Forest Growth Has Accelerated as the Air's CO2 Content Has Risen
McMahon, S.M., Parker, G.G. and Miller, D.R. 2010. Evidence for a recent increase in forest growth. Proceedings of the National Academy of Sciences USA: 10.1073/pnas.0912376107.
Therefore, using unique datasets 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 (which were derived from knowledge of when the stands had begun to regrow following major disturbances such as significant logging, various natural disasters that had decimated large patches of trees, or the clearing of trees to make room for agriculture that was ultimately abandoned), McMahon et al. "estimated biomass change, while controlling for stand regeneration." This they did within various parts of a temperate deciduous forest in the vicinity of the Smithsonian Environmental Research Center, Edgewater, Maryland (USA) by comparing recent (last 22 years or less) rates of biomass accumulation of the various stands with rates predicted for those age intervals by the overall growth function derived from the combined data of all of the stands. Then, last of all, they compared their findings with "over 100 years of local weather measurements and 17 years of on-site atmospheric CO2 measurements."
Based on their analysis, the three researchers report that "recent biomass accumulation greatly exceeded the expected growth caused by natural recovery," noting that in stands younger than 50 years the observed rate increase was generally at least one-third of total growth, and that in older stands it typically was "the majority of growth," even though past experience and the ensemble relationship of growth vs. age derived from the totality of their data suggest that "old forests should grow very little as they approach equilibrium." As for what could have caused the tremendous recent increases in forest plot growth rates detected by the Smithsonian scientists, they say that "increases in temperature, growing season [which is largely driven by temperature], and atmospheric CO2 have documented influences on tree physiology, metabolism, and growth," and they state that these global-change factors -- the magnitudes of which rose significantly over the course of their study -- may well have been "critical to changing the rate of stand growth observed across stands." Such findings and explanation also provides additional important evidence for the Greening of the Earth phenomenon that is quietly transforming the face of the planet as the earth's CO2 content continues to rise.
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