The temperatures of world rose about 2°F (1.1°C) since 1900 to date (NOAA, 2013). World’s climate is long-term shifts in temperatures and weather patterns, primarily as a result of human activities. The industrial revolution, burning of fossil fuels, increase of human population and harvesting of resources from forests, are the main causes of releasing dangerous and harmful greenhouse gases, importantly carbon dioxide (CO2), into the atmosphere. The amount of CO2 in the atmosphere passed 400 parts per million (ppm), its highest level since the Pliocene Epoch nearly 3 million years ago (National Geography, 2023). As greenhouse gas emissions trapped in the atmosphere of the Earth, they trap the sun’s heat, thus leads to global warming and climate change. Without any major reductions in these emissions, the increase in annual average global temperatures will be kept happening.

There are two main ways to stop the amount of greenhouse gases from emitted and increasing in the Earth: (1) stop releasing or reduce greenhouse gas emissions, and (2) increase the Earth’s ability to absorb greenhouse gas emissions from the atmosphere. These processes are called climate mitigation. There is not one single way to mitigate climate change. Instead, we will have to piece together many different solutions to stop the climate from warming.

Beyond the reduction of emissions (reduction of emission include the use of electrical vehicles, bioenergy instead of fossil fuels and adoption of green technology in industrials practices), there are a few other ways to reduce the amount of carbon in our atmosphere. The carbon removal strategies include familiar approaches like growing trees or forests as well as more novel technologies like direct air capture, which scrubs CO2 from the air and sequesters it underground. According to a World Resources Institute report released in 2020, forests have been one of the best carbon removal tactics for millions of years because of the trees and soil in the forest have the ability to sequester and store carbon as long as they stay standing (Khan, 2022). Mortaigne (2019) mentioned that the largest single place that’s removing CO2 from the atmosphere on an annual basis is forests.

Forests absorb CO2 from the atmosphere of Earth and store it in different repositories, which called carbon pools. The carbon pools include above ground trees (both living and dead biomass), below ground root systems, undergrowth, the forest floor and soils. Above ground trees have the highest carbon density, followed by soils and the forest floor. According to FAO’s Global Forest Resources Assessment 2020, the world’s forests store approximately 662 gigatonnes of carbon, with 44 percent in soil and forest floor (to one-meter depth), 42 percent in live biomass (above- and belowground), 8 percent in dead wood, and 5 percent in litter (FAO, 2020). In total, this is equivalent to nearly a century’s worth of current annual fossil fuel emissions. Tropical rainforests account for only 30 percent of global tree cover but contain 50 percent of the world’s carbon stored in trees. Tropical forests store most of their carbon in vegetation (biomass), and boreal forests store vast amounts of carbon in soils.

Forests capture and store different amounts of carbon at different speeds depending on the type of trees (coniferous and broad leaves), trees age and the number of trees in the forests. Young established forests have many trees and are excellent at absorbing carbon from atmosphere through photosynthesis. Young trees grow quickly and are able pull in carbon rapidly. Tree development from juvenility to maturity to senescence has been characterized in several ways, including chronological age, size and reproductive capacity. Young trees have a high ratio of photosynthetic area (leaf surface) to biomass. With this high ratio, they can generate a surplus of energy, which is used to fuel rapid growth. Nonetheless, not every small trees can survive into large tree. This may due to competition for sun-light, nutrition, and growing space but when these trees 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 refer to trees aged between 50 and 99 years are made up of trees around middle-aged, which are medium to large in stand size, healthy, their crowns will flatten out as limbs grow thicker and heavier 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. In this forests, certain of large trees occasionally die, but they are quickly replaced by younger trees who take advantage of the space left by the death trees. Preserving mature forests can play a vital role in removing CO2 from the atmosphere, says policy scientist William Moomaw (Montaigne, 2019). Since more trees are growing compared to those that are dying, the overall net productivity is positive and carbon capture is enhanced.

As forests age, plants grow and die to fill available space, so old-growth forests are more filled with carbon-storing plant matter than middle-aged forests. Old-growth forests have a more fixed, or less dynamic, carbon cycle within live and dead trees and the soil. Forests naturally experience loss of trees due to age, disease, weather, and competition. In old growth forests, large trees dominate by shading out small trees, 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. Nevertheless, old growth forests can continuously operate as carbon-rich banks.

3.1 Trees

Trees are without a doubt the best carbon capture technology and most cost-effective ways in the world. Growing trees require carbon dioxide from the atmosphere for photosynthesis and after the process they store that carbon and release oxygen. During the process of photosynthesis, they absorb CO2 out of the atmosphere, absorb sun light into their leaves into green-pigmented chloroplasts in cells, bind it up in glucose, and release oxygen. Trees use glucose to build wood, branches, and roots. This carbon remains locked up for the life span of the wood. 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 during respiration or when their leaves decompose, or their roots burn glucose that stored as starch to capture nutrients and water. As mentioned early, although trees do produce carbon dioxide during respiration and other form of carbon emission, they are net absorbers of CO2 because of the carbon store as starch in the tree trunk, root and branches.

