Background
The authors state "there are indications that forest biomass accumulation may be accelerating where nutrients and water are not limiting," citing the work of Myneni et al. (1997), Lewis et al. (2004), Lewis et al. (2009), Boisvenue and Running (2006), Delpierre et al. (2009), Salzer et al. (2009) and Chave et al. (2008); and, therefore, they felt it important to further investigate the subject because of the great significance such a phenomenon would hold for the planet's carbon balance and the future course of potential CO2-induced global warming.

What was done
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."

What was learned
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."

What it means
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." We agree, noting that their findings and explanation provide additional important evidence for the Greening of the Earth phenomenon that is quietly transforming the face of the planet.

References
Boisvenue, C. and Running, S. 2006. Impacts of climate change on natural forest productivity -- evidence since the middle of the 20th century. Global Change Biology 12: 862-882.

Chave, J., Condit, R., Muller-Landau, H.C., Thomas, S.C., Ashton, P.S., Bunyavejchewin, S., Co, L.L., Dattaraja, H.S., Davies, S.J., Esufali, S., Ewango, C.E.N., Feeley, K.J., Foster, R.B., Gunatilleke, N., Gunatilleke, S., Hall, P., Hart, T.B., Hernández, C., Hubbell, S.P., Itoh, A., Kiratiprayoon, S., LaFrankie, J.V., de Lao, S.L., Makana, J.-R., Noor, Md.N.S., Kassim, A.R., Samper, C., Sukumar, R., Suresh, H.S., Tan, S., Thompson, J., Tongco, Ma.D.C., Valencia, R., Vallejo, M., Villa, G., Yamakura, T., Zimmerman, J.K. and Losos, E.C. 2008. Assessing evidence for a pervasive alteration in tropical tree communities. PLoS Biology 6: 10.1371/journal.pbio.0060045.

Delpierre, N., Soudani, K., Francois, C., Kostner, B., Pontailler, J.-Y., Nikinmaa, E., Misson, L., Aubinet, M., Bernhofer, C., Granier, A., Grunwald, T., Heinesch, B., Longdoz, B., Ourcival, J.-M., Rambal, S., Vesala, T. and Dufrene, E. 2009. Exceptional carbon uptake in European forests during the warm spring of 2007: A data-model analysis. Global Change Biology 15: 1455-1474.

Lewis, S.L., Lopez-Gonzalez, G., Sonke, B., Affum-Baffoe, K., Baker, T.R., Ojo, L.O., Phillips, O.L., Reitsma, J.M., White, L., Comiskey, J.A., Djuikouo K., M.-N., Ewango, C.E.N., Feldpausch, T.R., Hamilton, A.C., Gloor, M., Hart, T., Hladik, A., Lloyd, J., Lovett, J.C., Makana, J.-R., Malhi, Y., Mbago, F.M., Ndangalasi, H.J., Peacock, J., Peh, K. S.-H., Sheil, D., Sunderland, T., Swaine, M.D., Taplin, J., Taylor, D., Thomas, S.C., Votere, R. and Woll, H. 2009. Increasing carbon storage in intact African tropical forests. Nature 457: 1003-1006.

Lewis, S.L., Phillips, O.L., Baker, T.R., Lloyd, J., Malhi, Y., Almeida, S., Higuchi, N., Laurance, W.F., Neill, D.A., Silva, J.N.M., Terborgh, J., Lezama, A.T., Vásquez Martinez, R., Brown, S., Chave, J., Kuebler, C., Núñez Vargas, P. and Vinceti, B. 2004. Concerted changes in tropical forest structure and dynamics: evidence from 50 South American long-term plots. Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 359: 421-436.

Myneni, R., Keeling, C., Tucker, C., Asrar, G. and Nemani, R. 1997. Increased plant growth in the northern high latitudes from 1981 to 1991. Nature 386: 698-702.

Salzer, M., Hughes, M., Bunn, A., Kipfmueller, K. 2009. Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes. Proceedings of the National Academies of Science USA 106: 20,346-20,353.