How Forests Stock Carbon — By Breathing

Forests are fantastic carbon sinks because they absorb more carbon than they release. But how? And which ones do it best?

Photo by Victor on Unsplash

Carbon: Forests’ Favorite Gas

Forests are one of the best natural carbon sinks in the world. But why do forests want to absorb carbon anyways?

Carbon is essential for the process of photosynthesis. Imagine this as plants breathing in carbon dioxide from the air (among other gases), binding it in sugar, and releasing oxygen back out. In forests, sugar is used by trees to make wood, branches, and roots.

The chemestry behind photosynthesis

Wood is an excellent carbon sink because it lasts for years as a standing tree and takes years to decompose once it dies. Trees store the majority of carbon in their heartwood, which is at the very center of the trunk, but also in their branches, leaves, roots, and eventually, transfer it to the soil¹. Trees mostly store carbon, but they release it too. When their leaves decay or their roots burn sugar to gather nutrients and water, or if they are cut or burned down, carbon is released.

Two important factors that determine how much carbon a forest can store are the type of forest and its age.

1. Boreal or Tropical?

Not every tree nor every forest stores carbon equally. For a long time, scientists thought that boreal forests, found in the far northern corners of the northern hemisphere, were the most efficient at storing carbon. But this has changed for two main reasons. First, we understand better how air moves around the Earth (global air patterns). And second, have a better grasp on how the carbon cycle works in entire ecosystems, not just individual trees. A recent NASA study suggests that tropical forests can absorb up to 1.4 billion metric tons of CO2 per year — which is 56% of all carbon dioxide absorbed in forests².

2. Carbon Sinks Across Generations

The age of a tree and a forest also has an important role to play in carbon sequestration. Old-growth forests range in age but are considered to be forests left intact by humans. On the other end of the spectrum, young forests are those that are growing back, naturally after a shock like a fire, or through human reforestation.

The Elders — Old Growth Forests

As long as they are not cut down, old-growth forests are excellent for keeping the carbon they already hold inside. Until recently, scientists believed that old-growth forests’ carbon stocks are relatively neutral. But findings from a ten-year study run by the University of Birmingham suggests that trees’ ability to absorb carbon is like a muscle: when more carbon is available, they can stretch it to absorb the excess³. This kick starts photosynthesis and converts the extra carbon into glucose to use as food.

The Adults — Secondary Forests

Secondary forests are neither young nor old, but they’ve been around long enough for a diversity of understory trees and plants to take root. These forests are valuable carbon sinks because they represent a period in the forests’ lifecycle when significant amounts of carbon are transferred into the soil, as trees die and rot on the forest floor.

The Children — Young Forests

Young is relative: forests are considered young until they reach 140 years old. But according to more research from the University of Birmingham⁴, more than half of carbon storage in forests is found in these young forests. For a young tree to thrive, it must spend years competing for space and sunlight, which pushes intense growth, and therefore stocks away lots of carbon.

Creating Massive Carbon Sinks by Regenerating Forests

Old-growth, secondary, and young forests are all essential in balancing carbon absorption on a global scale. At CarbonABLE, we understand the forest’s carbon cycle and see enormous potential for stocking carbon by regenerating young forests. We know that humanity only has less than three decades to take drastic climate mitigation action⁵. By focusing on regenerating forests, we focus on supporting the forests that can store carbon fast and help us re-establish a balance in the carbon cycle.

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