Forests are a vital defence against climate breakdown, because they absorb carbon dioxide (CO₂) from the atmosphere through photosynthesis and lock much of that away in trunks, branches, and roots for decades or even centuries. This forest carbon storage process makes them one of the world’s largest natural CO₂ sinks.
However, new research suggests this climate service of forest carbon storage may not be as reliable as scientists once believed, because trees power depends in part on how much carbon dioxide they can convert into wood, which keeps the planet-heating molecule out of the atmosphere.
The finding suggests that photosynthesis does not always lead to wood growth, because trees may not be able to store as much planet-heating carbon as hoped. Trees can continue photosynthesis even after they stop growing wood, according to a recent study conducted across 137 forest sites in the United States.
This means that although trees are still absorbing carbon dioxide, much of that carbon may no longer be stored in long-lasting wood. The findings not only reveal an important discovery, but also raise important questions about how much carbon forests will be able to store as global temperatures continue to rise.
Understanding Forest Carbon Storage
For decades, forests have been regarded as one of Earth’s most powerful natural allies in the fight against climate change. Forests help regulate Earth’s climate by removing carbon dioxide from the atmosphere. Through photosynthesis, trees use sunlight, water, and carbon dioxide to produce sugars that fuel their growth. Some of this carbon becomes:
- Tree trunks
- Branches
- Roots
- Woody tissues
Because wood can survive for decades or even hundreds of years, it serves as a long-term carbon reservoir. This is why protecting and restoring forests has become a central strategy in combating climate change.
Photosynthesis Does Not Always Mean Tree Growth
The new research, led by scientists from Columbia University’s Lamont-Doherty Earth Observatory, challenges a long-held assumption in climate science. Many climate models assume that: More photosynthesis automatically leads to more tree growth. The researchers found this assumption is not always true. Their observations showed that trees often stopped producing new wood several months before photosynthesis ended for the season. In other words:
- Trees continued absorbing carbon dioxide.
- Photosynthesis remained active.
- Wood production had already stopped.
This means the absorbed carbon was being used for purposes other than building wood.
What Did the Scientists Discover?
The researchers analyzed data from 137 forest sites across the United States. Their findings about forest carbon storage included:
- In eastern U.S. forests, approximately 36% of the annual carbon uptake occurred after wood growth had already stopped.
- In California forests, around 26% of annual carbon uptake happened after growth ceased.
Detailed observations at four study locations revealed that wood formation occurred only during favorable environmental conditions. Tree growth slowed or stopped when forests experienced:
- High temperatures
- Dry soils
- Water stress
- Increased atmospheric dryness
Meanwhile, photosynthesis continued, although at a somewhat reduced rate. This disconnect between photosynthesis and growth surprised many researchers.
Why Heat and Drought Matter
Wood production requires more than carbon. Trees also need:
- Adequate soil moisture
- Healthy water transport
- Favorable temperatures
- Active cell division
When temperatures become too high or drought intensifies, trees prioritize survival instead of growth. They redirect their energy toward:
- Maintaining living tissues
- Repairing damaged cells
- Supporting leaves
- Transporting water
- Coping with heat stress
As a result, less carbon is converted into long-lasting wood.
Climate Change Is Making the Problem Worse
Climate change is increasing the frequency of:
- Heatwaves
- Droughts
- Dry atmospheric conditions
- Water shortages
These stressful conditions shorten the period during which trees can grow. Researchers found that tree growth stopped almost immediately once hot and dry conditions developed, while photosynthesis continued for some time afterward. As global warming intensifies, these unfavorable conditions are expected to become more common. This means forests could absorb carbon dioxide without storing as much of it in durable wood.
Why This Matters for Climate Models
Current climate models often estimate future forest carbon storage using measurements of photosynthesis. If photosynthesis continues but wood growth declines, those models may significantly overestimate how much carbon forests can permanently remove from the atmosphere. In simple terms:
Carbon absorbed is not necessarily carbon stored.
This distinction is crucial for predicting future climate change. If forests store less carbon than expected, atmospheric carbon dioxide levels could rise faster than many projections currently suggest.
Where Does the Carbon Go?
When trees are not producing wood, the carbon absorbed during photosynthesis can be diverted into several short-term uses. These include:
- Producing new leaves
- Maintaining living cells
- Root metabolism
- Plant respiration
- Storage compounds used during stress
Unlike wood, these forms of carbon usually return to the atmosphere much sooner through decomposition or respiration. Therefore, they provide only temporary forest carbon storage.
Implications for Global Carbon Removal
Nature currently performs the overwhelming majority of the world’s carbon removal. Forests, wetlands, grasslands, and soils remove billions of tonnes of carbon dioxide every year. Technological carbon removal methods—including direct air capture and chemical processes—still account for only a tiny fraction of global carbon removal capacity. If forest carbon storage capacity becomes less efficient over the long term, achieving international climate goals could become even more challenging. This highlights the need to:
- Protect existing forests
- Restore degraded ecosystems
- Reduce fossil fuel emissions rapidly
- Develop additional carbon removal technologies
- Improve forest management under changing climates
Scientists Continue Their Research
The current study focused on forests across the United States.
Researchers are now investigating whether the same disconnect between photosynthesis and wood growth occurs:
- In tropical rainforests
- Boreal forests
- Temperate forests
- Other tree species worldwide
If similar patterns are found globally, climate models may need substantial revisions. Improving these models will help scientists better predict future carbon storage and design more effective climate policies.
What Can Be Done?
While the findings may sound concerning, they also provide valuable insight into how forests respond to environmental stress. To maintain healthy forest carbon storage, governments and conservation organizations can:
- Protect old-growth forests that store vast amounts of carbon.
- Reduce greenhouse gas emissions to limit global warming.
- Restore degraded forests and watersheds.
- Improve soil moisture conservation through sustainable land management.
- Prevent deforestation and forest fragmentation.
- Monitor forest health using modern technologies such as satellites and field observations.
Healthy forests remain one of humanity’s strongest natural defenses against climate change.
Final Thoughts
Forests continue to play an essential role in regulating Earth’s climate, but this new research reminds us that nature is more complex than many models assume. Trees may continue absorbing carbon dioxide even after they stop growing wood, meaning not all captured carbon remains locked away for decades. As hotter temperatures and prolonged droughts become more common, forests may lose some of their ability to store carbon over the long term.
Rather than diminishing the importance of forests, these findings emphasize the urgency of protecting them while simultaneously reducing fossil fuel emissions. A healthy climate will depend not only on how much carbon trees absorb but also on how much they can safely store for generations to come.
Key Takeaways
- Trees can continue photosynthesis after wood growth has stopped.
- Wood growth is more sensitive to heat and drought than photosynthesis.
- Less wood growth means less long-term carbon storage.
- Climate models may overestimate future forest carbon sequestration.
- Rising temperatures and droughts could reduce forests’ effectiveness as natural carbon sinks.
- Protecting forests and cutting greenhouse gas emissions remain essential climate solutions.
References
- Rao, M. P., et al. (2026). Seasonal decoupling of photosynthesis and wood growth in U.S. forests under increasing atmospheric moisture demand. (Study led by Columbia University’s Lamont-Doherty Earth Observatory.)
- Research summary reported by The Guardian on recent findings regarding forest carbon storage and climate modeling.
- IPCC Sixth Assessment Report (AR6): Climate Change 2023 – The Physical Science Basis.
