• For millions of years, carbon has been exchanged between the Earth's atmosphere and its land and oceans. Carbon dioxide (CO₂) in the air is continually converted by natural processes into organic compounds and carbonate rock minerals, while also dissolving into seawater.
• Forests play a crucial role in this cycle, as trees and plants absorb CO₂ from the air through photosynthesis, using the carbon to generate energy and to build the molecules required to make their roots, stems, branches, and leaves.
• The destruction of forests by humans over centuries has severely weakened their ability to regulate CO₂ levels, while also releasing vast amounts of stored carbon from degraded forest soils and destroying unique ecosystems.
• In recent decades, global efforts to protect and restore forests have intensified, originally motivated by the need to preserve biodiversity but now increasingly recognising the need to prevent further carbon emissions from these major sinks.
• At the same time, afforestation—planting trees on previously unforested land—is increasingly recognised as a means of removing CO₂ while providing a renewable source of timber and biomass for energy.
• However, large-scale tree planting faces challenges, including land availability, water use, and competition with other land uses such as agriculture, alongside the fact that artificial forests are typically less resilient to disease and fire than mature natural forests.
• As with all CO₂ removal methods, afforestation efforts funded through carbon markets must ensure accurate measurement of CO₂ removal while also accounting for emissions resulting from forestry activities.
• Equally important is understanding the permanence of CO₂ removal—what happens to the trees after they are harvested and how soon their stored carbon re-enters the atmosphere.

What are the processes by which forests pull CO₂ from the atmosphere, and put it back?

Trees and plants absorb CO₂ from the atmosphere through photosynthesis, converting it into glucose and oxygen:

The glucose produced during photosynthesis is used by the plant in two main ways.

1. It can be directly converted into the energy required for the plant's life processes through the process of respiration, resulting in the release of CO₂ back into the atmosphere:

2. Some glucose will be converted into more complex organic molecules to construct the plants' biomass as they grow:

Formation of cellulose - storing carbon in plant matter

It is the carbon contained in these molecules which accounts for the ability of all plants to act as carbon stores.

As trees and other vegetation die and decay, they release stored carbon back into the atmosphere. Decomposition is a natural process where dead plant material, including fallen leaves, branches, and entire trees, is broken down by bacteria and fungi, releasing CO₂ as they consume the plant matter. This balance between growth, respiration, and decay underscores the importance of both preserving existing forests and encouraging new forest growth for effective carbon management.

This illustrates that forests are dynamic systems, with trees at different stages of growth playing varying roles in carbon uptake and storage. Younger, rapidly growing trees are particularly good at absorbing CO₂ from the atmosphere. As these trees grow taller and thicker, they store large amounts of carbon in the organic material used to build their trunks, branches, and roots. 

The growth of natural forests across thousands of years has resulted in the removal of billions of tonnes of CO₂ from the atmosphere to be stored in the bodies of trees, other vegetation and soil. However, human intervention in recent centuries has led to the destruction of vast areas of forests, as the requirements of land for agriculture and the need for timber as a building material have been met by cutting down trees, causing the release of billions of tonnes of CO₂, as well as the loss of precious and unique forest habitat.

According to the Global Forest Resources Assessment 2020, the world lost 178 million hectares of forest from 1990 to 2020. The rate of net forest loss has decreased significantly over this period:

In the 1970s conservationists succeeded in drawing international attention to the plight of the world’s forests, especially the rate of deforestation which was taking place in the Amazon rainforest.  When the world’s other hot environmental issue of the late 20th century, global warming, began to emerge, the two topics became increasingly intertwined due to the increasing understanding of the role forests play in regulating the climate. Despite growing awareness, there has been a persistent lack of concerted international action to address either issue effectively.

How can we begin to reverse the damage inflicted on the world's forests over hundreds of years? Is it possible to restore at least some deforested land to its original condition or should we concentrate on preventing further destruction? From a carbon removal perspective, would it be better to create new plantations of trees with fast growing species which will pull CO₂ out of the air more rapidly than mature forests?

