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Earth's Forests: Carbon Sinks of the Future? Or Not!
Volume 10, Number 18: 2 May 2007

In an article published in Britain's New Scientist magazine a few years back, the thought that the planting of trees could sequester a significant portion of the carbon emitted to the air by anthropogenic CO2 emissions was labeled by Pearce (1999) "a dangerous delusion." Citing as the basis for his statement, a report of the United Nations' Intergovernmental Panel on Climate Change (IPCC), wherein it was claimed that scientists said "planned new forests, called 'carbon sinks,' will swiftly become saturated with carbon and begin returning most of their carbon to the atmosphere," he went on to quote Peter Cox of the British Meteteorological Office's Hadley Centre as stating "this is not something that may or may not happen," but something that is "more or less inevitable." Pearce then added that "the suggestion that planting trees means less atmospheric CO2 ignores simple logic," and in derisive denigration of the concept of biospheric carbon sequestration, he mockingly asked: "How did researchers get it so wrong?"

In describing the evidence for his out-of-hand dismissal of the trees-as-carbon-sinks concept, Pearce cited South Africa's Bob Scholes, who he described as "a leading light in the International Geosphere-Biosphere Programme's Global Carbon Project," stating that because (1) increasing CO2 concentrations have an ever-smaller effect on plant growth as they rise higher and higher, and because (2) plant respiration rates increase with rising temperatures (which Pearce assumed to be a consequence of increases in the air's CO2 content), CO2 fertilization rates "will flatten out while respiration rates soar," so that by 2050 "forests will have released much of what they have absorbed." But is this truly the iron-clad case Pearce made it out to be, and which many climate alarmists continue to promulgate as fact?

In a recent review of this very question, an international team of distinguished forest researchers (Hyvonen et al., 2007) - which was comprised of 22 scientists from nine different countries (Belgium, Denmark, France, Finland, Iceland, Italy, Sweden, the United Kingdom and the United States) - came to some interesting conclusions. Noting that certain forest ecosystem models do indeed suggest that forest carbon sinks "may gradually diminish in the medium term" because "photosynthesis will increase less as the CO2 concentration continues to rise, whereas respiration is expected to continue to increase with the rise in temperature," they gave as their considered expert opinion that "simplistic models forecasting that stand photosynthesis will be overtaken by stand respiration, purely on the basis of short-term responses of photosynthesis to CO2 and respiration to temperature, should be treated with great caution," adding that "because of current limitations on our understanding with respect to acclimation of the physiological processes, the climatic constraints, and feedbacks among these processes - particularly those acting at the biome scale - projections of C-sink strengths beyond a few decades [which they acknowledge are likely to remain positive] are highly uncertain."

The key question, in their estimation, "is whether the mature forests that are C sinks today will continue to be sinks as the climate changes." One way of addressing this question is to look at what has happened to forests that were already mature several decades ago, and that have experienced the concurrent increases in air temperature and atmospheric CO2 concentration of the past half-century or more, over which period climate alarmists claim both factors rose at unprecedented rates and to unprecedented levels.

Good examples are old-growth forests, such as those of Amazonia, which for most of the past century were believed to be close to dynamic equilibrium. In one of the first studies to dispel this long-held notion, 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. In doing so, they found that the growth rates of these forests had been rising ever higher since at least 1960, and that they had experienced an apparent acceleration in growth rate sometime after 1980. 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. Last of all, and 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.

In view of this wealth of pertinent positive findings, there is strong reason to reject the doom-and-gloom scenarios of forest carbon sinks turning into forest carbon sources in the decades ahead. If anything, real-world observations suggest that today's forest carbon sinks may well become even stronger carbon sinks as air temperatures and atmospheric CO2 concentrations continue to rise.

Sherwood, Keith and Craig Idso

Hyvonen, R., Agren, G.I., Linder, S., Persson, T., Cotrufo, M.F., Ekblad, A., Freeman, M., Grelle, A., Janssens, I.A., Jarvis, P.G., Kellomaki, S., Lindroth, A., Loustau, D., Lundmark, T., Norby, R.J., Oren, R., Pilegaard, K., Ryan, M.G., Sigurdsson, B.D., Stromgren, M., van Oijen, M. and Wallin, G. 2007. The likely impact of elevated [CO2], nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: a literature review. New Phytologist 173: 463-480.

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., 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.

Pearce, F. 1999. That sinking feeling. New Scientist 164 (2209): 20-21.

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.

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