How forests can combat climate change

The implementation of AB 32 — the state’s new law aimed at reducing greenhouse gas emissions – holds great promise for the future of California.

The new law provides an opportunity for the forest resource sector to be one of the most significant strategies to reduce emissions through carbon sequestration. That’s good news. It’s high time California encouraged the judicious use and management of its forestlands.

While burning fossil fuels spews carbon and other greenhouse gases into the atmosphere, forest management can reduce carbon pollution. It can also buy us time to develop more efficient technologies that ultimately minimize human contributions to global warming.

In particular, it’s the young trees and forests that are most efficient in taking up carbon. Not that old forests don’t help – they do. But when their capacity to remove carbon is measured against young forests, old forests come up short.

First, a little Biology 101: Trees take up carbon dioxide from the air and release oxygen in the process of photosynthesis. This is commonly referred to as carbon sequestration. The carbon is then stored in leaves, branches, stems, roots and soil. Trees also respire some carbon dioxide back to the atmosphere. In young forests, the uptake of carbon dioxide greatly exceeds the loss due to respiration. The reverse can be true for very old forests.

This prompts two questions: Can we enhance a forest’s capacity to store, or sequester, carbon? And if so, what’s the best way to do it?

The first answer is fairly straightforward. Increasing carbon storage in forests goes hand-in-hand with other forest management goals, such as providing essential wood products, enhancing watershed health, and maintaining biodiversity across the landscape. What’s good for forest health is good for carbon sequestration. Forest management can certainly increase carbon sequestration, especially when the carbon forests capture is stored long-term in wood products like lumber for building construction and furniture.

So how do we enhance carbon sequestration by forests? By improving growing conditions, controlling stand density, increasing tree vigor, examining rotation lengths, and encouraging the development of urban forests. The more rapidly leaves are produced, the more carbon is taken out of the atmosphere. The faster a tree grows, the more effective it is at removing carbon from the air. Creating ideal conditions for growing trees also creates ideal opportunities for carbon sequestration.

As forests become older the rate at which they take up carbon slows and the rate at which they lose carbon to the atmosphere through respiration and decomposition accelerates. Though older forests with many large trees may store more carbon than young forests, there is a trade off in that young forests are better at sequestering carbon.

The climate-helping character of young forests should be a boon to society because whether you’re interested in wood production or carbon sequestration, the forest-management approaches are similar: you want to increase leaf area, maintain forest health, and accelerate growth. That means thinning forests to remove the less vigorous trees, leaving the rapidly growing trees. Replanting the land with fast-growing trees quickly restores the forest canopy and continues the process of sequestering carbon.

The same forest-management techniques that maintain healthy forests and sequester carbon offer another climate-change benefit: they reduce the threat of high-intensity wildfires that release tremendous amounts of carbon into the air in a single, catastrophic event.

Although there’s currently no market incentive in place to manage forestland to store carbon, California established the Climate Action Registry in 2001, and the California Air Resources Control Board is refining protocols pursuant to adoption of AB 32. When finalized, these guidelines will likely reward forest management activities that increase carbon sequestration. Without financial incentives it is unlikely that forests will reach their full carbon sequestration potential.

But this much is certain: rapidly growing trees sequester carbon more quickly and efficiently than old ones. That fact should stay front and center in policy discussions. If we want to maximize carbon sequestration and storage, we need forest management that results in healthy forests of all ages on the landscape. That means sustainable forestry, and plenty of young forests.

John A. Helms, Ph.D. is professor emeritus of forestry at the University of California, Berkeley, and the immediate past-president of the Society of American Foresters.

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