A new study from Columbia University researchers challenges a key assumption in climate models: that more photosynthesis automatically means more wood growth. The team studied 137 sites across the US and found trees stopped growing months before photosynthesis stopped, meaning a significant portion of absorbed carbon goes to short-lived uses like foliage rather than long-term wood storage. At eastern US sites, about 36% of yearly carbon uptake occurred after trees stopped growing in late summer. In California, it was about 26%. The researchers found wood growth halts quickly under hot, dry conditions, while photosynthesis continues at a reduced rate. As heatwaves and droughts become more common, this decoupling could reduce the power of forests as carbon sinks. The findings have direct implications for carbon accounting and nature-based offset projects. If Earth system models overestimate future forest carbon sequestration, it could affect everything from national climate targets to corporate net-zero strategies that rely heavily on tree planting.
