Are old forests carbon neutral?

Until recently, ecologists often assumed that older forests consume the same amount of carbon that they release into the atmosphere, making them carbon neutral. But new evidence suggests this is likely not the case. Old forests may actually continue to accumulate carbon for hundreds of years, making them important sites for carbon storage.

When forests are young, they grow rapidly, gobbling up lots of carbon from the atmosphere during photosynthesis. They use this energy from the sun to grow and then store the carbon in forest soil and vegetation. Forests release carbon both when plants breathe and when microbes decompose dead forest matter.

As the forest canopy thickens and fills in, it blocks the sun from reaching the forest floor and stimulating new growth. In theory, as a forest ages and needs less carbon for growth, carbon removed from the atmosphere should eventually balance the amount released through respiration and decomposition, rendering the forest carbon neutral.

A September study in the journal Nature suggests that carbon “deposits” actually exceed “withdrawals” in old forests because they continue each year to accumulate more carbon than originally thought in soil and vegetation.

In the Nature study, an international team of scientists synthesized data from over 500-old-growth forest sites ranging in age from 150 to 800 years. Although the results show that the uptake of carbon does slow over time, even 800-year-old forests swallow more carbon than they spew out.

The amount of carbon stored in a forest is like the amount of money in your bank account. If you continue to deposit more money than you withdraw, your bank account will grow.

“How much is in your bank account and how much your bank account changes each month are two entirely different questions,” says Eric Davidson, a senior scientist and forest ecologist at Wood’s Hole Research Center in Massachusetts.

In other words, old growth forests may not be growing as fast and thus consuming carbon at the same rate as young forests. Instead, because of their age, old forests tend to contain more carbon than young forests. They have a bigger bank account and thus are valuable carbon sinks, or carbon storage areas.

Identifying old forests as carbon sinks may help explain why deforestation causes such high carbon emissions. Deforestation alone accounts for more than 20 percent of global carbon emissions each year. Cutting down more old forests to make space for development and agriculture could increase this amount.

Beverly Law of Oregon State University, a forest ecologist and an author of the study, is particularly concerned about the preservation of old forests for this reason. She hopes that more research on forest dynamics will prompt global policy-makers to protect one of our best lines of defense against global climate change—nature’s own carbon sinks.

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Using cow poo to create energy.

China’s environmental future may depend on reclaiming some of its past.