Trees may store significantly less atmospheric carbon than climate models currently predict because photosynthesis does not always result in wood growth. Researchers at Columbia University’s Lamont-Doherty Earth Observatory found that trees often stop producing wood months before photosynthesis ceases, a decoupling that could lead to overestimations of how much carbon forests can actually sequester.
Why is there a gap between photosynthesis and tree growth?
Most existing Earth system models operate on the assumption that photosynthesis and tree growth are tightly linked. However, a study of 137 sites across the United States suggests this link is breaking down. Mukund Palat Rao, a carbon cycle scientist at the Lamont-Doherty Earth Observatory and lead author of the study, noted that just because a tree is performing photosynthesis does not mean it is building more wood.
The research highlights a distinction between carbon uptake and long-term storage. While photosynthesis pulls carbon dioxide from the air, that carbon can be used for “transient” purposes, such as maintaining foliage or fueling internal biological processes. For carbon to be stored for decades or centuries, it must be converted into wood. When trees use carbon for temporary functions instead of structural growth, their ability to act as a long-term carbon sink diminishes.
The data shows significant regional differences in this phenomenon:
- Eastern United States: Approximately 36% of yearly carbon uptake occurred after wood growth had already stopped in late summer.
- California: Roughly 26% of yearly carbon uptake occurred after the growth period ended.
How do heatwaves and droughts impact carbon sequestration?
Rising global temperatures and increasing aridity are directly limiting the windows during which trees can grow. Detailed measurements at four specific study sites showed that wood growth is largely restricted to periods characterized by low temperatures and low aridity.
As heatwaves and droughts become more frequent, these optimal growth windows shrink. According to Rao, the physiological response to heat is immediate. “The moment you have dry and hot conditions, growth activity stops pretty instantly, while photosynthesis seems to continue at a slightly decreased rate,” Rao said.
This means that during a drought, a forest might still appear to be active in terms of gas exchange, but it is failing to lock that carbon away in its structure. This “decoupling” makes forests less effective as a defense against climate change than previously thought.
What does this mean for future climate modeling?
The findings suggest that current climate projections may be too optimistic regarding the capacity of land-based carbon sinks. If models continue to use photosynthesis levels as a proxy for wood growth, they risk overestimating how much CO2 forests will absorb throughout the 21st century.
This discrepancy creates a potential gap in global carbon budgets. If natural sinks provide less support than expected, the pressure on other carbon removal methods will increase. A recent report indicated that humanity may need to deploy carbon-removal technologies even faster than the current expansion of solar energy to compensate for these atmospheric shifts.
Frequently Asked Questions
Does more photosynthesis always mean more tree growth?
No. Research from Columbia University shows that trees can continue photosynthesis even after they have stopped growing wood, especially during hot or dry periods.
Why is wood growth more important than photosynthesis for the climate?
Photosynthesis is the process of absorbing CO2, but wood growth is the process of storing that carbon long-term. Carbon used for leaves or internal processes returns to the atmosphere much faster than carbon stored in wood.
How do rising temperatures affect forests?
Higher temperatures and increased drought frequency can cause growth activity to stop almost instantly, even if the tree continues to absorb some carbon through photosynthesis.
Are current climate models inaccurate?
The researchers suggest that models assuming a “tight coupling” between photosynthesis and growth may overestimate how much carbon forests can sequester as the planet warms.
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