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Forest Growth Rates
Volume 8, Number 16: 20 April 2005

In a lecture presented at the University of Minnesota nearly ten years ago, Idso (1995) laid out the evidence for a worldwide increase in the growth rates of earth's forests that had been coeval with the progression of the Industrial Revolution and the rising CO2 content of the atmosphere.  The development of this concept began with the study of LaMarche et al. (1984), who analyzed annual growth rings of two species of pine tree growing near the timberline in California, Colorado, Nevada and New Mexico and thereby discovered large increases in growth rate between 1859 and 1983, which rates exceeded what might have been expected from climatic trends but were consistent with the global trend of atmospheric CO2.  The developmental journey continued with a study of ring-width measurements of Douglas fir trees in British Columbia, Canada, that also revealed a marked increase in growth in the trees' latter decades (Parker et al., 1987), leading the principal investigator of the project to state that "environmental influences other than increased CO2 have not been found that would explain this [phenomenon]."  West (1988) reported much the same thing with respect to long-leaf pines in Georgia, i.e., that their annual growth increments had begun to rise at an unusual rate about 1920, increasing by approximately 30% by the mid-1980s; and he too stated that "the increased growth cannot be explained by trends in precipitation, temperature, or Palmer Drought Severity Index," leaving the rising CO2 content of the atmosphere as the likely cause of the increase in productivity.

Contemporaneously, stands of Scots pines in northern Finland were found to have experienced growth increases ranging from 15 to 43% between 1950 and 1983 (Hari et al., 1984; Hari and Arovaara, 1988).  As to the cause of this phenomenon, the researchers stated that "CO2 seems to be the only environmental factor that has been changing systematically during this century in the remote area under study," and it was thus to this factor that they looked for an explanation of their observations.

The next major development in the continuing saga was the finding of Graybill and Idso (1993) that very long ring-width chronologies (some stretching back nearly 1800 years) of high-altitude long-lived bristlecone, foxtail and limber pine trees in Arizona, California, Colorado and Nevada all developed an unprecedented upward growth trend somewhere in the 1850s that continued as far towards the present as the records extended.  In this case, too, like the ones that preceded it, 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 ever-increasing productivity over the prior century and a half.

Perhaps the most striking evidence of all for the significant growth enhancement of earth's forests by the historical increase in the air's CO2 concentration was provided by the study of Phillips and Gentry (1994).  Noting that turnover rates of mature tropical forests correlate well with measures of net productivity (Weaver and Murphy, 1990), the two scientists assessed the turnover rates of 40 tropical forests from around the world in order to test the hypothesis that global forest productivity was increasing in situ; and they found that the turnover rates of these highly productive forests had indeed been rising ever higher since at least 1960, with an apparent pan-tropical acceleration since 1980.  In discussing what might be causing this phenomenon, they stated that "the accelerating increase in turnover coincides with an accelerating buildup of CO2," and as Pimm and Sugden (1994) stated in a companion article, 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."

Four years later, a group of eleven researchers headed by Phillips (Phillips et al., 1998) reported another impressive finding.  Working with data on tree basal area (a surrogate for tropical forest biomass) for the period 1958-1996, which they obtained from several hundred plots of mature tropical trees scattered about the world, they found that average forest biomass for the tropics as a whole had increased substantially.  In fact, they calculated that the increase amounted to approximately 40% of the missing terrestrial carbon sink of the entire globe.  Hence, they suggested that "intact forests may be helping to buffer the rate of increase in atmospheric CO2, thereby reducing the impacts of global climate change," as Idso (1991a,b) had earlier suggested, and they identified the aerial fertilization effect of the ongoing rise in the air's CO2 content as one of the factors responsible for this phenomenon.  Other contemporary studies also supported their findings (Grace et al., 1995; Malhi et al., 1998), verifying the fact that neotropical forests were indeed accumulating ever more carbon; and Phillips et al. (2002) continued to state that this phenomenon was occurring "possibly in response to the increasing atmospheric concentrations of carbon dioxide (Prentice et al., 2001; Malhi and Grace, 2000)."

As time progressed, however, it became less and less popular (i.e., politically correct) to report positive consequences of rising atmospheric CO2 concentrations; and the results of Phillips and company began to be repeatedly questioned (Sheil, 1995; Sheil and May, 1996; Condit, 1997; Clark, 2002; Clark et al., 2003).  In response to the most recent of these challenges to their work, we published a rebuttal in our Editorial of 18 Jun 2003.  And now, Phillips, joined by 17 other researchers (Lewis et al., 2005b), including one who had earlier criticized his conclusions, has published a new analysis that vindicates his and his colleagues' earlier analyses.

One of the primary concerns of critics of Phillips' work has been the fact that his meta-analyses have included sites with a wide range of tree census intervals (2-38 years), which they contend could be confounding or "perhaps even driving conclusions from comparative studies," as Lewis et al. (2005b) describe it.  However, in their detailed study of this potential problem, which they conclude is indeed real, they find that re-analysis of Phillips' published results "shows that the pan-tropical increase in stem turnover rates over the late 20th century cannot be attributed to combining data with differing census intervals."  Or as they state more obtusely in another place, "the conclusion that turnover rates have increased in tropical forests over the late 20th century is robust to the charge that this is an artifact due to the combination of data that vary in census interval (cf. Sheil, 1995)."

Lewis et al. (2005b) additionally note that "Sheil's (1995) original critique of the evidence for increasing turnover over the late 20th century also suggests that the apparent increase could be explained by a single event, the 1982-83 El Niño Southern Oscillation (ENSO), as many of the recent data spanned this event."  However, as they continue, "recent analyses from Amazonia have shown that growth, recruitment and mortality rates have simultaneously increased within the same plots over the 1980s and 1990s, as has net above-ground biomass, both in areas largely unaffected, and in those strongly affected, by ENSO events (Baker et al., 2004; Lewis et al., 2004a; Phillips et al., 2004)."

In conclusion, we note that these most recent developments continue to support the view that there has indeed been an increase in forest growth rates throughout the world that has gradually accelerated over the years in concert with the historical increase in the air's CO2 concentration; and, therefore, we fully expect this trend to continue into the future.

Sherwood, Keith and Craig Idso

References
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