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Old Trees: The Bigger They Are, The More Carbon They Sequester
Stephenson, N.L., Das, A.J., Condit, R., Russo, S.E., Baker, P.J., Beckman, N.G., Coomes, D.A., Lines, E.R., Morris, W.K., Ruger, N., Alvarez, E., Blundo, C., Bunvavejchewin, S., Chuyong, G., Davies, S.J., Duque, A., Ewango, C.N., Flores, O., Franklin, J.F., Grau, H.R., Hao, Z., Harmon, M.E., Hubbell, S.P., Kenfack, D., Lin, Y., Makana, J.-R., Malizia, A., Malizia, L.R., Pabst, R.J., Pongpattananurak, N., Su, S.-H., Sun, I.-F., Tan, S., Thomas, D., van Mantgem, P.J., Wang, X., Wiser, S.K. and Zavala, M.A. 2014. Rate of tree carbon accumulation increases continuously with tree size. Nature 507: 90-93.

The 38 authors - hailing from 14 different countries: Argentina, Cameroon, China, Colombia, the Democratic Republic of the Congo, France, Germany, Malaysia, New Zealand, the Republic of Panama, Spain, Taiwan, the United Kingdom and the United States of America - write that "a widely held assumption is that after an initial period of increasing growth, the mass growth rate of individual trees declines with increasing tree size." But they then go on to demonstrate that this assumption is wrong.

What was done
Stephenson et al. say they "conducted a global analysis in which [they] directly estimated mass growth rates from repeated measurements of 673,046 trees belonging to 403 tropical, subtropical and temperate tree species, spanning every forested continent."

What was learned
"For all continents," the international team of researchers found that "aboveground tree mass growth rates (and, hence, rates of carbon gain) for most species increased continuously with tree mass (size)," while further noting that "the rate of mass gain increased with tree mass in each model bin for 87% of species, and increased in the bin that included the largest trees for 97% of species." In fact, they say that "even when we restricted our analysis to species achieving the largest sizes (maximum trunk diameter >100 cm; 33% of species), 94% had increasing mass growth rates in the bin that included the largest trees."

What it means
In further discussing their findings, Stephenson et al. say that "large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees," adding that "at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree." And in this regard, they say their findings "further indicate that the extraordinary growth recently reported in an intensive study [Sillett et al., 2010] of large Eucalyptus regnans and Sequoia sempervirens, which included some of the world's most massive individual trees, is not a phenomenon limited to a few unusual species." In fact, they state that "rapid growth in giant trees is the global norm" and that "it appears to hold regardless of competitive environment."

Sillett, S.C., Van Pelt, R., Koch, G.W., Ambrose, A.R., Carroll, A.L., Antoine, M.E. and Mifsud, B.M. 2010. Increasing wood production through old age in tall trees. Forest Ecology and Management 259: 976-994.

Reviewed 28 May 2014