Forest carbon: surprising dynamics worldwide

Decisive details are currently revealing surprising dynamics around the world.

Anyone who sees forests merely as a green backdrop underestimates their impact. Long-term measurements and satellite assessments suggest that particular forest types and age groups store substantially more carbon than many models have previously indicated. At the same time, significant risks remain: droughts, deforestation and poor nutrient management could reverse this trend.

What new studies reveal about forest carbon

Forests remove carbon dioxide from the atmosphere and retain the carbon it contains in their roots, trunks and branches, sometimes for centuries. Their exact storage capacity is shaped by the climate, nutrient supply, species mix and, above all, their age. Several recent analyses now provide a far more nuanced picture.

Record carbon storage in the United States

Over the past two decades, forests in the United States have stored more carbon than at any other point during the last century. Researchers attribute this to an interaction between natural processes and forestry practices. Rising temperatures, changing rainfall patterns and CO2 fertilisation all play a role. However, the age of forest stands remains the most powerful factor.

According to specialist information, forests at their peak growth rate store around 89 million tonnes of additional carbon each year.

This effect outweighs many other influences. It demonstrates why allowing forest areas to mature, rather than harvesting them prematurely, is so important.

Human influence: reforestation versus deforestation

Human activity also alters the overall balance. Carbon stores increase where forests are allowed to age or where new trees are planted. At the same time, deforestation erodes these gains.

• Deforestation reduces stored carbon by around 31 million tonnes per year.
• Reforestation programmes restore roughly 23 million tonnes per year.
• Overall, the balance remains positive, but it is fragile.

If droughts become more frequent or deforestation accelerates in some regions, the pattern could be reversed within only a few decades.

Nitrogen as an accelerator - with risks

Tropical forests recovering after clear-felling or the abandonment of farmland often lack one essential ingredient: nitrogen. Without it, biomass production stalls even where light and rainfall are sufficient.

When regenerating tropical forests receive enough nitrogen, their biomass grows almost twice as quickly during the first ten years.

How much additional CO2 could be absorbed?

Estimates indicate that such forests could absorb up to 820 million tonnes of CO2 more each year, for an entire decade. This is equivalent to around 2 percent of global greenhouse gas emissions. Faster regeneration of tropical land would therefore buy time for industrial emissions to be reduced more decisively.

When too much becomes harmful

An excess of nitrogen carries risks. In forests that are already saturated, additional nitrogen can cause soil respiration to decline sharply. This biological process breaks down organic material. If it collapses, the nutrient cycle of the entire ecosystem suffers. In addition, inappropriate fertiliser use can release nitrous oxide, a highly potent greenhouse gas. Careful monitoring is essential.

Boreal and secondary forests as overlooked carbon stores

In the far north, boreal forests expanded their range by 12 percent between 1985 and 2020, covering around 844,000 square kilometres. On average, the tree line shifted northwards by 0.29 degrees of latitude.

Young boreal stands under 36 years old already store between 1.1 and 5.9 petagrams of carbon. Once they reach maturity, a further 2.3 to 3.8 petagrams could be added - an amount comparable with several years of emissions from a major industrialised country.

Protecting existing young forests outperforms new planting

Protecting secondary forests that are already regrowing is up to eight times more effective per hectare than relying on new planting alone.

This changes the priorities. Areas that are already recovering deserve consistent protection from fire, grazing pressure and renewed clearance. Tree species and mixtures must suit the local site conditions; otherwise, the potential is lost.

What this means for policy and practice

Anyone seeking to store carbon through forests needs a combination of protection, management and intelligent expansion. Three measures stand out.

Three high-impact measures

• Allow stands to age: Less clear-felling, longer rotations and deliberately varied forest structures increase carbon storage during peak growth.
• Safeguard regeneration: Protect secondary forests from disturbance, control invasive species and encourage natural regeneration.
• Manage nutrients: Act according to local conditions in the tropics, favouring legume-rich mixtures and soil building rather than blanket fertilisation.

Context and terminology

What is a petagram?

One petagram (Pg) is equal to one billion tonnes. 1 Pg of carbon equates to 3.67 Pg of CO2 because the weight of oxygen is included.

Carbon versus CO2 - the difference

Studies sometimes report figures in carbon (C) and sometimes in carbon dioxide (CO2). For comparison, 1 tonne of C is equivalent to 3.67 tonnes of CO2. Anyone offsetting emissions should keep the unit firmly in mind.

Risks and side effects

Forest fires, pests and extreme drought can release large quantities of CO2 in a short time. Boreal peatlands store substantial carbon in their soils; if permafrost thaws, methane and CO2 emissions could follow. In the tropics, poor nutrient management raises the risk of nitrous oxide emissions. This is why early-warning systems, fire prevention, locally suitable tree-species mixtures and robust monitoring of outcomes are required.

Practical examples and opportunities for German-speaking countries

Forestry operations in central Europe can apply these findings directly: longer rotation periods, more deadwood and mixed forests instead of monocultures. Agroforestry systems combine timber, fruit and arable farming, creating additional carbon sinks. In urban areas, rows of trees reduce heat-island effects while also storing carbon. Certificates can help provide funding, but only where measurement is reliable, carbon storage is permanent and clear exclusion criteria are in place, so that the balance is genuinely accurate.

Ultimately, not every newly planted sapling has the same value. Age, location, nutrients and protection determine how effectively a forest relieves pressure on the climate - and for how long.