Climate Change and Tree Growth: New Trees Can’t Handle As Much Carbon As We Thought, Study Shows

Photosynthesis is driven by atmospheric carbon dioxide, so trees are often planted to capture and store the increasing carbon emissions we face today. It turns out that our reliance on them may be overestimated, according to a recent study.

To better understand future tree growth related to increased carbon emissions, researchers from the University of Illinois at Urbana-Champaign and the University of Bristol in the United Kingdom modeled future land cover and use. They discovered that forests are limited on how much carbon they can absorb, because they will also be storing nitrogen, according to a news release.

"Forests take up carbon from the atmosphere, but in order for the plants to fix the carbon, it requires a certain amount of nitrogen," Prasanth Meiyappan, graduate student who worked alongside atmospheric sciences professor Atul Jain, said in the release. "If that ratio of carbon to nitrogen isn't right -- even if you add many times more carbon than it gets currently -- the forests cannot absorb the extra carbon."

For their study, the researchers specifically examined secondary forests – areas that are regrowing after being destroyed by deforestation, wood harvest or fires. These areas are generally slow-growing because nitrogen in the soil is released when the original forests are disturbed.  Most forests are actually considered secondary forests. 

"The carbon that is lost from a forest during harvest or fires can be replaced over time if the forest regrows, so net carbon loss is minimal. If forest regrowth is limited due to a lack of nitrogen, then net carbon emissions are higher," House explained.

Plant matter, such as leaves and sticks, is also removed for burning, which further limits nitrogen. The Intergovernmental Panel on Climate Change (IPCC) has projected that bioengineering will be needed in order to meet the world's increased energy consumption. 

"Residue decomposes over time and releases nitrogen for the plants," Jain said in a statement. "By taking it out, it further tilts the nitrogen-to-carbon ratio in the future."

Since most IPCC models don't account for nitrogen, researchers believe projections are underestimated by 90 to 150 percent.

"If net land-based emissions are underestimated, it means stronger mitigation action would need to be taken in other sectors such as energy to meet the same mitigation targets," House added.

Researchers hope that this study will aid in future projections accurately modeling the strength of land carbon uptake.

Their study was recently published in the journal Global Biogeochemical Cycles. 

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