In terms of forest conservation and reforestation, human intervention is aimed at protecting and repairing existing natural forests. These are forests which have developed over hundreds or possibly thousands of years with minimal human intervention, except potentially for the presence of indigenous communities who practice sustainable forest management. These forests support incredibly complex ecosystems of plants, animals, and microorganisms adapted to the local conditions, while the presence of a diverse mix of tree species enhances the forest's resilience against diseases that may target a single species.
While many trees in natural forests are mature and grow more slowly, they continue to sequester carbon, albeit at a slower rate than younger forests. Importantly, natural forests represent an enormous carbon reservoir, storing vast amounts of carbon in both vegetation and soils.

Preserving existing forests and rejuvenating degraded ones often requires minimal human intervention - simply halting activities on the land and allowing natural processes to resume may suffice. Control measures may be necessary for invasive flora and fauna. For instance, naturalists at Yellowstone National Park found that reintroducing wolves—an apex predator that helps regulate herbivore populations—enabled vegetation to thrive, resulting in a remarkable recovery of plant life.

However, from the standpoint of those seeking rapid outcomes—such as capturing CO₂ from the air to sell carbon credits—this minimal-intervention approach faces one key challenge: nature needs time to do its work. Financing a reforestation project through the sale of carbon credits for carbon which may not be absorbed for decades can be a hard sell, not helped by stories of unscrupulous developers exaggerating, or even fabricating the carbon storage potential of forests in order to justify selling carbon credits.

Meanwhile, afforestation is a technique aimed at establishing new forests where none previously existed. The motive may be commercial, growing so-called plantation forests to provide timber for construction or wood for burning at bioenergy plants. Other afforestation projects may have environmental goals, such as preventing or reversing mass desertification, or to increase river and groundwater levels to improve conditions for local farmers.
More recently, the concept of planting new forests for the express purpose of CO₂ removal, where the CO₂ estimated to have been captured can be sold in the form of carbon credits to companies and individuals who wish to offset their own emissions.
Unlike natural forests, which evolve over centuries and often contain a diverse array of tree species adapted to local conditions, plantation forests usually consist of a single species or a limited number of species selected for their commercial value, and not necessarily native to the area.

How effective plantation forests are at reducing CO₂ levels depends on several factors:

• Will the trees eventually be harvested, to be replaced by younger faster growing trees?
• If harvested, will the wood be used for construction, or burned to produce energy?
• If the trees are destined to be sent to fuel bioenergy plants, will the CO₂ emissions from burning the wood be released into the atmosphere or captured and permanently stored underground? (see BECCS)
• How much CO₂ will be produced from mechanized activities associated with growing, cutting down and transporting?

While re-establishing degraded forests with diverse species of trees and encouraging native wildlife to return seems intuitively like a positive move for the environment on all fronts. Meanwhile, planting new forests where none previously existed, using a lesser variety of possibly non-native trees, can also be beneficial as long as it doesn’t come at the expense of other types of natural landscapes which have taken thousands or millions of years to evolve.

However from the perspective of creating artificial forests with the specific aim of removing CO₂ from the air, how significant is their impact?

Estimates of how much CO₂ is absorbed by forests range significantly, depending on the age of forests and the types of tree. A 2018 report entitled "Global carbon dioxide removal rates from forest landscape restoration activities" has estimated the range of CO₂ sequestration rates for forests varies depending on the specific ‘forest landscape restoration’ (FLR) activity:

According to the report, the amount of CO₂ sequestered by various forest types in one year during the first 20 years of each project is estimated to be:

• Planted forests and woodlots: 4.5 to 40.7 tonnes
• Natural regeneration: 9.1 to 18.8 tonnes
• Agroforestry: 10.8 to 15.6 tonnes
• Mangrove restoration: Up to 23.1 tonnes

So how do these figures stack up when compared to human emissions of CO₂?

Current annual emissions of CO₂ from human activity are estimated at around 35 billion tonnes, equating to 96 million tonnes per day.

Some rough calculations tell us that close to 2.5 million hectares of incredibly efficient forest —an area around the size of New Jersey or the island of Sardinia absorbing 40 tonnes of CO₂ per hectare per year—would take a year to account for just one day's global CO₂ emissions. While no forest is going to maintain that rate of CO₂ absorption, it does give an idea of the scale of forestation which would be required to make a dent in the current elevated CO₂ levels.

