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Forests as a carbon sink – An overview

Malaysia is home to approximately 21 million hectares of land under tree cover, covering about 65% percent of the nation’s total land area. Forests are found throughout the country and contain more than 1000 species of trees. Although significant regional changes have occurred, the total area of forestland has been fairly stable for the past few decades (FDPM, 2021). 94 percent of Malaysia’s forestland is state-owned, as stipulated under the Federal Constitution, which stipulates land is a state-matter. The remaining 6 percent is privately owned, and under native people customary land ownership (FDPM, 2021).

The country’s forests (including standing planted trees) can sequester (absorb) and store a tremendous amount of carbon, and have significant potential to do more. According to a report by the World Resources Institute (2019), that the nation’s forests and forest products offset nearly 15 percent of domestic carbon dioxide emissions by storing 150 million metric tons of carbon dioxide per year, which is indeed a sizeable amount. Forests absorb carbon dioxide from the atmosphere and store it in different repositories, called carbon pools, which include trees (both living and dead), root systems, undergrowth, the forest floor and soils. Live trees have the highest carbon density, followed by soils and the forest floor. Harvested wood products and landfills also store carbon. When a carbon pool decomposes or is burned, it releases carbon as carbon dioxide back into the atmosphere. Invasive insects and diseases, drought, wildfires and urban development, all of which can be compounded with a changing climate, can affect the amount of forestland and the rate of carbon sequestration and storage (McNulty et al. 2017).
The role of trees
Trees are without a doubt the best carbon capture technology in the world. When they perform photosynthesis, they pull carbon dioxide out of the air, bind it up in sugar, and release oxygen. Trees use sugar to build wood, branches, and roots. Wood is an incredible carbon sink because it is mostly made of carbon (about 50%), it lasts for years as a standing tree, and takes years to break down after the tree dies. While trees mainly store carbon, they do release some carbon, such as when their leaves decompose, or their roots burn sugar to capture nutrients and water.

Let's examine a real example, a Meranti Sarang Punai (Shorea sp.) can live for 150 years; all that time it is pulling carbon out of the air and storing it. When the tree dies, it takes decades for the tree to rot. While it is slowly breaking down, the rotten tree is still keeping carbon out of the atmosphere. Generally, forests capture and store different amounts of carbon at different speeds depending on the average age of the trees in the stand and the number of trees in the stand. Young forests have many trees and are excellent at capturing carbon. Young trees grow quickly and are able pull in carbon rapidly. Not every small sapling becomes a large tree due to competition for light, resources, and growing space, but when they die and decompose little carbon is released. The trees that remain continue to grow and sequester more carbon as the forest matures. Established or mature forests are made up of "middle-aged trees", which are medium to large, healthy, and have a large root system. Middle-aged trees grow slower than young trees, but the amount of carbon sequestered and stored is relatively greater. Some of large trees occasionally die, but they are quickly replaced by younger trees who take advantage of the new space. Since more trees are growing compared to those that are dying, the overall net productivity (how many trees grow versus how many die) is positive and carbon capture is enhanced (McNulty et al. 2017).

Old-growth forests have a more fixed, or less dynamic, carbon cycle within live and dead trees and the soil. In old growth forests, large trees dominate by shading out small saplings, so recruitment of young trees and net productivity is near zero. Still, the carbon is well contained within the big trees, slowly rotting logs, thick leaf litter and soil. Large individual trees may take up as much carbon as an individual middle-age tree, but since there are fewer trees in an old growth stand, the rate of carbon sequestration is slower.
The importance of soils
The carbon that is sequestered in forests comes in many forms. For example, forest soils contain plant roots, leaf litter, and other dissolved organic material. The amount of carbon stored in forest soils is variable, and how much carbon soil can sequester is dependent on many local factors like local geology, soil type, and vegetation. In some forests, like in the rain forests of Malaysia, the soil holds relatively little carbon and the trees store more carbon. This is because some soil types, like clay soils, can bind up a large amount of carbon, whereas sandy soils are not able to bind much carbon. Soils with more organic material (bits of wood, decaying leaves, or dead creatures) can store more carbon because organic material easily binds loose carbon molecules and the organic material itself is stored carbon. Soils that are frozen for a good part of the year or have a high-water table can also store large amounts of carbon because decomposition is slow (McNulty et al. 2017).
Carbon cycle and climate change mitigation
Like all things natural, the carbon in forests eventually gets released into the atmosphere through the process of respiration, decomposition, and combustion. The rate at which these processes occur can vary across regions and forest types. There is growing global interest in using forests to help mitigate climate change. Forests that grow quickly and store carbon for long periods of time are well suited for this goal.

The tropical rain forest is often considered a good place for carbon sequestration and storage because it is full of big trees that grow rapidly. But research has found that the carbon moves in and out of tropical systems very quickly compared to temperate zone forests. Whole trees rapidly decompose in the hot humid climate and the soils can be low in organic matter. Also, the near constant rain helps to break down organic material and wash away soil and nutrients. So while tropical forests are good at capturing carbon, they are also just as good at releasing the carbon in a short time frame (McNulty et al. 2017).

In contrast, forest plantations in the tropics are also growing in importance, which may serve as an alternative carbon sink. In the final analysis, it is apparent that the forests, whether natural of planted forests have equally important roles in serving as carbon sinks, and will continue to become man’s strongest weapon to fight the global climate change.

References
FDPM (2021). State of Malaysian Forests. Forest Department of Peninsular Malaysia, Kuala Lumpur.

WRI (2019) State of Forests – A Global Assessment. World Resources Institute, Washington, USA.

McNulty, M.C., McGregor, T., and Calvin, P.E. (2017). Carbon Assessment of Forests – Case Studies. Geography, 14(3): 14-31.

Tarikh Input: 12/07/2023 | Kemaskini: 12/07/2023 | masridien

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