Evidence for a recent increase in forest growth is questionable

In a recent article, McMahon et al. (1) examined forest-plot biomass accumulation across a range of stands in the mid-Atlantic United States and suggest that climate change and trends in atmospheric CO2 explain an increase in forest growth. To show this increase, they fit a simple model to live above-ground forest biomass (AGB) as a function of stand age, and then propose that the derivative of this model is the expected rate of ensemble biomass change (Graphic). They conclude that biomass changes within census plots that exceed the ensemble expectation constitute recent increases in growth rates. We disagree with this conclusion, and instead, we suggest that (i) Graphic is incorrectly equated with forest growth, because it ignores past mortality that could explain the difference in rates, (ii) stated trends in plot-species composition could account for plot AGB trajectories that differ from ensemble-model expectations, and (iii) the authors’ model and confidence bounds (CIs) are overly conservative (2), making overlap of rates unlikely.