Is it just a matter of planting as many trees as possible?
While large-scale tree planting is crucial for carbon sequestration, it's not as simple as planting trees indiscriminately. Effective forest restoration requires careful planning and consideration of various factors to ensure long-term success and maximum benefits.

The importance of creating resilient forests
Resilient forests are better equipped to withstand climate change impacts, pests, and diseases. This involves planting a diverse mix of native species adapted to local conditions. Biodiversity enhances forest resilience and provides additional ecosystem services beyond carbon sequestration.

Where should new forests be established?
Prioritize areas that were previously forested but have been degraded or deforested. Avoid planting trees in naturally treeless ecosystems like grasslands or wetlands, as this can harm biodiversity and potentially release more carbon than it sequesters. Consider factors such as soil quality, water availability, and proximity to existing forests.

Engaging local and indigenous communities
Successful forest restoration relies on the support and involvement of local communities. Engage them early in the planning process, incorporate traditional knowledge, and ensure they benefit from the restoration efforts. This can include creating jobs, improving livelihoods, and maintaining access to forest resources.

Set realistic and transparent targets
Set clear, realistic goals for carbon sequestration and biodiversity, and be transparent about how progress will be measured. This helps build trust with stakeholders and supports a more accurate evaluation of the project’s outcomes.

Measure and monitor carbon sequestration and biodiversity impacts
Implement robust monitoring systems to track carbon sequestration rates and biodiversity changes over time. This data is crucial for verifying the project's effectiveness, making necessary adjustments, and reporting outcomes to stakeholders and potential funders.

Take into account the full carbon lifecycle
Consider the entire carbon lifecycle of the restoration project. This includes emissions from site preparation, tree nurseries, and ongoing management activities. A comprehensive approach ensures that the net carbon benefit is accurately assessed.

Always prioritize broader conservation efforts
While reforestation is valuable, it should complement, not replace, efforts to conserve existing forests. Protecting mature forests often provides greater immediate carbon and biodiversity benefits than newly planted areas. Integrate reforestation into broader landscape-level conservation strategies for maximum impact.

Forests play an indispensable role in environmental and climate management, serving as vital carbon sinks and essential ecosystems. However, centuries of deforestation and land conversion have inflicted serious damage, leaving humanity with a substantial ecological debt.

While establishing new plantation forests can help ease pressure on natural forests, providing timber as well as biomass for energy—ideally with carbon capture—it is important to understand the sheer scale of tree planting which would be needed if we were to rely on it alone to solve the climate crisis. The land and water required would be immense, and the vulnerability of plantation forests to disease and wildfires means that long-term carbon storage is not guaranteed, particularly if trees are harvested and the stored carbon is subsequently released.

Planting trees is not a quick or simple fix, and forests alone cannot solve climate change. However, when done responsibly and as part of a broader strategy, reforestation and afforestation can still play a valuable role in reducing CO₂ levels.

• Forests and Climate - World Wildlife Fund
• How many new trees would we need to offset our carbon emissions? - MIT Climate Portal
• Primer on Forest Carbon - How Canada’s Boreal Forest can be a powerful solution for climate change Nature United Canada
  
• Carbon Sequestration - USDA Forest Service
• Forests and Decarbonization – Roles of Natural and Planted Forests - .frontiersin.org
• Study: Protected forests are a climate powerhouse - Conservation International
• Forests and Climate - IUCN
• Global carbon dioxide removal rates from forest landscape restoration activities - PubMed
• How to Grow a Forest - Get out of the way - National Geographic
• How Forests Store Carbon - Penn State Extension
• Forest Carbon 101 - The Nature Conservancy
• Mapping the World’s Forests: How Green is our Globe? - World Economic Forum
• How Forests Fight Climate Change - Rainforest Alliance
• Forest Carbon Stocks - World Resources Institute
• Deforestation & Climate Change - Global Forest Watch
• Natural Climate Solutions - The Nature Conservancy

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