Soil nutrition affects carbon sequestration in forests
Are forests carbon sinks or not? It's a question that keeps many scientists busy, and their answers are as diverse as there are forest types. Researchers studying the same type of forest - tropical rainforests for example - arrive at conflicting conclusions, with one group saying the trees are net CO2 contributors, whereas others find the opposite to be true. Going further, and beyond the mere carbon storage capacity of forests, other scientists are looking at the net contribution of forests to global warming. Here too, results are complex: some forest types (rainforests) actually cool the planet because they produce clouds that reflect light back into the atmosphere, whereas forests at mid-latitudes don't produce this cloud-induced 'albedo-effect' and result in a net warming. Planting trees in these mid-latitudes, as many 'carbon-offsetting' programs do, is probably not a good idea (earlier post).
Now a related, fascinating question that is being studied by several research groups, has to do with the future: what will happen to forests if atmospheric carbon dioxide levels are much higher than today? In other words, suppose the level of CO2 in the atmosphere keeps increasing - which it does in reality -, then what about the capacity of forests to store some of it?
On December 11, United States Dept. of Agriculture (USDA) Forest Service (FS) scientists from the FS Southern Research Station (SRS) unit in Research Triangle Park, NC, along with colleagues from Duke University, published two papers in The Proceedings of the National Academy of Science (PNAS) that provide a more precise understanding of how forests respond to increasing atmospheric concentrations of carbon dioxide (CO2), the major greenhouse gas driving climate change.
Building on preliminary studies reported in Nature, the researchers found that trees can only increase wood growth from elevated CO2 if there is enough leaf area to support that growth. Leaf area, in turn, is limited by soil nutrition; without adequate soil nutrition, trees respond to elevated CO2 by transferring carbon below ground, then recycling it back to the atmospheric through respiration:
biomass :: bioenergy :: biofuels :: energy :: sustainability :: carbon dioxide :: CO2 :: carbon sequestration :: forests :: climate change :: global warming ::
"With sufficient soil nutrition, forests increase their ability to tie up, or sequester carbon in woody biomass under increasing atmospheric CO2 concentrations," says Kurt Johnsen, SRS researcher involved in the project. "With lower soil nutrition, forests still sequester carbon, but cannot take full advantage increasing CO2 levels. Due to land use history, many forests are deficient in soil nutrition, but forest management -- including fertilizing with nitrogen -- can greatly increase growth rate and wood growth responses to elevated atmospheric CO2."
The studies took place at a Free Air Carbon Enrichment (FACE) study established by the U.S. Department of Energy on the Duke Forest in Durham, NC. In FACE studies, groups of trees are circled by rings of towers that provide CO2 to increase atmospheric concentrations of the gas around the selected trees. At the Duke FACE experiment, half of each ring was fertilized with nitrogen to study the effect of added soil nutrients on tree growth under elevated CO2.
The researchers further tested their hypotheses using data from FACE sites in Wisconsin, Colorado, and Italy. In the articles, the scientists identify critical areas needing further study, but the overall consistency they found across these diverse forests bodes well for developing accurate models to predict the ability of the world's forests to sequester carbon.
"Forests play a critical part in sequestering carbon, and may play a role in mitigating the elevated levels of carbon dioxide associated with climate change," says Johnsen. "To predict how much forests can sequester, we need accurate ways to predict what happens to carbon within forest systems and how this partitioning is affected by environmental conditions."
Picture: Aerial view of free-air carbon dioxide enrichment (FACE) rings at Duke Forest, Durham, NC. Credit: Will Owens.
More information:
The two articles can be accessed online at PNAS and on the SRS website:
-Aboveground sink strength in forests controls the allocation of carbon below ground and its [CO2]-induced enhancement (or at the SRS website)
-Canopy leaf area constrains [CO2]-induced enhancement of productivity and partitioning among aboveground carbon pools. (Or at the SRS website).
Now a related, fascinating question that is being studied by several research groups, has to do with the future: what will happen to forests if atmospheric carbon dioxide levels are much higher than today? In other words, suppose the level of CO2 in the atmosphere keeps increasing - which it does in reality -, then what about the capacity of forests to store some of it?
On December 11, United States Dept. of Agriculture (USDA) Forest Service (FS) scientists from the FS Southern Research Station (SRS) unit in Research Triangle Park, NC, along with colleagues from Duke University, published two papers in The Proceedings of the National Academy of Science (PNAS) that provide a more precise understanding of how forests respond to increasing atmospheric concentrations of carbon dioxide (CO2), the major greenhouse gas driving climate change.
Building on preliminary studies reported in Nature, the researchers found that trees can only increase wood growth from elevated CO2 if there is enough leaf area to support that growth. Leaf area, in turn, is limited by soil nutrition; without adequate soil nutrition, trees respond to elevated CO2 by transferring carbon below ground, then recycling it back to the atmospheric through respiration:
biomass :: bioenergy :: biofuels :: energy :: sustainability :: carbon dioxide :: CO2 :: carbon sequestration :: forests :: climate change :: global warming ::
"With sufficient soil nutrition, forests increase their ability to tie up, or sequester carbon in woody biomass under increasing atmospheric CO2 concentrations," says Kurt Johnsen, SRS researcher involved in the project. "With lower soil nutrition, forests still sequester carbon, but cannot take full advantage increasing CO2 levels. Due to land use history, many forests are deficient in soil nutrition, but forest management -- including fertilizing with nitrogen -- can greatly increase growth rate and wood growth responses to elevated atmospheric CO2."
The studies took place at a Free Air Carbon Enrichment (FACE) study established by the U.S. Department of Energy on the Duke Forest in Durham, NC. In FACE studies, groups of trees are circled by rings of towers that provide CO2 to increase atmospheric concentrations of the gas around the selected trees. At the Duke FACE experiment, half of each ring was fertilized with nitrogen to study the effect of added soil nutrients on tree growth under elevated CO2.
The researchers further tested their hypotheses using data from FACE sites in Wisconsin, Colorado, and Italy. In the articles, the scientists identify critical areas needing further study, but the overall consistency they found across these diverse forests bodes well for developing accurate models to predict the ability of the world's forests to sequester carbon.
"Forests play a critical part in sequestering carbon, and may play a role in mitigating the elevated levels of carbon dioxide associated with climate change," says Johnsen. "To predict how much forests can sequester, we need accurate ways to predict what happens to carbon within forest systems and how this partitioning is affected by environmental conditions."
Picture: Aerial view of free-air carbon dioxide enrichment (FACE) rings at Duke Forest, Durham, NC. Credit: Will Owens.
More information:
The two articles can be accessed online at PNAS and on the SRS website:
-Aboveground sink strength in forests controls the allocation of carbon below ground and its [CO2]-induced enhancement (or at the SRS website)
-Canopy leaf area constrains [CO2]-induced enhancement of productivity and partitioning among aboveground carbon pools. (Or at the SRS website).
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