How temperate forests are quietly losing their carbon sink capacity

Forest inventory plots in Europe and North America have tracked aboveground carbon storage for decades. When the most recent measurement cycles are aligned and re-analyzed under a consistent allometric model, a striking pattern emerges: the rate of carbon accumulation is decelerating, and not uniformly. Stands that historically gained the most biomass per hectare per year are now the ones decelerating fastest.

This is not yet visible in coarse satellite indices because canopy greenness can persist even as net primary productivity declines. The signal lives in the wood, not in the leaves.

What the long-term plots show

The plot networks involved — ICP Forests in Europe, the USDA Forest Service Forest Inventory and Analysis program in the United States, equivalent systems in Canada, the UK, and Japan — were designed in different decades for different purposes. Aligning them required decisions about allometric equations, plot stratification, and the treatment of mortality. Different reasonable choices yield different magnitudes for the deceleration. None yield no deceleration.

The signal is strongest in stands aged 40 to 100 years — the cohort that drove much of the late-20th-century terrestrial carbon sink. Younger stands are still accumulating. Older stands were never accumulating much. The middle cohort is the one carrying the productivity loss.

Three reinforcing drivers

Three drivers, each individually documented, appear to be reinforcing each other.

Drought frequency. Continental interiors have seen an increase in years where soil moisture limits stomatal conductance for a substantial fraction of the growing season. Even when those years are not classified as "drought" in conventional indices, they reduce annual carbon uptake.

Pest range expansion. Bark beetles and defoliating insects are expanding their range polewards and upslope. The overlap with stressed stands shortens the recovery window between disturbance events. Stands that historically had decades to recover between outbreaks now have years.

Shifted growing seasons. Warming has lengthened the growing season at the leaf level — earlier budburst, later senescence — but soil moisture has not kept pace. The marginal day adds little carbon and may add water stress.

The novel finding is the reinforcement. A drought-weakened tree is a more attractive host for beetles. A defoliated stand transpires less, locally raising air temperature and worsening drought. A longer growing season under fixed soil-water budgets accelerates moisture depletion.

Why the standard models miss this

Most coupled climate–vegetation models treat disturbance as a stochastic background process rather than a state variable that interacts with the climate. They also calibrate growth-response curves on the same historical period in which the carbon sink was strengthening — so they project that strength forward. The IPCC AR6 Working Group I report (Chapter 5, on the global carbon cycle) explicitly flags the persistence of the land sink as one of the larger uncertainties in century-scale projections.

A new generation of models is starting to incorporate disturbance feedbacks explicitly, with promising early results. The current public-facing carbon-budget projections, however, mostly do not yet include them.

What this means for policy

If the temperate sink is weaker than assumed, the implicit carbon budget compatible with 1.5°C narrows further. Land-use commitments that depend on forest sequestration as a hedge become more fragile, and the case for accelerating direct emissions reductions strengthens.

It also raises the bar for any policy that monetizes forest carbon. If the underlying biophysical assumption — that mature temperate forests will continue to sequester at recent rates — is wrong, the carbon credits issued against that assumption are systematically over-counted. Markets currently priced on the older assumption will need to be re-priced.

This is not a counsel of despair. The deceleration is detectable in part because the monitoring networks are good. The corrective is not to give up on forest carbon but to model it honestly, including the disturbance feedbacks that are now visible in the data.

Sources

1. IPCC — Climate Change 2021: The Physical Science Basis (AR6 WG I), Intergovernmental Panel on Climate Change, 2021. Chapter 5 covers the global carbon cycle and land-sink persistence.
2. USDA Forest Service — Forest Inventory and Analysis Program. Multi-decade U.S. inventory plot network underlying the temperate-sink trend analyses.
3. ICP Forests — International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests. The European long-term forest monitoring network.
4. FAO — The State of the World's Forests, Food and Agriculture Organization of the United Nations. Global forest extent and condition reporting.
5. UNEP — Emissions Gap Report 2024, United Nations Environment Programme. Frames the policy implications of land-sink revisions for net-zero pathways.