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Winter 1998

Science Bits

LTERs go urban

In-depth environmental studies have traditionally taken place in areas where humans have done little to interfere with the natural cycles of life. Breaking away from this traditional focus, the Long-term Ecological Research (LTER) program at NSF has added two new sites in highly populated urban areas: Baltimore, Maryland, and Phoenix, Arizona. These sites, launched last year, expand the LTER to 21 locations.

"The idea is revolutionary in ecology," says Grace Brush, a professor at the Johns Hopkins University who is part of the Baltimore team. "But the time has come to consider humans as part of ecosystems."

Brush's studies focus on the impact 18th-century beaver trapping had on the forests that flourished where the city now sits. Trappers' hunting habits may have dramatically changed the landscape by eliminating the beavers and their dams, altering the flow of local streams and the mix of vegetation nearby. By studying the pollen, seeds, tiny animals, and chemicals preserved in sediment, Brush hopes to learn how natural resources were affected by trapping, farming, and finally the evolution of Baltimore into a bustling metropolis. Other Baltimore LTER studies will include the effects of the variable tree canopy on stormwater runoff and urban wildlife habitat, and the potential ecological impact of converting vacant lots to nurseries and vegetable gardens.

As the beavers disappeared, so did their dams, Brush explains. "As a result, streams would have been more free-flowing, resulting in less flooding, drier land adjacent to the streams, and different vegetation. Because there were no longer dams and stagnant water, marshy wetland areas near the streams disappeared. Sediment in the water would be transported farther downstream, perhaps as far as the Chesapeake Bay."

Brush's theory is supported by her discovery of sedge pollen in the oldest portions of some of the first sediment cores her team has extracted from the Gwynns Falls Watershed. Sedge plants grow in marshes, indicating that the land beside the streams was once much wetter, probably because beaver dams were diverting water over the banks.

The Baltimore LTER research team of 36, led by Stewart Pickett at the Institute of Ecosystem Studies in Millbrook, New York, includes ecologists, sociologists, educators, geographers, and economists. A comparable interdisciplinary team led by Charles Redman and Nancy Grimm of Arizona State University will examine the Phoenix LTER.

"The purpose of this study is to help people become aware of and understand the environment within urban areas," Brush says. "We are beginning to think of interrelations between people and nature, and how each influences the other. Hopefully, what we learn here will be applicable elsewhere."

The Johns Hopkins University, Arizona State University

Smoke boosts positively charged lightning

Smoke from the massive forest fires that plagued Mexico earlier this year caused three times the normal number of positively charged lightning strikes to ground in the southern plains of the United States, according to an article in the October 2 issue of Science.

The smoke that migrated up the Mississippi River valley "appeared to have a substantial influence on the electrical characteristics of thunderstorms over the central United States," says Earle Williams of the Massachusetts Institute of Technology, one of the paper's coauthors. "In some way, the smoke from these fires significantly altered the electrical characteristics of a wide variety of storm types during all phases of their life cycles." The primary author of the paper is Walter Lyons of the Yucca Ridge Field Station in Fort Collins, Colorado.

Ordinarily, thunderstorms produce negative flashes to the ground. Positive flashes occur only 10% of the time. "For two months this spring, these shifted to positive dominance, which is very bizarre," says Williams. Another instance of unusually high numbers of cloud-to-ground flashes of positive polarity occurred during the severe fires in Yellowstone National Park in 1988. Between April and June, nearly half a million flashes in the southern plains were positively charged.

These thunderstorms also produced abnormally high numbers of mesospheric optical sprites, the fleeting red glows that appear between 30 and 90 km above the ground in association with thunderstorms. One mesoscale convective system over Nebraska, on 20 May 1998, produced 380 sprites, the highest single-storm total recorded in more than 100 storms and six years of observations.

Smoke provides cloud condensation nuclei, particles of matter on which cloud vapor condenses. When a large amount of smoke, soot, contaminants, or aerosols is in the atmosphere, the elevated numbers of CCNs can affect droplet size, which in turn can affect various aspects of the charge separation mechanisms, according to the paper's authors. But there is currently no hard-and-fast explanation for the physical mechanism behind this phenomenon.

The research team also includes Thomas Nelson of the Yucca Ridge Field Station and John Cramer and Tommy Turner of Global Atmospherics in Tuscon, Arizona. The research was conducted in part with support from NSF, NASA, and the U.S. Department of Energy.

