A forest can absorb more carbon without growing more wood that year

A forest can take in more carbon this year without adding more wood this year. That distinction matters whenever a forest is described as absorbing carbon: carbon entering through leaves is not automatically carbon stored in trunks.

Researchers have seen the puzzle in measurements from 31 forest sites. Estimates of annual photosynthesis often have only a weak relationship with measurements of annual wood growth. Earlier research interpreted that mismatch as evidence that something besides the supply of carbon was widely limiting trees’ ability to grow. A study published Tuesday in Nature Plants accepts the annual mismatch but questions what it can prove.

The two measurements follow different parts of a tree’s carbon story. Photosynthesis brings carbon into plants. Some carbon is used in respiration, some can be stored, and some becomes new growth. At the study sites, researchers compared estimates associated with flux towers, which track carbon exchange between ecosystems and the air, with tree-ring records of wood growth. FLUXNET’s account of its data processing makes an easily missed point: the photosynthesis figure is itself an estimate, calculated by separating measured ecosystem carbon exchange into photosynthesis and respiration. It is not a direct reading of how much carbon became wood.

Now consider two questions that sound alike but test different ideas. In years when photosynthesis is higher, is wood growth higher too? The answer in the site observations was often weak or unclear. Over a much longer period, does more photosynthesis go with more woody biomass? To investigate that question, the new team examined an ensemble of vegetation models. In those simulations, woody biomass rose in proportion to photosynthesis over 1951–2020, even though year-to-year photosynthesis and wood growth were weakly linked in more than half the models.

The distinction is much like watching a household’s deposits and the amount spent on home improvements. A larger deposit need not produce more renovation that same year. Yet a long-running change in deposits could still matter to what the household builds over time. For trees, respiration, storage and growth are among the destinations for captured carbon. The analogy illustrates why an annual mismatch alone cannot identify which destination—or which limit on growth—is responsible.

There is an important boundary around the result. The long-term relationship comes from models, not a decades-long direct demonstration that the 31 forests stored carbon in wood at that rate. And the site observations in the new paper came from the earlier study’s compilation; reusing them does not provide an independent set of forests confirming the annual pattern. The model comparison instead tests an inference drawn from that pattern: if annual photosynthesis and wood growth move separately, must growth be broadly constrained? In these models, no.

That leaves a more precise way to read a forest-carbon claim. Ask whether it concerns carbon taken up, wood added in a particular year, or biomass accumulated over many years. Each answers a different question. The new study gives reason to be cautious about diagnosing a growth limit from a single timescale; it does not show that forests can keep storing carbon without limit.