As part of Celebrating Science during Earth Week, we are focusing on the science behind trees. King County and partners will plant one million trees by 2020 across King County in both urban and rural areas.
Jennifer Vanderhoof, Ecologist with King County Water and Land Resources Division writes an Earth Week exclusive.
How trees store carbon?
Before we talk about trees, it’s important to know that all known life on Earth is made from carbon.
The subject of chemical compounds that contain carbon is called organic chemistry because all known organisms, or living things, are made up of water and carbon compounds. (Which means that no matter how many pesticides are in your apples, they are still technically organic!)
Carbon forms the major component of all living things including trees. Trees and wood are 50 percent carbon by weight (the other half? It depends on the species, but generally 42 percent oxygen, 6 percent hydrogen, 1 percent nitrogen, and 1 percent other elements).
Carbon may also take the inorganic form of coal, charcoal, and diamonds. Further, fossil fuels, such as coal, oil, and natural gas, all contain large amounts of carbon that was originally organic carbon in the form of plants and animals decomposed over millions of years.
Burning fossil fuels releases large amounts of carbon as CO2 plus other greenhouse gases into the atmosphere faster than natural processes. The problem of trying to deal with the excess CO2 is one of the reasons that planting trees has become so important: because trees store carbon as they grow.
Often when people talk about the various means of absorbing carbon, they refer to “carbon sequestration.” Carbon sequestration is a process where CO2 is pulled from the atmosphere and stored for a long period of time. CO2 can be stored in many places: oceans, wetlands, soil, and forests, to name a few. It is through photosynthesis that atmospheric CO2 is converted into sugar and cellulose and stored in the tree’s wood, leaves, branches, and roots. Oxygen is released back into the atmosphere as a by-product of photosynthesis, which animals greatly appreciate because they depend on it for survival.
What type of trees store more carbon and why?
In general, the larger the tree, the more carbon it stores, because half of a tree’s mass is carbon. However, some trees get larger faster so will sequester carbon at a faster rate, but over the life of a tree it may not sequester as much as a tree that grows more slowly but ultimately gets much larger.
If we are planting trees in the Pacific Northwest, chances are extremely high that at some point in the past 200 years, there were trees in that location before. If we are planting a forest, like we do in many of our restoration projects, the intent is that the area will remain forested indefinitely. So even if some trees in the forest dies, other trees will grow up to take their place. In the long term, the amount of carbon sequestered by that forest should remain about the same on average once the trees have reached maturity.
What is unique to the region’s forests in their ability to store carbon?
The wet temperate conifer forest types growing along the Pacific Coast from northern California to southeast Alaska have a high density of large trees. These trees have the highest average carbon per acre in the nation—averaging 92.9 metric tons of carbon per acre. And in fact, old-growth forests of the Pacific Northwest store more carbon per unit area than any other forest, anywhere on Earth. Which means there is an incredible amount of potential to sequester carbon by planting trees today that will eventually grow to become old-growth forests of the future.