Webber (2022) in his technical note published on 2022, saying that in a single year, a mature live tree can absorb more than 22 kg of CO2. Carbon constitutes approximately 50% the dry mass of trees and when wood from these trees is used to produce wood products the carbon is stored for life in that product.

3.2 Soils

The carbon that is sequestered in forests, beside through the growing of trees, its can comes in other form. In temperate forest ecosystems, the amount of carbon stored in soils is often greater than the amount stored aboveground in living and dead plant biomass. Forest soils contain plant roots, leaf litter, and soil organic matter. The amount of carbon stored in forest soils show more variation than the above ground biomass (live or dead trees), and the amount of carbon soil can sequester is dependent on many local factors like local geology, carbon inputs from vegetation, carbon losses from decomposition, soil type, and vegetation. In some forests, like those mountains (volcanoes) nutrient-rich soil types , the soil holds more carbon than the trees, but in other forests, like the sandy forest, the soil holds relatively little carbon and the trees store more carbon. This is because some soil types, like organic soils, can bind up a large amount of carbon, whereas sandy soils are not able to bind much carbon. Soils with more organic material 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.

Carbon and other green house gases (methane) within forests are captured and released on a cycle. Trees in the forests absorb carbon of the atmosphere to make glucose during photosynthesis, but they also release CO2 back into the atmosphere through respiration and decomposition. Thus, forests can be net sinks through sequestration or net sources of carbon, depending on the trees age, health condition and susceptibility to biological decay, wildfires and other disturbances and on how the forest being managed. 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. Mature forests store more carbon but at very slow pace of sequestration than younger managed forests. As old trees die or are lost to insects, storms, or fire, they release their carbon back to the atmosphere. However, harvested wood from these forests when converted into wood and paper products can store the carbon permanently.

The carbon sequestered by trees from the atmosphere can be stored for decades in paper and wood products such as flooring, furniture and construction timber. Thus, a wood-based carbon pool exists through the chain of forests in the form of finished wood products. While these wood products continue to store carbon, the forests from which they were harvested, will regrow with young trees and thereby sequester additional carbon from the atmosphere. Increasing the use of wood in long-lived applications is therefore another strategy for climate-change mitigation and green economic development.

The rain forest either Amazon or Tropical is often considered the perfect place for carbon sequestration and storage because it is full of big trees that grow rapidly. But recent research has found that the carbon moves in and out of tropical systems very quickly compared to temperate zone forests (University of Leeds, 2020). Inside the rain forest, the dead trees decompose rapidly in the hot humid climate and the soils can be low in organic matter due to the constant heavy rain that helps to break down organic material and wash away nutrient rich top-soil. So while tropical forests are good at capturing carbon, they are also just as good at releasing the carbon in a short time frame.

Forest management is the key to offer trees to sequester more carbon through planting of fast growing species, changing the age structure and tree density in the stand. The standard protocols or voluntary methods used in a carbon offset project are often based on sustainable forestry and are designed to increase total carbon gains over time. For example, partial cutting is a practice that increases forest carbon sequestration rates and maintains higher carbon storage in soils compared to clear-cuts. In stands where the trees are many different ages, there is continuous recruitment of younger trees, but older trees also remain as carbon-sink that will help hold carbon for longer periods.

In order to make the good decision on forest management about how to improve growth and tree regeneration for climate mitigation, Bellassen and Luyssaert, (2014) proposed to that forestry management should prioritize 'win–win' strategies, which increase both forest stocks and timber harvest, through certain silviculture measures. In sustainable managed forests, some trees can be removed using single tree or group selection harvesting methods. Nonetheless, removing individual trees in the forest can disturb the soils in that particular area. The disturbance of soils scan turn soils from a carbon sink to a carbon source over the time. Thus, to prevent and minimize soil disturbance in these stands, the best practice in forest management is to extend the cutting rotation period. For example, a mangrove forest that has been traditionally thinned every 15 years and 20 years followed by clear felling at 30 years, could be thinned ever 20-25 years and clear felling at 40 years, so the soils have time to recover between entries.

Forests of different ages play vital roles in carbon sequestration from the atmosphere and storing in the stands. Old forests which consider as carbon store have accumulated more carbon than younger forests. On the other hand, young forests grow rapidly, sequestering much more CO2 every year from the atmosphere than an older forest covering the same area. Trees and soils keep a lot of carbon trapped in the forest, pulling it from the atmosphere. However, carbon stored in forest soils depends on type of soils. Sustainable managing forests by avoiding large carbon emissions from the loss of old trees while rapidly removing CO2 from the atmosphere through young forest growth is the recommended practice for both storage and sequestration benefits. In addition, when trees are sustainably harvested, wood continues to store carbon. Carbon content moves through different levels during their life cycle. Beside, the wood products provide an added benefit when they are used in place of more energy-intensive ones that require more fossil fuel emissions.

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