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20th-Century Accelerated Growth of Longleaf Pine Trees: A Belated Review of a Paper Published in 1993
West, D.C., Doyle, T.W., Tharp, M.L., Beauchamp, J.J., Platt, W.J. and Downing, D.J. 1993. Recent growth increases in old-growth longleaf pine. Canadian Journal of Forest Research 23: 846-853.

What was done
The authors developed tree-ring chronologies for both "young" (96-136 years of age, mean = 125 years) and old (161-396 years of age, mean = 250 years) longleaf pine (Pinus palustris Mill.) trees in one of the last remaining old-growth stands of this species in the United States, where the chronologies terminated in 1987. Located in Thomas County, Georgia, they say the study site was "relatively free of point-source emissions and represents an area of low concentrations of anthropogenic atmospheric contaminant deposition," adding that "there are no known effects of site-specific environmental changes occurring within the stand."

What was learned
West et al. note that "the expected annual ring-width increment of undisturbed trees is a negative exponential function with time," but they report that the trees in their study "began a positive departure from this trend approximately 30-50 years ago," i.e., prior to 1987, and that "the beginning of the positive response began in the 1920s for many individual trees," so that trees in 1987 were reaching larger sizes more rapidly than did equally-old trees 50 years earlier. Determining that climate variables were "deficient in explaining the growth trends observed," they suggested that "increased atmospheric CO2 is a possible explanation for initiation of the observed trend."

What it means
In discussing their findings, West et al. note that other researchers before them had found much the same thing they had observed, and that these other scientists had come to much the same conclusion. They state, for example, that "LaMarche et al. (1984) found that upper tree line bristlecone pine in central Nevada and eastern California showed a considerable increase in radial growth during the past 100 years and attributed the positive response to the global CO2 increase," and that "Hari et al. (1984) reported a similar growth increase in trees from southern and central Finland and suggested a related CO2 response."

Additional Evidence and Implications
Many researchers not cited by West et al. have also detected 20th-century increases in tree growth that they could attribute to nothing ... except the concomitant increase in the air's CO2 concentration. In British Columbia, Canada, for example, Parker (1987) determined that Douglas fir trees had experienced a marked increase in growth over prior decades and that "environmental influences other than increased CO2 have not been found that would explain [the phenomenon]." Likewise, Hari and Arovaara (1998) discovered growth increases ranging from 15 to 43% between 1950 and 1983 in stands of Scots pine in northern Finland; and they report that "CO2 seems to be the only environmental factor that has been changing systematically during this century in the remote area under study."

One of the most striking of these other studies was that of Graybill and Idso (1993), who in studying high-altitude long-lived bristlecone, foxtail and limber pine trees in Arizona, California, Colorado and Nevada observed a sharp upward growth trend that began about the middle of the 1850s and continued up to the ends of the several records. Finding no comparable variations anywhere else in the records, which continued uninterrupted back in time for close to two millennia, they concluded that the upward swings in the growth rates of the several chronologies were truly unique and the result of some pan-regional or global factor. What is more, comparisons of the chronologies with temperature and precipitation records ruled out the possibility that either of these climatic variables played a significant role in enhancing the trees' growth rates, strongly implicating the historical rise in the air's CO2 content as the factor responsible for their increasing productivity over the past 150 years.

Close on the heels of Graybill and Idso's findings came those of Phillips and Gentry (1994), who studied turnover rates of forty tropical forests from all around the world. Interestingly, they discovered that the turnover rates of these highly productive tropical forests had been rising even higher since at least 1960, with an apparent pan-tropical acceleration since 1980; and they noted in their discussion of possible causes that "the accelerating increase in turnover coincides with an accelerating buildup of CO2." And to drive this point home, Pimm and Sugden (1994) stated in a companion article that it was "the consistency and simultaneity of the changes on several continents that lead Phillips and Gentry to their conclusion that enhanced productivity induced by increased CO2 is the most plausible candidate for the cause of the increased turnover."

Subsequently, several other studies of tropical forests around the world have confirmed the global nature of 20th-century accelerating tree growth; and they have continued to associate it with the ongoing increase in the atmosphere's CO2 concentration (Phillips et al., 1998, 2004; Laurance et al., 2004a,b, 2005; Lewis et al., 2004a,b). Hence, it would appear there is overwhelming evidence that the tree-ring chronologies employed by Mann et al. (1998, 1999) and Mann and Jones (2003) to create the "hockeystick" temperature curve that climate alarmists have used as the basis for claiming 20th-century global warming was unprecedented over the past two millennia were likely responding more to the truly unprecedented historical increase in the atmosphere's CO2 concentration than to its more subdued increase in temperature, and, therefore, that the hockeystick temperature reconstruction significantly over-inflates 20th-century global warming.

Graybill, D.A. and Idso, S.B. 1993. Detecting the aerial fertilization effect of atmospheric CO2 enrichment in tree-ring chronologies. Global Biogeochemical Cycles 7: 81-95.

Hari, P., Arovaara, H., Raunemaa, T. and Hautojarvi, A. 1984. Forest growth and the effects of energy production: A method for detecting trends in the growth potential of trees. Canadian Journal of Forest Research 14: 437-440.

Hari, P. and Arovaara, H. 1988. Detecting CO2 induced enhancement in the radial increment of trees: Evidence from the northern timberline. Scandinavian Journal of Forest Research 3: 67-74.

LaMarche Jr., V.C., Graybill, D.A., Fritts, H.C. and Rose, M.R. 1984. Increasing atmospheric carbon dioxide: Tree ring evidence for growth enhancement in natural vegetation. Science 223: 1019-1021.

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.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Parker, M. L. 1987. Recent abnormal increase in tree-ring widths: A possible effect of elevated atmospheric carbon dioxide. In: Jacoby Jr., G.C. and Hornbeck, J.W. (Eds.), Proceedings of the International Symposium on Ecological Aspects of Tree-Ring Analysis. U.S. Department of Energy, Washington, DC, pp. 511-521.

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.

Reviewed 13 June 2007