Massachusetts Institute of Technology

Scientists study how long trees can absorb high levels of CO2

The growth rate of loblolly pine trees in a North Carolina forest increased by 12% when the trees were exposed for one growing season to carbon dioxide levels projected for 50 years in the future, according to data gathered by scientists from four institutions.

The findings--presented in August at the Ecological Society of America's annual meeting--were from the first year of work at the Forest-Atmosphere Carbon Transfer and Storage Experiment in a forest near Durham, North Carolina.

While this work may appear to offer hope for the ability of trees to absorb increasing amounts of carbon dioxide from fossil-fuel combustion and deforestation, researchers caution that the growth rate likely cannot be sustained. In fact, it already appears to be stabilizing.

"Depending on trees may not be a usable mitigation policy," said Evan DeLucia, a plant biologist at the University of Illinois. "Trees have large reserves of nutrients, but it is likely that the growth stimulation we saw will drop in time. It may abate to zero in ten years, as the trees adapt to the higher carbon dioxide concentrations and growth exceeds the capacity of the soil to provide limiting nutrients.

"We may need to put more focus on the issue of soil management so carbon dioxide can be stored in the ground, " DeLucia said. "The key is long-term, locked-up storage below ground. Everything we see above ground will end up back in the atmosphere in one or a few human lifetimes. All of it will die and decompose. Trees are short-term carbon storage. Carbon must go into soil to remove it from the atmosphere."

DeLucia worked with postdoctoral student Shawna Naidu and project codirectors William Schlesinger of Duke University and George Hendrey of Brookhaven National Laboratory. They and other principal investigators from the University of Illinois, Duke, Brookhaven, and West Virginia University are interested in measuring forests' overall carbon budget.

So far, DeLucia said, trees can use higher levels of carbon in photosynthesis, but the question is how much more can they use and ultimately store. "This is the first experiment that lets us address this question in the real world."

University of Illinois, Duke University, West Virginia University

Researchers measure the temperature of solar "sneezes"

For the first time, scientists are able to determine accurately the temperature of individual solar "sneezes," small explosions on the sun called impulsive solar flares. The researchers used the Solar Energetic Particle Ionic Charge Analyzer (SEPICA) instrument on board NASA's Advanced Composition Explorer (ACE) spacecraft to observe a series of flares of this type in August 1998.

"These measurements are a first step to understanding how solar flares accelerate particles from the Sun to extremely high velocities," said Eberhard Moebius of the University of New Hampshire. Moebius presented the research at the American Geophysical Union conference in December.

SEPICA derived the flare temperature by measuring the electric charge on high-speed atoms shot from the flares. At high temperatures, electrons can be removed from atoms, giving the atoms a positive electric charge and allowing magnetic fields present in flares to accelerate them to high speeds. As temperatures rise, atoms lose more electrons until they have none left, a condition known as "stripped."

"The atoms of various elements detected, from hydrogen to silicon, had been completely stripped, and iron was almost fully stripped," said Joseph Mazur of the Aerospace Corporation, who contributed to the research. "This corresponds to a flare temperature of about 18 million degrees Fahrenheit." For comparison, the surface of the sun is only 10,000 degrees.

"In the past, very often we were not sure whether these energetic particles came directly from solar flares or were accelerated between the Sun and the Earth," said Moebius. Now, "We have the means to exactly time their arrival and hence infer the acceleration site, even for these very interesting small flares.

"Exactly how magnetic fields within flares accelerate particles and release energy is unknown. Strange things happen in them," Moebius reported. "For example, for some reason, impulsive solar flares prefer to accelerate helium 3 atoms. The concentration of helium 3, a rare isotope of helium, in matter ejected from these flares is as much as 1,000 times greater than its average concentration throughout the rest of the universe."

After their acceleration in a flare, ions rush along magnetic field lines extending from the Sun into interplanetary space, like race cars confined to a track. The Ultra Low Energy Isotope Spectrometer (ULEIS) instrument on ACE has identified many of these small flares when particles arrive directly along the interplanetary magnetic field lines.

"The sensitive particle detectors on ACE tell us details about the interplanetary magnetic field. If the field were uniform, showers of particles from different flares would all last about the same time, approximately a day or so as the slower particles trail the fast ones," Mazur explained. "Occasionally, however, we see a particle shower from one flare suddenly 'turn off,' while the shower from a different flare continues unaffected. Apparently some unknown feature of the interplanetary magnetic field must have severed the magnetic pathway between one of the events and ACE without severing the other. ACE provides another tool for diagnosing the structure of this unseen field."

University of New Hampshire

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