Ground-based studies, satellite data mesh to provide more accurate forest biomass data
By Bob Berwyn
SUMMIT COUNTY — With global warming and beetle-killed forests near the top of many environmental agendas, the role of forests as carbon sinks, where heat-trapping greenhouse gases are stored, has become increasingly important.
U.S. forests absorb about 750 million metric tons of carbon dioxide each year, and the U.S. Forest Service says managing forest resources to optimize carbon storage is essential in mitigating the effects of climate change. But millions of acres of trees have been killed by beetles, with millions more expected to die in the next few years. Those forest areas no longer absorb and store carbon dioxide — they actually become net producers of greenhouse gases as the wood decays.
Managing forest lands to mitigate increased emissions of heat-trapping greenhouse gases is an important part of an overall strategy to adapt to climate change, but requires accurate information. A pair of recent research projects may help establish some of that baseline data.
In a ground-based project, Rutgers University biologists and U.S. Forest Service researchers teamed up to develop a computer model that can predict when a forest changes from being a carbon sink to being a net carbon generator instead. The results will help pinpoint the effectiveness of trees in offsetting carbon releases that contribute to higher atmospheric temperatures and global climate change.
The experiments included detailed measurements of tree sap flow and photosynthetic gas exchange at different canopy levels in a New Jersey oak and pine forest. That helped calibrate a model simulating the amount of carbon the tree canopy absorbs and releases into the atmosphere — about 1,240 grams of carbon per square meter of canopy area based on a seasonal average.
The biologists compared those numbers with data from the same stand of trees measured during an infestation of gypsy moths, when the trees were completely defoliated. The infestation occurred when the stand was at a peak seasonal carbon uptake. Based on the comparison, the scientists were able to show that the average carbon absorption rates for the growing season dropped 25 percent, to around 940 grams of carbon per square meter of canopy area.
The decline meant that the stand was no longer a net carbon sink — instead, it ended up adding more carbon back to the atmosphere than it had absorbed.
It doesn’t take that kind of fine-tuned measurement to understand the beetle-killed lodgepole pine forests are quickly turning into huge net carbon producers.
Understanding that big-picture impact requires a global perspective, and scientists with the European Space Agency are working on creating a large-scale boreal forest biomass inventory that will enable scientists to better understand the carbon cycle and to more accurately predict Earth’s future climate.
The inventory is based on computer modeling and data from the ESA’s Envisat satellite that allows for the retrieval of boreal forest biomass data on a more detailed scale than previously available.
The European study focused on boreal forests, spanning Russia, northern Europe, Canada and Alaska, and comprised of interrelated habitats including forests, lakes, wetlands, rivers and tundra. Boreal forests cover about 14.5 percent of the Earth’s surface (about the size of the contiguous 48 states) and are considered to be one of the most significant carbon sinks in the world. Including their soil, boreal forests, store about 33 percent more carbon per acre as tropical forests. They are considered global hotspots, where scientists are measuring areas of increased warming, which represent possible important tipping points for abrupt climate change.
But biomass remains one of the big unknowns in the carbon cycle, said Prof. Christiane Schmullius, of the Friedrich Schiller University in Jena, Germany. “Since no biomass maps exist with a high level of accuracy, we do not know how much is changing and cannot do calculations with any certainty. With this new algorithm, it is the first time that we have something in hand that may be a first step to a global biomass map,” Schmullius said.
Being able to map changes in boreal forests — like Summit County’s dying lodgepole stands — at a detailed level will help land managers make better decisions. It could, for example, be better from a global warming standpoint to harvest dead trees and convert them into low-emission ethanol or gasify them instead of letting them rot and emit greenhouse gases for years and years.
The researchers compared the satellite data to figures obtained by on-the-ground research at test sites in Scandinavia, Siberia and Canada. The new methodology will help track relatively quick changes in the biomass-carbon cycle after disturbances like pine beetle infestations, clear-cutting and forest fires.
One of the ESA researchers, Russian Prof. Anatoly Shvidenko, cited the catastrophic fire that occurred in Russia during 2003 as an example of why it’s important to to estimate the amount of organic matter in forests.
“The direct carbon emissions in Russia due to the wildfires in 2003 were about 270 million tons. Compare that to the 250-million-ton goal of emission reductions for the entire Kyoto Protocol, and it becomes very clear why it is extremely important to know what we have in terms of forest biomass,” Shvidenko said. “The maps are extremely important for providing effective forest fire protection monitoring and other important forest management activities,” he added.
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