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Land-atmosphere connection

NCAR Scientists launch long-term field project in Colorado forest

December 10, 2008 | NCAR scientists launch long-term field project in Colorado forest In a forest outside Colorado Springs, a group of researchers is investigating how forests impact weather and air quality. The team even suspects that the pine beetles eating their way through the West’s forests are altering local weather patterns.

As Alex Guenther (ESSL/ACD/TIIMES) explains it, forests help control the atmosphere. “There’s a big difference between the impacts of a living forest and a dead forest,” he says. “With a dead forest, we may get different rainfall patterns.”

Alex, a biogeochemist, is one of the principal investigators on BEACHON (pronounced beacon), the Bio-hydro-atmosphere Interactions of Energy, Aerosols, Carbon, H2O, Organics, and Nitrogen. During this NCAR–led field project, researchers are exploring how trees and other vegetation influence rainfall, temperature, smog, and other aspects of the atmosphere. The goal is to learn more about cloud formation, climate change, and the cycling of gases and particles between the land and the atmosphere.

BEACHON involves several dozen NCAR researchers in collaboration with university, government, and international colleagues. The project, launched this summer, is scheduled to continue for four years across a region from southern Wyoming to northern New Mexico.

Although it’s not the first field project to measure emissions from vegetation, BEACHON’s extent and duration will allow researchers to study emissions during different seasons and measure yearly changes. It is especially unique in that the researchers will look at the feedbacks of climate on vegetation, with an eye toward questions such as how drought affects the emissions of particles that control clouds, which in turn produce rain that impacts vegetation. A broad range of scientists will lend expertise, including biologists who study plant physiology, atmospheric chemists, hydrologists, and more. The team’s arsenal for making observations includes ground-based instruments and sensors, computer models, and possibly an aircraft or helicopter.

“BEACHON will give us a very comprehensive picture of a forest’s impact on the atmosphere,” Alex says. “But at this point, we don’t know what the project will reveal. We may end up with more questions than answers.”

In the field

One of the main field sites for BEACHON is located 28 miles northwest of Colorado Springs in the Manitou Experimental Forest, part of the U.S. Forest Service’s Rocky Mountain Research Station. The research team’s goal during the summer 2008 phase, which ran July 21–September 19, was to prepare the site for long-term observations, set up infrastructure, test instruments, and begin addressing science questions.

The team constructed a 100-foot tower to measure emissions above the forest canopy. On the ground, scientists began sampling aerosols and measuring trace gases (ozone, carbon monoxide, nitrogen oxide, and sulfur dioxide). Over the course of two days, they launched balloon-borne radiosondes every three hours to measure temperature, humidity, and winds.

“We’re trying to set up a core suite of measurements that will be there for several years,” explains site coordinator Jim Smith (ESSL/ACD). The researchers will use these measurements to view long-term trends in exchanges between the atmosphere and forest, as well as to determine if air from the Front Range influences the Manitou site.

As an aerosol scientist, Jim hopes that BEACHON will shine light on how emissions of hydrocarbons from forest vegetation (and possibly soil) affect local climate. These hydrocarbons play a major role in atmospheric chemistry. “To me, the most intriguing question is the climatic significance of the birth of these very small particles into the atmosphere,” Jim says. “It’s the kind of question that benefits from being in one place year in and year out.”

The team is studying the ground at the Manitou site as well as the air. Hydrometeorologist Dave Gochis (RAL) is using sensors to measure soil moisture and temperature, as well as deploying rain gauges. His goal is to monitor the impact of precipitation and climate forcing on soil hydrology, the availability of water to plants, and the impact of moisture stress on plant emissions.

The site will be quieter over the winter, though researchers will continue making measurements remotely and make occasional site visits. Jim and Tom Karl (ACD) plan to look at how snow affects emissions of trace gases. “We think snow could be acting as a trap for some hydrocarbons, so that when the snow melts, it represents an emissions source,” Jim says.

Living, breathing vegetation

Plants interact with the atmosphere in a variety of ways. They take in and emit chemicals and gases, and absorb the Sun’s heat. Tiny airborne particles from plants rise into clouds and seed them by providing surfaces for water droplets to adhere to and grow into raindrops. Plants emit chemicals known as volatile organic compounds that interact with human-caused pollution to form smog, which affects air quality and local temperatures. Carbon dioxide that is emitted in large quantities from dead forests joins carbon dioxide from human activities to influence the amount of the Sun’s heat that reaches Earth.

The magnitude of the tree loss from the pine beetle epidemic is enough to disrupt local weather patterns and air quality. Preliminary computer modeling for BEACHON by Fei Chen (RAL) suggests that beetle kill can lead to temporary temperature increases of about 2–4°F, partly due to a decrease in the ability of trees to cool the atmosphere by transpiring water—similar to how people cool their bodies by sweating. Scientists also believe that beetles stimulate trees to release more particles and chemicals into the atmosphere as they try to fight off the insects. This worsens air quality, at least initially, by increasing ground-level ozone and particulate matter.

Land use changes

Wildfires, clearcutting, and new development also change the atmosphere through vegetation removal. The impacts in each case can vary significantly, depending on the remaining vegetation and changes to soil conditions. If cloud and precipitation patterns change for a decade or more, the land cover can in turn be altered.

In arid places such as the Rockies, the exchange of gases and particles between Earth’s surface and the atmosphere is especially critical since even slight changes in precipitation can have significant impacts on the region.

“Here in the western United States, it is particularly important to understand these subtle impacts on precipitation,” Alex says. “Rain and snow may become even more scarce in the future as the climate changes, and the growing population wants ever more water.”