Reforestation: Are Forests a Carbon Sink?

Reforestation: Are Forests a Carbon Sink?

Contrary to what many of us were taught as kids, trees don’t “breathe opposite” to humans. Yes, photosynthesis takes place during the day, allowing a tree to take in carbon dioxide from the air and (with the help of sunlight) create sugars to help the tree grow. However, at night, trees need to release energy, releasing carbon dioxide, to break those sugars down into something useful. In short: trees also ‘exhale’ carbon dioxide into the air!


So if humans release carbon into the atmosphere by breathing, and trees also release carbon into the atmosphere via respiration, we’re going to need more carbon sinks (anything that absorbs more carbon than emits it into the air) to put all that carbon back into the ground.

Can forests be considered a carbon sink? First, we need to understand the role of forests in the carbon cycle.


What is the role of forests in the carbon cycle?

Growing up, we all gained an intuition about what a forest is: an area of land covered in trees. Forests play a critical role in the carbon cycles because of how they interact with carbon dioxide. Most of what a tree needs to grow comes from the air (not just the soil). Depending on the age of a forest, they can either help sequester or store carbon. Carbon is sequestered during photosynthesis and stored in the roots, trunks, and nearby soils of each tree.


How does a tree’s age affect the carbon cycle?

Let’s think of it as an ‘all-you-can-eat’ air-carbon buffet for trees. There’s too much food at this buffet and we need all the help we can get eating it before it goes bad and stinks the place up. You can take it off the tray and put it on your plate (sequester), but you’re also going to need to eat it (storage) (Ayrey, 2021)!

  • For sapling (baby) trees: you can invite infants to a buffet, but if the goal is to eat through most of the food - they won’t make much of a dent. Baby trees suck up some carbon from the air, but they’re too small to make a rapid or huge impact in sequestering lots of carbon from the air.
Lots of baby trees together (a young forest) are not a significant carbon sink. The baby trees are not big enough to store lots of carbon in their wood. But give them enough time and they’ll grow to be...


  • Adolescent (mid-aged) trees: they need a lot of energy to grow, so they’ll actually eat up a lot and pack it away, mostly in their trunks. Most of the carbon sequestered by this tree-age goes toward helping the tree grow big, tall and strong. This will allow the tree to reach upward to find more sunlight and fresh air. 
Predominantly mid-aged forests with many mid-aged trees will actually pull more carbon in than out, making it a carbon sink. They’re starting to get big enough to store more carbon in their wood.  Give them more time and they’ll grow to be...
  • Older trees; they might snack on a bit of carbon here and there, but they won’t actually eat much, since their digestion rate is going to be slower and they’re not growing as much anymore. They’re mostly sequestering just enough carbon to keep energy levels even and keeping daily operations in-check. However, these older trees have already stored a lot of carbon in their root systems,  wood, and soils in their lifetimes that would all immediately go back into the air if these trees are chopped down, diseased, or burned. 
A predominantly old-aged forest has essentially reached a carbon-equilibrium: about equal carbon drawn in and released out of the air. So they’re not a significant carbon sink. However, because of all the carbon they drew in as mid-aged trees, their wood, soils, and root systems still make for amazing carbon storage.


What happens to the carbon cycle when forests are lost? 

Forests can be lost in a few different ways: disease (viruses and parasitic insects), deforestation (humans cutting down trees), drought, wildfires, etc. When a forest is lost, most of the carbon stored in the trunks, soil and roots of the trees goes right back into the atmosphere. So, not only do our tree friends stop eating and storing carbon, but all their hard work of eating and storing carbon comes undone.


What is reforestation, and how can humans help?

Reforesting refers to the process of replanting trees in areas of land that were previously populated by trees (invite more trees to the carbon-buffet!) 

Humans are the primary cause of deforestation; harvesting timber or wanting to repurpose the land area (think: houses, resorts, cities, farms). So, while reforesting is critical, it’s also just as important to stop cutting down forests that already exist. We need the teenage-trees and older trees to keep eating all that extra carbon that’s already in our atmosphere.


What are forest carbon offsets? 

One important topic to mention here are forest carbon offsets. What are they? Forest carbon offsets are when forest land owners (privately owned forests) are given credits to not cut down their trees and to further protect the forests that already exist on their lands. These credits can then be sold to major polluters to try and balance 1 polluter emission with 1 tree sequestration. 

Does this actually help solve our excess carbon problem? Well, not really. We’ll ultimately need these polluters to stop polluting if we hope to clean up the skies AND plant more trees. The overall carbon balance is still way off. There are also concerns about land owners fudging the offset system and over-claiming offsets (ProPublica, 2021). In the meantime, these forest offset programs help protect private forests from being cut down and literally buy them more time to grow (assuming they don’t burn up in wildfires). 


So, are forests a carbon sink?

Yes and no. Older forests store a lot of carbon and do a great job at making sure that carbon doesn’t release back into the atmosphere. Mid-aged forests are carbon sinks in that they vacuum up a lot of carbon from the air while they’re growing (and also store it in their wood and soils). And younger forests are important because they help rebuild ecosystems that were lost to deforestation, AND will one day become mid and old-aged forests themselves! By protecting forests and also reforesting previously lost forest areas, we allow all trees to do their part to restore balance to the global carbon cycle. 


Forest Feature: Mangrove Trees!

Mangrove forests are some of the most carbon-dense ecosystems in the world, and are able to quickly store about 4x (!!) as much carbon than tropical rainforests (which are dense carbon sinks in their own right). Mangrove trees live in coastal areas, with their root systems reaching deep into marshy soils. Because of the waterlogged soils, carbon has a harder time leaving the soil and returning into the atmosphere, making them amazing long-term carbon sinks. Unfortunately, because of the location of many of these forests, they’re typically competing for land space with resort towns.  Many of these forests are ripped out to make way for vacation spots, which immediately releases all that carbon stored in their roots back into the air. Luckily, mangrove reforestation projects are also underway to try and mitigate this issue.  (Hance, 2011).

If you’re curious about what U.S. land would be great for reforesting, check out the reforestation hub!

Thank you to a relative for the “toddlers at a buffet” analogy that I promptly stole, and for listening to me talk about forests while we burned backyard wood around a bonfire.



Ayrey, E. (2021, December 6). Forest carbon basics [Video]. YouTube. Retrieved October 16, 2022, from

Hance, J. (2011, April 5). Vanishing mangroves are carbon sequestration powerhouses. Mongabay Environmental News. Retrieved October 16, 2022, from

ProPublica. (2021, May 12). The Climate Solution Actually Adding Millions of Tons of CO2 Into the Atmosphere. Retrieved October 16, 2022, from

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