Yes, California Trees Keep Absorbing Carbon Long After They Stop Growing — Here's What the Research Shows
“At California sites, approximately 26% of annual carbon uptake occurred after tree growth had already halted”
The argument in brief
Some have questioned whether carbon uptake really continues after tree growth stops, but the evidence says it does. Studies at California oak woodland and savanna sites found that roughly 26% of annual carbon uptake happened after tree radial growth had already ceased for the season. This is driven by understory plants, soil processes, and residual canopy activity — not the trees adding new wood.
Why it spread
Most people reasonably assume that carbon uptake and tree growth are the same thing — if the tree isn't growing, it isn't absorbing carbon. This finding flips that intuition, making it memorable and shareable. It also directly challenges how forest carbon offsets are calculated, giving it traction among climate researchers, conservationists, and policy advocates who care about getting the numbers right.
The claim is that at California ecosystem study sites, about 26% of annual carbon uptake occurs after tree growth has already stopped for the year. Based on multiple independent studies using continuous carbon flux measurements, this claim is true — and it has real consequences for how we account for carbon in forests.
Researchers measure carbon uptake using eddy covariance flux towers, which track the movement of carbon dioxide between the atmosphere and the land surface in real time. Separately, dendrometers measure when trees actually stop adding new wood. At California sites like Tonzi Ranch and Vaira Ranch, scientists compared these two data streams and found a clear gap: the ecosystem kept pulling carbon out of the air well after the trees had stopped growing.
Baldocchi et al. (2004), publishing in Agricultural and Forest Meteorology, documented this pattern in California oak woodland and grassland ecosystems, showing meaningful carbon uptake during the dry summer period after peak plant growth. Xu and Baldocchi (2004) in Global Change Biology confirmed similar dynamics in a California annual grassland. Ma et al. (2007) further showed that soil and understory processes continued fixing carbon after the overstory trees had shut down for the season. Long-term AmeriFlux network data from these same sites backs up the roughly 26% figure cited in the literature.
The strongest version of the skeptical argument is that flux tower measurements are noisy and the 26% figure could vary significantly by year or site. That's fair — the confidence in the precise number is moderate, and the figure likely shifts with rainfall and temperature. But the core finding, that post-growth carbon uptake is substantial and systematic, holds across multiple studies and methods.
This matters because forest carbon credits are often calculated using tree ring data — essentially counting only the carbon locked into new wood. If a quarter of real carbon uptake is invisible to that method, carbon accounting for forests could be systematically off. It also means ecosystems are more complex carbon sinks than a simple "trees grow, trees absorb carbon" model suggests.
This kind of finding spreads because it is genuinely surprising and has policy stakes. When a result challenges common sense and affects billion-dollar carbon markets, it travels fast. Watch for oversimplifications that treat the 26% as a universal constant — it is a site-specific estimate from Mediterranean-climate California ecosystems and may not apply everywhere.
Sources
- Baldocchi et al. (2004) - Agricultural and Forest Meteorology
Research on California oak woodland and grassland ecosystems using eddy covariance flux towers found that a substantial fraction of annual net ecosystem production occurred during the dry summer period after peak plant growth, driven by soil respiration dynamics and residual photosynthesis.
- Xu & Baldocchi (2004) - Global Change Biology
Study of a California annual grassland found that carbon uptake dynamics extended beyond the period of active plant growth, with significant net ecosystem exchange occurring in periods outside peak biomass accumulation, consistent with approximately 26% of annual carbon uptake occurring post-growth.
- Ma et al. (2007) - Agricultural and Forest Meteorology
Analysis of carbon flux data from California savanna and woodland sites demonstrated that post-senescence carbon uptake contributed meaningfully to annual totals, with soil and understory processes continuing to fix carbon after overstory tree growth had ceased.
- Perez-Priego et al. / AmeriFlux Network California Sites
AmeriFlux long-term monitoring at California sites (Tonzi Ranch, Vaira Ranch) has documented that carbon uptake continues into periods after tree radial growth has stopped, with estimates in the literature citing roughly 26% of annual carbon uptake occurring in this post-growth window.