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Continued Accelerated Growth of Amazonian Forests
Volume 8, Number 36: 7 September 2005

For most of the past century it was believed that old-growth forests, such as those of Amazonia, should be close to dynamic equilibrium.  Just the opposite, however, has been repeatedly observed over the past two decades.

In one of the first studies to illuminate this new reality, Phillips and Gentry (1994) analyzed the turnover rates - which are close correlates of net productivity (Weaver and Murphy, 1990) - of forty tropical forests from all around the world.  They found that the growth rates of these already highly productive forests had been rising ever higher since at least 1960, and that they had experienced an apparent acceleration in growth rate sometime after 1980.  Commenting on these findings, Pimm and Sugden (1994) reported that the consistency and simultaneity of the forest growth trends that Phillips and Gentry had documented on several continents led them to conclude that "enhanced productivity induced by increased CO2 is the most plausible candidate for the cause of the increased turnover."

A few years later, Phillips et al. (1998) analyzed forest growth rate data for the period 1958 to 1996 for several hundred plots of mature tropical trees scattered around the world, finding that tropical forest biomass, as a whole, increased substantially over the period of record.  In fact, the increase in the Neotropics was equivalent to approximately 40% of the missing terrestrial carbon sink of the entire globe.  Consequently, they concluded that tropical forests "may be helping to buffer the rate of increase in atmospheric CO2, thereby reducing the impacts of global climate change."  And, again, they identified the aerial fertilization effect of the ongoing rise in the air's CO2 content as one of the primary factors likely to be responsible for this phenomenon.

More recently, Laurance et al. (2004a) reported accelerated growth in the 1990s relative to the 1980s for the large majority (87%) of tree genera in 18 one-hectare plots spanning an area of about 300 km2 in central Amazonia, while Laurance et al. (2004b) observed similarly accelerated tree community dynamics in the 1990s relative to the 1980s.  And once again, it was suggested, in the words of Laurance et al. (2005), that these "pervasive changes in central Amazonian tree communities were most likely caused by global- or regional-scale drivers, such as increasing atmospheric CO2 concentrations (Laurance et al., 2004a,b)."

Expanding upon this theme, Laurance et al. (2005) say they "interpreted these changes as being consistent with an ecological 'signature' expected from increasing forest productivity (cf., Phillips and Gentry, 1994; Lewis et al. 2004a,b; Phillips et al., 2004)."  They note, however, that they have been challenged in this conclusion by Nelson (2005), and they thus go on to consider his arguments in some detail, methodically dismantling them one by one.

At the end of the day, it thus appears that a large body of scientists (see the references cited below) agrees that a wealth of scientific data confirms the reality of the ever-increasing productivity of earth's tropical forests, especially those of Amazonia; and they tend to agree that the concomitant rise in the air's CO2 content has had much to do with this phenomenon.  We also agree, noting that an even greater wealth of laboratory and field data demonstrates that rising forest productivity is exactly what one would expect to observe in response to the stimulus provided by the ongoing rise in the atmosphere's CO2 concentration (see many of the Headings listed under Trees in our Subject Index).

Sherwood, Keith and Craig Idso

Laurance, W.F., Nascimento, H.E.M., Laurance, S.G., Condit, R., D'Angelo, S. and Andrade, A.  2004b.  Inferred longevity of Amazonian rainforest trees based on a long-term demographic study.  Forest Ecology and Management 190: 131-143.

Laurance, W.F., Oliveira, A.A., Laurance, S.G., Condit, R., Dick, C.W., Andrade, A., Nascimento, H.E.M., Lovejoy, T.E. and Ribeiro, J.E.L.S.  2005.  Altered tree communities in undisturbed Amazonian forests: A consequence of global change?  Biotropica 37: 160-162.

Laurance, W.F., Oliveira, A.A., Laurance, S.G., Condit, R., Nascimento, H.E.M., Sanchez-Thorin, A.C., Lovejoy, T.E., Andrade, A., D'Angelo, S. and Dick, C.  2004a.  Pervasive alteration of tree communities in undisturbed Amazonian forests.  Nature 428: 171-175.

Lewis, S.L., Malhi, Y. and Phillips, O.L.  2004a.  Fingerprinting the impacts of global change on tropical forests.  Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 359: 437-462.

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.  2004b.  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.

Nelson, B.W.  2005.  Pervasive alteration of tree communities in undisturbed Amazonian forests.  Biotropica 37: 158-159.

Phillips, O.L., Baker, T.R., Arroyo, L., Higuchi, N., Killeen, T.J., Laurance, W.F., Lewis, S.L., Lloyd, J., Malhi, Y., Monteagudo, A., Neill, D.A., Núñez Vargas, P., Silva, J.N.M., Terborgh, J., Vásquez Martínez, R., Alexiades, M., Almeida, S., Brown, S., Chave, J., Comiskey, J.A., Czimczik, C.I., Di Fiore, A., Erwin, T., Kuebler, C., Laurance, S.G., Nascimento, H.E.M., Olivier, J., Palacios, W., Patiño, S., Pitman, N.C.A., Quesada, C.A., Saldias, M., Torres Lezama, A., B. and Vinceti, B.  2004.  Pattern and process in Amazon tree turnover: 1976-2001.  Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 359: 381-407.

Phillips, O.L. and Gentry, A.H.  1994.  Increasing turnover through time in tropical forests.  Science 263: 954-958.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J.  1998.  Changes in the carbon balance of tropical forests: Evidence from long-term plots.  Science 282: 439-442.

Pimm, S.L. and Sugden, A.M.  1994.  Tropical diversity and global change.  Science 263: 933-934.

Weaver, P.L. and Murphy, P.G.  1990.  Forest structure and productivity in Puerto Rico's Luquillo Mountains.  Biotropica 22: 69-82.