How Forest Age Shapes the Global Carbon Balance

Forests are often described as the “lungs of the planet,” but their ability to absorb or release carbon dioxide (CO₂) depends on more than just their size. A new study published in Nature Ecology & Evolution highlights how the age of forests—and the transitions between young, mature, and old-growth states—play a critical role in determining whether forests act as carbon sinks or sources.

The research, led by Simon Besnard and an international team from institutions including the GFZ Helmholtz Centre for Geosciences, the Max Planck Institute for Biogeochemistry, and the University of Bristol, underscores that not all forest growth contributes equally to climate mitigation. The study raises a pressing question: how do global shifts in forest age structure align with changes in the planet’s net carbon balance?

Why Forest Age Matters for Climate Change

Forests are dynamic carbon reservoirs. Young forests absorb carbon rapidly as they grow, while old-growth forests store vast amounts of accumulated carbon. However, disturbances such as logging, fires, and droughts are altering this balance.

The research stresses that simply increasing forest cover does not guarantee a stronger carbon sink. If mature or old forests are replaced by younger stands, the immediate loss of stored carbon can outweigh short-term gains from faster growth in younger forests. “Not all forest gains translate into climate gains,” the authors caution.

How the Study Was Conducted

The team combined cutting-edge datasets to track the relationship between forest age and carbon balance on a global scale. These included:

• Global forest age maps (GAMI v2.0)
• Satellite-based biomass estimates (ESA-CCI v4)
• Atmospheric CO₂ flux data (inversions)

By applying ensemble modeling and spatial analysis across TRANSCOM regions (a widely used framework for global carbon cycle studies), the researchers quantified how different types of forest transitions—such as replacement by young stands or uninterrupted aging—affect carbon fluxes.

Importantly, the analysis also projected future scenarios through 2050, comparing “business-as-usual” disturbance trends with targeted conservation strategies.

Key Findings: A Global Covariation

The study revealed a consistent global pattern:

• Young forest replacement boosts carbon uptake in the short term, thanks to rapid growth.
• Replacing old-growth forests leads to carbon losses because of the immediate release of carbon stocks and the slower recovery time.
• Uninterrupted aging weakens carbon sinks as growth slows in mature stands.

Spatial analysis showed that the fraction of replaced forests was a strong predictor of net CO₂ fluxes between 2010 and 2020. Projections for 2050 demonstrated that protecting old-growth forests while supporting natural regeneration could sustain or even increase global carbon stocks. In contrast, continued large-scale disturbances risk flipping some regions from carbon sinks into carbon sources.

Policy and Practical Implications

For policymakers, the message is clear: the quality of forests matters as much as their quantity. Protecting old-growth forests is essential for maintaining vast carbon reserves, while restoration in degraded landscapes can provide long-term mitigation benefits if protected from future disturbances.

The findings also highlight the need for more nuanced approaches in carbon accounting and climate reporting. Current carbon markets and climate pledges often focus on tree planting or forest cover expansion, but without considering forest age dynamics, these measures may misrepresent real climate impacts.

In Latin America, for example, secondary forests show strong potential for carbon capture. Yet ongoing deforestation of ancient Amazonian forests risks undermining regional and global climate stability.

Looking Ahead

As global climate strategies intensify, integrating forest age into land-use policies and carbon accounting systems is no longer optional—it is urgent. Conservation efforts must prioritize saving carbon “capital” stored in old forests, while restoration strategies should focus on enabling young forests to grow into future reserves.

The study ultimately delivers a dual warning and opportunity: safeguarding mature forests while encouraging long-term regeneration is key to keeping forests as allies in the fight against climate change.

Topics of interest

Climate