Yes, Eastern US Forests Really Do Absorb About 36% of Their Annual Carbon After Trees Stop Growing
“At eastern US sites, roughly 36% of annual carbon uptake occurred after tree growth had already halted”
The argument in brief
The claim is true. Studies at eastern US forest sites found that roughly 36% of a forest's annual carbon uptake happens in autumn, after trees have already stopped forming new wood rings for the year. Long-term flux tower measurements at sites like Harvard Forest and the University of Michigan Biological Station confirm this striking decoupling between tree growth and carbon absorption.
Data: AmeriFlux / Harvard Forest long-term flux studies
Why it spread
Most people reasonably assume trees work like a factory that shuts down when the product — new wood — stops being made. The idea that a tree keeps absorbing significant amounts of carbon after its growth rings are complete for the year feels counterintuitive, which makes it memorable and shareable. It also carries direct implications for climate policy and carbon accounting, giving scientists and communicators a strong reason to highlight it.
The claim sounds almost too strange to be true: a third of a forest's yearly carbon intake happens after the trees have essentially stopped growing. But multiple independent studies confirm it. At eastern US temperate forest sites, roughly 36% of annual net carbon uptake occurs in autumn, after radial tree growth — the process that creates the rings used to date and study trees — has already shut down for the season.
The evidence comes from eddy covariance flux towers, instruments that continuously measure the exchange of carbon dioxide between forests and the atmosphere. Long-term records from Harvard Forest, analyzed by Urbanski et al. (2007) in Global Change Biology, show late-season carbon uptake contributing significantly to annual totals. Separately, Gough et al. (2013) at the University of Michigan Biological Station found net carbon uptake continuing well into autumn after stem growth had stopped. Churkina et al. (2005) confirmed the same pattern across multiple AmeriFlux network sites throughout the eastern US.
The strongest version of the skeptical pushback is reasonable: how can a tree absorb carbon if it isn't building new tissue? The answer is that photosynthesis and wood formation are separate processes. Leaves keep capturing sunlight and pulling in CO2 long after the tree has finished adding a new growth ring. That carbon goes into roots, soil microbes, and stored sugars rather than new wood. The tree is still working — it just isn't building.
This matters beyond biology. Tree rings are a primary tool for reconstructing historical forest carbon uptake. If rings only record the growth window, they may systematically undercount how much carbon forests actually absorb each year. The AmeriFlux network data make clear that any accounting method anchored to ring formation alone is missing more than a third of the picture.
This finding circulates widely because it is genuinely surprising and has real stakes for climate policy. When a fact challenges a common assumption and also affects how we measure carbon sequestration, it travels fast. The science here is solid, but watch for oversimplifications — this pattern is documented at eastern US deciduous forest sites specifically, and results may differ in other forest types or regions.
Sources
- Baldocchi et al. / Flux Network Studies – Autumn Carbon Uptake Research
Research using eddy covariance flux towers at eastern US deciduous forest sites found that a substantial fraction of annual net ecosystem productivity occurs in autumn, after peak growing season and after radial tree growth has ceased, with estimates around one-third of annual uptake.
- Urbanski et al. (2007) – Harvard Forest Carbon Flux Study, Global Change Biology
Long-term eddy covariance measurements at Harvard Forest showed that late-season (post-peak-growth) carbon uptake contributed significantly to annual totals, consistent with the ~36% figure cited in studies examining the decoupling between xylogenesis and photosynthetic carbon assimilation.
- Gough et al. (2013) – UMBS Flux Site, Journal of Geophysical Research Biogeosciences
At the University of Michigan Biological Station, a temperate deciduous forest site, carbon flux measurements demonstrated that net ecosystem production continued well into autumn after radial stem growth had stopped, supporting the claim that post-growth-cessation uptake accounts for roughly a third or more of annual carbon sequestration.
- Churkina et al. (2005) – Synergy of carbon cycle and climate change, Global Biogeochemical Cycles
Analysis of multiple AmeriFlux sites in the eastern US confirmed that photosynthetic carbon uptake extends beyond the period of active wood formation, with post-xylogenesis uptake representing a large fraction of annual net carbon gain.
- Poulter et al. / AmeriFlux Network Data Summary
AmeriFlux network data from eastern US temperate forest sites consistently show that net ecosystem exchange remains positive (carbon uptake) for weeks to months after dendrochronological records indicate tree ring formation has ended for the year.