Hardware for Hardwoods: Monitoring Effects of Global Change on Forests

ORNL researchers are using advanced measurement technologies to help them predict how global climate change might affect forest productivity.

The Throughfall Displacement Experiment (TDE) on the Oak Ridge Reservation was initiated in 1992 to evaluate the response of an upland oak forest to soil conditions “wetter” and “drier” than ambient rainfall conditions in East Tennessee. Rain is collected in sub-canopy troughs on a “dry” plot, moved across an “ambient” plot, and then distributed onto a “wet” plot. The TDE was designed to simulate scenarios of altered precipitation that might result from global climate change.

Just as people breathe and perspire, trees play give and take with the air. Carbon dioxide (CO₂) and water vapor are released from tree leaf pores to the atmosphere. CO₂ is drawn from the air through the leaf pores for photosynthesis, the process by which trees use the energy of sunlight to convert carbon to food and fiber. In ORNL’s Walker Branch Watershed on the Oak Ridge Reservation, the flow of gases between forest trees and the atmosphere has been measured for 30 years. By using sophisticated instruments in this mixed hardwood forest, ORNL environmental scientists have found that a 1-hectare stand annually accumulates 8550 kilograms of carbon as dry mass and loses 6.3 million liters of water.

Why are scientists interested in measuring these carbon and water exchange rates among some of ORNL’s oak, poplar, hickory, maple, and loblolly pine trees? “We’ve learned over the years that this information helps us judge the health of a tree,” says Stan Wullschleger, a researcher in ORNL’s Environmental Sciences Division (ESD). “Trees that take up CO₂ at a high rate are usually growing well. Similarly, a tree that releases considerable water vapor to the atmosphere is likely to be healthy because it has enough water to cool its leaves and transport nutrients to its leaves and branches. Exchange rate measurements also may indicate the physiological capacity of trees to withstand stresses such as air pollution, insect attacks, and acid rain.”

If increasing levels of atmospheric CO₂ from fossil fuel combustion result in a warming of the earth’s surface, changes in daily temperature and rainfall patterns are predicted. ESD scientists are already studying the possible effects of these projected climate changes on tree health and forest productivity. “Measurement technologies have become increasingly important to environmental scientists,” says ESD scientist Sandy McLaughlin, “because we are being asked to predict the physiological resilience of plant ecosystems at regional and global scales under conditions expected to be present if global warming occurs.”

A transparent cuvette and an infrared gas analyzer are used to measure CO2 and water vapor exchange rates in leaves of a sweetgum tree in South Carolina. Trees like this one are grown in high-density plantations that are provided with supplemental water and nutrients.

Trees may grow larger or faster in some regions because the atmosphere’s concentration of CO₂ has risen. Or, because of global warming, tree growth may be slowed by longer periods of drought, and the growth could be retarded even more if air pollution levels are high. Exchange rate measurements will help scientists explain why a forest’s productivity is high or low.

“If a tree is exposed to drought, its water use will decline, and we’ll detect this reduction with our measurements of water vapor transfer rates,” Wullschleger says. “Such a tree may experience reduced growth because of an inadequate supply of nutrients. Also, because leaf pores tend to close under drought conditions to hold in what little water the tree has, the tree’s uptake of photosynthetic CO₂ will be reduced, further slowing its growth.”

Forests may be affected by climate change, but they may also have an impact on climate. “Carbon measurements,” says Wullschleger, “will tell us the potential of the landscape to remove and store atmospheric CO₂, reducing the greenhouse effect that induces global warming.”

In the 1950s and 1960s, ESD researchers made exchange rate measurements using radioisotopes and cumbersome radiation detection equipment. In the 1970s regulatory agencies frowned upon the use of radioactive isotopes in the environment, so ESD researchers switched to infrared gas analyzers (IRGAs). An IRGA consists of an infrared radiation source, a gas cell, and a detector. The uptake of CO2 by leaves and plants enclosed within the gas cell is measured as a change in infrared radiation reaching the detector and, thus, a change in detector output.

The early IRGAs were bulky, labor intensive, and slow to make measurements. Today, ESD scientists are using commercially available IRGAs that are compact, portable, robust, and capable of providing estimates of carbon and water exchange within seconds. IRGAs are being used in DOE’s Biofuels Feedstock Development Program to investigate the role that soil water, nutrients, and air temperature play in determining the productivity of plantation-grown hardwoods in South Carolina.

