Why Where Trees Grow Matters More Than How Many Are Planted

Governments, businesses and foundations are currently competing to promise the planting of billions of trees. It sounds like a straightforward answer to the climate crisis. Yet a recent study makes clear that the key issue is not how many saplings are put into the ground, but precisely where they grow. Some regions cool the planet substantially, while others may, paradoxically, make it warmer.

Why trees do not protect the climate equally well everywhere

At first glance, the calculation appears simple: trees remove carbon dioxide from the atmosphere and store carbon in timber, soil and vegetation. More forest, less CO₂ – problem solved. The new research corrects this oversimplified view.

The researchers show that forests affect the climate through several physical and biological mechanisms, which can reinforce or cancel out one another. Three factors are particularly important: carbon storage, the reflectivity of the Earth’s surface and evaporation.

Afforestation can cool the Earth – or warm it. The difference depends heavily on the location and the type of landscape converted into forest.

CO₂ storage: the familiar climate benefit of forests

Through photosynthesis, trees absorb carbon dioxide and use it to create biomass. Some of this carbon is held in trunks, roots and leaf litter, while another portion is retained in the soil. This storage process slows further temperature rises, provided the land is not cleared or burned again.

Trees grow especially quickly in tropical areas and produce large amounts of biomass. Their CO₂ balance is therefore particularly favourable: substantial uptake, relatively limited seasonal variation and stable stores – at least if widespread fires, deforestation and soil degradation are avoided.

Albedo: dark forests and bright surfaces

Less widely recognised is the effect known as albedo. This refers to the proportion of sunlight that a surface reflects back into space. Bright surfaces, including snow, ice and pale dry soils, reflect a great deal of radiation. Darker surfaces – such as coniferous forests or damp soils – absorb more energy and heat up more strongly.

When a dark forest develops on a bright, snow-covered surface, its reflectivity falls sharply. The ground absorbs more heat and local air temperatures rise. This warming effect can partly, or even entirely, offset the positive CO₂ benefit.

Evaporation: the landscape’s natural air conditioning

The third component is evaporation, known technically as evapotranspiration. Trees draw water from the soil and release it into the air through their leaves. This process consumes energy and therefore cools the surrounding area, much like perspiration on skin.

In tropical and humid regions, this “air conditioning” effect is particularly powerful: dense forests transfer enormous quantities of water into the atmosphere, reducing temperatures locally and regionally. In very dry areas, the effect is weaker simply because too little water is available.

Why tropical forests are a climate bonus – and boreal forests are not always

The study evaluates measurement data and model simulations from different climate zones, finding a clear pattern: the impact of afforestation depends greatly on geographical location.

Tropics: low risk, strong cooling effect

In tropical regions such as the Amazon, Central Africa and South-East Asia, the positive effects clearly outweigh the negative ones. All three mechanisms operate in the same direction there:

• high CO₂ uptake driven by rapid growth
• strong evaporation, resulting in significant cooling
• comparatively limited albedo loss because the ground is already relatively dark

Allowing degraded land in these regions to return to forest delivers an especially large climate benefit per hectare. The researchers show that the same cooling effect can be achieved with roughly half the forest area if sites are deliberately selected in such high-potential regions.

High latitudes: afforestation can backfire

The picture is very different at northern latitudes, including Canada, Scandinavia and Siberia. In winter, the ground there is often covered by snow, which reflects large amounts of sunlight. If forests grow on this land, dark canopies conceal the bright covering. The region absorbs more energy and the surrounding area warms.

In some scenarios, the climate benefit is even reversed: afforestation makes a net contribution to warming despite the trees storing CO₂. Large-scale schemes in snow-rich areas are particularly exposed to this risk.

Long-distance effects: forests influence weather far from where they grow

Forests do not act only locally. They alter wind patterns, the exchange of moisture between land and atmosphere, and consequently rainfall and temperature patterns in distant regions. A major afforestation project may therefore change weather hundreds or thousands of kilometres away – either positively or negatively.

