Are old forests carbon neutral?

Asks Rob from Los Angeles, CA

November 10, 2008
Old forests like this one are pulling more than their weight in carbon storage. [Credit:  Frank K.]
Old forests like this one are pulling more than their weight in carbon storage. [Credit: Frank K.]

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.

Also on Scienceline:

Pine beetles are threatening forests and potentially contributing to global warming.

Using cow poo to create energy.

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

About the Author



Scott says:

Great job! It’s nice to see old growth forests finally getting the recognition they deserve.

cynthia says:

Now that we have great carbon sinks, how do we utilize them without great danger to the environment?

Thanga says:

Such a good info. Lets join hands in saving our earth.

Doug says:

Good post. For those who want to learn more, here is a report on “Forests, Carbon & Global Warming” explaining how climate change is likely to affect forests as well as how forest conservation may help mitigate climate change. The report also helps debunk some of the flawed arguments used by logging advocates.
or see the polished pdf version without footnotes:

And here is a slide show clarifying many misconceptions about forests, logging, and carbon: http://www.slideshare.net/dougoh/forest-carbon-climate-myths-presentation/

Bob Roberts says:

The only problem is you cannot stop or even significantly alter climate change – that’s all nonsense! Humans don’t contribute to such processes in a way that can be detected or measured. The recent warming has ended despite continued deforestation (which I agree is bad) and continued fossil fuel use (which is NOT bad). Arbuments by people like DOUG that we can “mitigate” climate change are ridiculous. Right now the Earth is CLEARLY cooling. Despite FALSE claims the Arctic would soon be ice-free the ice there is clearly gaining. At the same time Arctic ice reached a minimum, Antarctic ice set a new record for extent but did we hear about THAT every 5 minutes? No, of course not, all we heard was about how the Larsen and other ice shelves were collapsing – WHICH THEY DO ANNUALLY! Himalayan glaciers were supposed to all melt by 2035 – ANOTHER LIE! In fact I challenge you to come up with one “climate crisis” the greeniacs came up with that was NOT a lie.

Mike Mc says:

Now wait a minute… are we talking sinks here or are we talking removing CO2 from the atmosphere? Something being C-neutral isn’t going to do squat for our current situation… we’re looking for sinks. Considering younger forests take in and use up more CO2 than old growth, wouldn’t it behoove us to select cut forests to keep younger generations of trees coming up and using CO2?

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