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Update on Tropical Forest Productivity
Volume 9, Number 30: 26 July 2006

There are two major changes that are occurring throughout the world's tropical forests. First, and most obvious, deforestation has led to vast amounts of carbon being released to the atmosphere. Second, and much less obvious, within remaining intact forests there has been an acceleration of forest dynamism that has led to vast amounts of carbon being extracted from the atmosphere. In this editorial we focus on the latter phenomenon, as summarized by Lewis (2006).

The increasing dynamism and productivity of intact tropical forests has a long history, with Lewis reporting that "across the paleotropics forest dynamism has been steadily increasing for five decades (Phillips and Gentry, 1994)." Among 50 old-growth plots scattered across tropical South America, for example, he notes that "stem recruitment, stem mortality, and biomass growth, and loss, all increased significantly (Lewis et al., 2004a)." In addition, he reports that "over approximately the last 20 years, long-term monitoring of 59 plots showed that above-ground biomass increased by 0.6 ± 0.2 tonnes C ha-1 a-1, or a relative increase of 0.50 ± 0.17% a-1 (mean ± 95% confidence interval; Baker et al., 2004a)." This rate of increase, to quote him again, "is slightly higher than that documented by Phillips et al. (1998)." Thus, there is no question that "over the past two decades," according to Lewis, "these forests have shown concerted changes in their ecology, becoming, on average, faster growing - more productive - and more dynamic, and showing a net increase in above-ground biomass," all of which rates of increase are greater than the previously documented increases in the rates of these phenomena. What is more, Lewis says that "preliminary analyses also suggest the African and Australian forests are showing structural changes similar to South American forests (Lewis et al., in preparation)."

What is causing this suite of concerted changes? Lewis states that "the results appear to show a coherent fingerprint of increasing net primary productivity across tropical South America, caused by a long-term increase in resource availability (Lewis et al., 2004a,b)." So what "resources" might be involved? Lewis gives four possibilities - increases in solar radiation, air temperature, nutrient deposition and atmospheric CO2 concentration - but after analyzing each of them in detail, he concludes that "the most parsimonious explanation is the increase in atmospheric CO2, because of the undisputed long-term historical increase in CO2 concentrations, the key role of CO2 in photosynthesis, and the demonstrated positive effects of CO2 fertilization on plant growth rates including experiments on whole temperate-forest stands (Ainsworth and Long, 2005)," or as he states in another place in his review, the explanation resides in "the anthropogenic increase in atmospheric carbon dioxide concentrations, increasing forest net primary productivity leading to accelerated forest growth and dynamics."

In light of the voluminous and undeniable real-world observations reported by Lewis, it must be acknowledged that where tropical forests have not been decimated by the direct destructive actions of man, such as the felling and burning of trees, forest productivity has been growing ever greater with the passing of time, rising hand-in-hand with the increasing CO2 content of the air; and it has been doing so in spite of all concomitant changes in atmospheric, soil and water chemistry, as well as "dreaded" 20th-century global warming, which is claimed by climate alarmists to have been unprecedented over the past two millennia. Real-world evidence also suggests we have the anthropogenic-induced increase in the air's CO2 content to thank for this beneficent state of affairs, which further suggests - to us at least - that if humanity will but cease its direct physical assaults upon earth's tropical forests, we should have nothing to fear about their future well-being but ill-founded fear itself, which could well drive us to irrationally deprive them of that which appears to have supported the phenomenal increase in productivity they have experienced over the past half-century. Nevertheless, many people, including Lewis, believe that the forests' biological response to rising CO2 may saturate sometime in the future, and that the predicted climatic effects of anthropogenic CO2 emissions may ultimately overpower this positive effect and lead to a significant downturn in tropical forest productivity, once again highlighting the need to resolve this latter most important issue.

Sherwood, Keith and Craig Idso

References
Ainsworth, E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165: 351-372.

Baker, T.R., Phillips, O.L., Malhi, Y., Almeida, S., Arroyo, L., Di Fiore, A., Erwin, T., Higuchi, N., Killeen, T.J., Laurance, S.G., Laurance, W.F., Lewis, S.L., Monteagudo, A., Neill, D.A., Núñez Vargas, P., Pitman, N.C.A., Silva, J.N.M. and Vásquez Martínez, R. 2004a. Increasing biomass in Amazonian forest plots. Philosophical Transactions of the Royal Society of London Series B - Biological Sciences 359: 353-365.

Lewis, S.L. 2006. Tropical forests and the changing earth system. Philosophical Transactions of the Royal Society B 361: 195-210.

Lewis, S.L., Malhi, Y. and Phillips, O.L. 2004b. Fingerprinting the impacts of global change on tropical forests. Philosophical Transactions of the Royal society B 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. 2004a. 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.

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.