For example, more forest in the tropical belt can affect the supply of moisture to other continents, alter monsoon systems, or weaken or intensify heatwaves. Climate policy that considers only local effects overlooks these long-distance impacts and therefore fails to plan for reality.

Why climate policy must move beyond simply “counting trees”

For a long time, afforestation programmes were regarded as an uncomplicated climate tool. One country pledges to plant billions of trees, companies offset flights through forest projects, and many climate plans make generous allowances for future CO₂ sinks. The new analysis delivers a poor verdict on this logic.

It is not the sheer number of saplings that determines the climate benefit, but the combination of location, forest type and long-term stability.

Targeted land instead of symbolic policy

The researchers argue that afforestation needs to be managed far more carefully. Priority should go to areas with high cooling potential – especially tropical, humid and degraded land that was previously forested. New monocultures on land with high albedo or limited water availability, by contrast, perform considerably worse.

For governments and companies, this means that reporting hectares of afforestation is not enough. What matters is whether projects are located where forests genuinely have a net effect on global temperature.

Monocultures – cheaper, but risky

The study also criticises uniform plantations consisting of one or only a few tree species. They often grow faster and can be managed industrially, but they carry serious risks:

• high vulnerability to pests and diseases
• greater fire risk from uniform stands that are often easily combustible
• lower biodiversity among plants and animals
• generally shallower and less stable soil carbon stores

Instead, specialists recommend mixed forests adapted to their particular region. Although they grow more slowly, they cope better with disturbance and store carbon more durably and reliably over the long term.

Limits of afforestation – without stopping emissions, the effect fades away

The study also tempers excessive expectations. Even in optimistic scenarios, in which vast areas worldwide are purposefully and sensibly reforested, model calculations show that the global average temperature falls by only around 0.25 degrees by 2100. This is noticeable, but it is not a miracle cure.

Meanwhile, emissions from coal, oil and gas, industry and transport continue to rise. Unless these sources decline quickly and substantially, no amount of afforestation worldwide can offset the additional warming.

What afforestation projects can do better now

Clear guidance follows for future programmes. Anyone seeking to use afforestation as a climate tool should observe several basic principles:

• Assess the site: Before a project begins, analyse exactly how albedo, evaporation and CO₂ storage will behave, including snow cover and soil moisture.
• Prioritise tropical and subtropical regions: The same hectare generally produces a greater cooling effect there than at high latitudes.
• Restore previously forested land: Reforesting damaged ecosystems achieves more than planting sensitive natural areas for the first time.
• Plant mixed forests: Diversity improves resilience, reduces fire risks and stabilises carbon stores.
• Plan for the long term: Forests operate over decades. Projects require long-term funding, protection from clearing and monitoring.

Although these criteria limit the number of projects that can be marketed quickly, they substantially increase the real climate impact achieved for every euro invested. For companies promoting themselves with climate promises, this makes it harder to generate “green” certificates with minimal effort.

Terms worth knowing – and what they mean in practice

Anyone assessing afforestation projects will repeatedly encounter technical terms. Three are central:

• Albedo: A measure of how strongly a surface reflects sunlight. High albedo (snow, pale desert) cools, while low albedo (forest, water) warms.
• Evapotranspiration: The combination of water evaporating from soils and plants. It cools the surrounding air and creates more comfortable temperatures in humid areas.
• Carbon sink: A system that permanently absorbs more CO₂ than it releases. Intact forests, peatlands and oceans are considered key sinks.

In practice, this means that a “good” afforestation project strengthens all three factors in a sensible balance – creating a resilient carbon sink, improving local evaporation and reducing albedo as little as possible.

For public debate, the study represents a change of direction. The image of the tree as a universal climate saviour is too simplistic. Forests remain an important ally in the fight against warming, but only when they are established in the right places, with suitable species and as part of a policy that consistently cuts fossil-fuel emissions. Those who decorate climate promises with large planting figures without seriously assessing the site are engaging in image management rather than climate protection.