Several people have contacted us about a news item on p.20 of the October 1998 issue of Scientific American that suggests that warmer nights may be slowing tropical rainforest growth and raising atmospheric CO2 levels. Their concerns are perhaps best summarized by the simple question "Are these suggestions correct?"
Answer:
These suggestions may very well be correct, but they could just as easily be wrong. In their favor are a decade of aboveground growth measurements of six different tree species at the La Selva Biological Station in central Costa Rica, plus a preliminary calculation of the amount of carbon dioxide released from tropical land areas in the 1980s and 1990s. Also supporting them are two negative linkages between the growth rates of the trees the La Selva researchers studied and air temperature: (1) tree growth rates seemed to slow in warmer years, and (2) growth appeared to be slower when daily minimum air temperatures were higher.
Weighing in against the suggestions, or at least raising some important cautionary flags, are a number of other observations. First is the fact that the growth measurements come from but a single spot on the planet, which may or may not be representative of the rest of the world. That it may not be characteristic of earth's other rainforests is suggested by the landmark study of Phillips and Gentry (1994), who assessed the turnover rates -- which correlate well with net forest productivity -- of 40 tropical forests from around the globe. They found that the turnover rates of these already highly productive forest ecosystems have been rising at a substantial rate since at least 1960, with an apparent planet-wide acceleration since 1980, which includes much of the same time period as the La Selva study. Furthermore, Pimm and Sugden (1994) have noted 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," which conclusion clearly flies in the face of the suggestion of the La Selva researchers that "the growth-slowing effect of increased temperature in tropical regions is now stronger than any beneficial fertilization effect from rising carbon dioxide." Their sentiment is also called into question by the recent review of global carbon cycle research prepared by Tans and White (1998), who report that measurements of the vertical transport of CO2 over forest ecosystems conducted in a number of different places over the last several years all tend to indicate that forests are substantial sinks for CO2.
Another potential problem with the La Selva study is that only aboveground growth was measured. As the air's CO2 content rises, plants typically produce more extensive root systems (Idso and Kimball, 1991; Rogers et al., 1992), particularly in response to conditions that may limit aboveground growth (Norby et al., 1992; Idso and Idso, 1994); and the La Selva study didn't even scratch the surface in this regard.
With respect to the preliminary calculations of Keeling and his associates that are reported to reveal "a colossal unexplained tropical terrestrial source of carbon dioxide," we will need to see more details. Keeling is almost always right in everything he does; but sometimes "preliminary" assessments are significantly altered before they make their way to press.
In spite of what these calculations may ultimately show, there are some well-founded reasons for doubting the implications of the Scientific American news report on this point, or at least its implications for the globe as a whole. Just four years ago, for example, Keeling (1994) noted that "according to measurements made on air collected at an array of stations from the Arctic to the South Pole, the concentration of atmospheric CO2 during the past several years (which coincides with much of the period over which the La Selva study was conducted) has risen globally at a slower rate than expected from past measurements." [See Carbon Dioxide Trends: (The Last 1,000 Years)]. He further noted that this slowdown, which was unprecedented in 36 years of observations, was accompanied by an anomaly in the 13C/12C ratio of CO2 that indicated that the slower rate was "caused by an anomalous uptake of CO2 by the terrestrial biosphere of about 6 billion metric tons of carbon between August 1988 and August 1993." Fourteen months later, Keeling et al. (1995) reaffirmed this assessment of the world's carbon cycle; stating again that the terrestrial biosphere was responsible for most of the slowdown in the rate of rise of the air's CO2 content; and the recent review of Tans and White (1998) depicts "a terrestrial biosphere nearly balanced (globally) with respect to carbon."
Nevertheless, tropical forests only account for about a third of all land plant photosynthesis, and things can always change, as in the case of atmospheric methane, which has nearly stopped rising altogether (Dlugokencky et al., 1998). Consequently, as we stated at the start of our answer, the La Selva researchers could be correct. But we doubt it. As the Scientific American news report notes, "the new information from Costa Rica has not yet been published in a peer-reviewed journal, so it remains to be seen whether the scientific community will accept it." (See the Volume 1, Number 2 Editorial Commentary). Of course, even a large consensus of scientists can be wrong -- and a couple of reviewers even more so. But for the present, meticulous adherence to established scientific procedures is the only means we have of moving forward on these important global issues in a credible manner.
References
Dlugokencky, E.J., Masarie, K.A., Lang, P.M. and Tans, P.P. 1998. Continuing decline in the growth rate of the atmospheric methane burden. Nature 393: 447-450.
Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.
Idso, S.B. and Kimball, B.A. 1991. Effects of two and a half years of atmospheric CO2 enrichment on the root density distribution of three-year-old sour orange trees. Agricultural and Forest Meteorology 55: 345-349.
Keeling, C.D. 1994. A study of the abundance and 13C/12C ratio of atmospheric carbon dioxide and oceanic carbon in relation to the global carbon cycle. In: Riches, M.R. (Ed.), Global Change Research: Summaries of Research in FY 1994. U.S. Dept. of Energy, Washington, DC, pp. 109-110.
Keeling, C.D., Whorf, T.P, Wahlen, M. and van der Pilcht, J. 1995. Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980. Nature 375: 666-670.
Norby, R.M., Gunderson, C.A., Wullschleger, S.D., O'Neill, E.G. and McCracken, M.K. 1992. Productivity and compensatory responses of yellow-poplar trees in elevated CO2. Nature 357: 322-324.
Phillips, O.L. and Gentry, A.H. 1994. Increasing turnover through time in tropical forests. Science 263: 954-958.
Pimm, S.L. and Sugden, A.M. 1994. Tropical diversity and global change. Science 263: 933-934.
Rogers, H.H., Peterson, C.M., McCrimmon, J.N. and Cure, J.D. 1992. Response of plant roots to elevated atmospheric carbon dioxide. Plant, Cell and Environment 15: 749-752.
Tans, P.P. and White, J.W.C. 1998. The global carbon cycle: In balance, with a little help from the plants. Science 281: 183-184.