Forests and their soils contain the majority of the earth's terrestrial carbon stocks. Changes in patterns of tree growth can have a huge impact on atmospheric cycles, biogeochemical cycles, climate change, and biodiversity. Recent studies have shown increases in biomass across many forest types. This increase has been attributed to climate change. However, without knowing the disturbance history of a forest, growth could also be caused by normal recovery from unknown disturbances. Using a unique dataset of tree biomass collected over the past 22 years from 55 temperate forest plots with known land-use histories and stand ages ranging from 5 to 250 years, we found that recent biomass accumulation greatly exceeded the expected growth caused by natural recovery. We have also collected over 100 years of local weather measurements and 17 years of on-site atmospheric CO 2 measurements that show consistent increases in line with globally observed climate-change patterns. Combined, these observations show that changes in temperature and CO 2 that have been observed worldwide can fundamentally alter the rate of critical natural processes, which is predicted by biogeochemical models. Identifying this rate change is important to research on the current state of carbon stocks and the fluxes that influence how carbon moves between storage and the atmosphere. These results signal a pressing need to better understand the changes in growth rates in forest systems, which influence current and future states of the atmosphere and biosphere. T he movement of carbon in our atmosphere, oceans, and terrestrial ecosystems is critical to predicting how climate change may influence the natural systems on which humans rely (1-4). Changes in ecosystems can, in turn, feed back into global atmospheric cycles through evapotranspiration, net ecosystem CO 2 exchange, and surface albedo and roughness, which complicates predictions about future climate states (1, 5-7). Key evidence that global changes may affect the functioning of forests is shown in changes in forest biomass over time, which can have important implications for whether or not forests accumulate biomass at a rate that would alter current trends of atmospheric carbon cycling (8).In densely forested regions across the globe, forests can recover rapidly from agricultural fields, logged stands, or areas cleared because of natural disturbances as long as remnant patches or seed banks remain. Across forest types, the period of recovery consists of a rapid rise in above-ground biomass (AGB) followed by a leveling off as the canopy fills in and biomass shifts from the sum of many small stems to fewer, larger canopy trees. The rate and asymptote of this pattern of biomass recovery can differ across stands because of nutrient availability and species composition or can differ between regions because of climate and disturbance regimens; however, the functional form of this response remains similar across forest types and regions (9, 10).There are indications that forest biomass accumulation...

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