Forest ThreatNet

Scientists ponder carbon, water, and climate in managed forests

By Stephanie Worley Firley, EFETAC

Planting more trees and allowing degraded land to return to natural forest cover are widely discussed options for mitigating climate change impacts. Trees take in carbon dioxide—the greenhouse gas whose rising atmospheric levels have been implicated in increasing average global temperatures—and provide long-term carbon storage. However, some recent studies have suggested that greater forest coverage might actually warm the planet.

These studies cite albedo, the amount of sunlight reflected off earth’s surfaces that is not absorbed as heat. With the albedo effect, lighter colored surfaces, such as snow and ice, reflect more sunlight (high albedo) than darker surfaces, such as asphalt, fields, and forests (low albedo). According to these studies, tropical areas of the world could become cooler as a result of reforestation, but temperate areas of North America and Europe may not respond in the same manner.

While forests indeed have a relatively low albedo, reforestation does not necessarily contribute to global warming, conclude scientists from the Southern Research Station, North Carolina State University, and University of Toledo in newly published research. Their findings come from data collected in two coastal North Carolina loblolly pine plantations where the interacting effects of forest management on carbon and water cycles have been examined since 2004. “We observed that 18-year-old trees on one research site do absorb more heat than recently planted trees on the other site. But low albedo and higher heat levels actually seem to increase the rates of forest evapotranspiration—a process that consumes energy in converting liquid water to vapor—rather than heat the surrounding air,” says Ge Sun, EFETAC research hydrologist and the study’s lead author. “We argue that forest regrowth or establishment is not likely to increase the energy flow needed to heat the atmosphere as some other studies speculate—at least in conditions where water is not very limited.”

This study is part of a larger collaboration through FLUXNET, a global network of towers instrumented to take measurements of water, carbon dioxide, and energy as they are exchanged between an ecosystem and the surrounding atmosphere. In contrast to other FLUXNET sites, these loblolly plantations are managed ecosystems. These plantations are also unique because they are former forested wetlands on organic soils with shallow groundwater originally drained for timber production. “This type of site has been historically underrepresented by FLUXNET,” says Sun.

The research will continue with scientists tracking the changes in water and energy balances over the entire rotation of the plantations under various climatic conditions. “So far, Mother Nature has offered excellent opportunities for this long term study. For example, 2007 and 2008 were exceptionally dry years, and 2009 was a wet year. These data will offer insights into how droughts affect the conclusions derived from our previous studies,” says Sun. “We also hope our landscape-scale research contributes to answering bigger questions, such as how land use change affects regional climate.”

Top photo:EFETAC North Carolina State University cooperating researcher Asko Noormets downloads data from a tower on a recently planted loblolly pine plantation.

Right: Eighteen-year-old loblolly pines on the coastal North Carolina research site.