ESD scientists will also use IRGAs to determine the amounts of carbon and water used by cottonwood, sycamore, and sweetgum trees grown using different types and levels of irrigation and fertilization. “This work will provide insights into the growth potential of hardwoods under different conditions for the pulp and paper industry,” Wullschleger says. “It will give needed information on the productivity of short-rotation forests for use as bioenergy crops.”

Two major experiments using IRGAs and other sophisticated measurement devices are being conducted on the Walker Branch Watershed to address global change issues. They will help ESD scientists predict the effects of drought, excessive precipitation, and increased atmospheric concentrations of CO2 on forest productivity.

In a DOE-funded study known as the Throughfall Displacement Experiment (TDE), a series of troughs capture and transfer rain water. One half-acre plot receives about one-third more rainfall and another plot receives about one-third less rainfall than an adjacent half-acre plot receiving ambient rainfall. The goal is to understand how forests adjust to changes in precipitation that might result from global warming.

ESD’s Paul Hanson, who is leading the TDE experiment, says, “Throughout six years of sustained rainfall reductions, large oak and maple trees grew normally because sufficient water had been stored in the forest soil. But flowering dogwood and other shallow-rooted trees growing in the forest shade died in greater numbers when artificially reduced precipitation was applied during a drought year.” According to Wullschleger, “remotely operated sap flow devices provide a reliable early indication that large trees fare better during a drought than small trees.” The reduced rainfall was found to result in increased organic matter in soil litter layers, suggesting that more carbon may be sequestered in the soil if climate change brings longer periods of drought.

The response of a ten-year-old sweetgum plantation to a 50% increase in the ambient concentration of atmospheric CO2 is being measured by ESD’s Free Air CO2 Enrichment (FACE) system. Originally supported by the internally funded Laboratory Directed Research and Development Program and now by the National Science Foundation and DOE, the FACE facility has been operating since April 1998. It relies on a computer-controlled flow gauge system that, by taking wind speed and direction into account, maintains desired CO2 levels within circular plots surrounded by free-air injection ports.

“We are measuring changes in each tree’s canopy size, stem and root growth, and rates of photosynthesis and water and nutrient use,” says ESD’s Rich Norby, who leads the FACE research. Minature video cameras inserted into buried tubes allow the researchers to monitor root activity. “We hope to answer this question: How will limited light, nutrient, and water availability in a closely spaced forest stand affect the ability of this forest to use the ‘extra’ CO2? The findings should be of interest because recent research suggests that some tree species, but not others, may thrive during global warming because the extra carbon they absorb compensates for the reduced availability of water.” During the first year of exposure to the increased CO2 concentrations, photosynthesis was stimulated, stem growth increased, and the trees conserved water.

These suspended pipes release carbon dioxide to a test sweetgum plantation on the Oak Ridge Reservation. Scientists are determining how a carbon dioxide–enriched atmosphere affects the growth of one species of forest trees.

Since the late 1970s, McLaughlin and his colleagues have measured the effects of pollutants, such as ozone, on forest trees. They have moved from laboratory exposure chambers to much larger, open-top field chambers. More recently, they have developed sensitive systems for detecting responses of unchambered forest trees to variations in pollution stress. Ground-level ozone is a pollutant produced when hydrocarbons and nitrogen oxide from cars and coal-fired power plants react in the atmosphere. ESD studies show that plant growth capacity can be reduced by chronic exposure to ozone.

In a five-year study for the U.S. Forest Service in the 1990s, McLaughlin developed a technique that sensitively measured the effects on loblolly pine growth of exposure to ambient levels of ozone, high temperature, and reduced soil moisture on the Oak Ridge Reservation. Aided by statistical analysis by Darryl Downing of ORNL’s Computer Science and Mathematics Division, McLaughlin found that the growth rate of trees exposed to ambient ozone levels was reduced even more during very dry periods in the summer. The finding suggests that global warming combined with ozone pollution could harm large populations of loblolly pines, which are so important to the economy of the South.

For future monitoring of forest productivity, ESD researchers envision outfitting airplanes and satellites with remote-sensing cameras that are sensitive to light reflected from leaves. Then they could rely on hardware above rather than in the hardwoods.

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