods," said Hanson.
However, underscoring the complexity of future climate modeling, there are a variety of possible variables that have not been completely addressed yet.
For instance, ORNL is not manipulating temperature, and Hanson emphasized that a greenhouse gas, carbon dioxide, can actually act as a fertilizer for plant growth.
"Climate variables may be counteracted by increasing levels of carbon dioxide ... and that makes it all the more difficult to ascertain [the true cumulative effects of climactic change]," said Hanson.
In other words, it may be possible for plants to compensate for lack of precipitation by increased intake of carbon dioxide.
"They may be getting less water, but they're still producing the same amount of carbon," said Hanson.
In spite of this potential for plants to compensate, tests of the past two years indicate reduced vegetative growth will be the result of a drier climate.
"The first indication of a drier climate is reduced growth--we saw a 20 percent mortality rate in the summer of 1995," said Hanson.
The summer of 1995 was unusually hot and dry , but the amount of rainfall still fell within the working parameters of a 30 percent reduction in precipitation.
"I must emphasize, responses tend to be localized. Many species are well-buffered against change," said Hanson.
Many scientists are still uncertain as to the eventual outcome of global climate change, due to the complex nature of the earth's natural climactic processes. However, there is an increasing consensus that the increase in the amount of "greenhouse" gases in the atmosphere could result in many environmental effects. The direst of predictions range from the desertification of the Amazon river basin to the gradual melting of polar icecaps and flooding of low-lying coastal areas, such as New Orleans, La. or Amsterdam, Holland.
As far as East Tennessee forest lands are concerned, researchers at ORNL have been able to narrow down the range of potential fluctuations in precipitation, and their respective outcomes.
"We measure a lot of physiological processes to see ... how well the trees are doing. Leaf processes, photosynthesis, transpiration, so we can know how increased or decreased precipitation is going to affect the plants," said Wullschleger.
Like his colleague, Wullschleger is hesitant to expound too drastically on the potential regional effects of climate change, mainly because no one really has any idea what the earth may have to offer in coming years. He said, however, that a distinct change in forest composition could be one result of the understory dieback.
"The main characteristic of a forest is the trees that grow best. The indications are that forest composition could change. Maybe dogwoods die and maples are stunted -- then we may have a pure oak forest, which is different from what Tennessee has now. A main question: are plants with altered stress physiology more susceptible to disease or insects?"
The Fraser firs in the upper elevations of the Smokies have all but been eliminated by the Balsam Wooly Adelgid. Some speculate that natural immune protections were subverted by stress due to acid precipitation and high levels of pollution within the park.
Threats to wildlife are even more difficult to predict, though the majority of mammalian browse is to be found on or near the forest floor. Even the smallest critter in the forest could be affected, said Wullschleger.
"There are some people asking: how will insects be affected under future climate change? Will insect populations increase or decrease?"
As answers are found, more questions are raised, indicated Wullschleger, but the basics are falling into place.
"In general, large trees seem to be more resilient, more able to cope ... but there is the chance of altered growth potential."
In fifty years, said Wullschleger, "our forests might look a lot different."