Tropical Plant Responses to Increases in Atmospheric C02 Concentrations

From Sec. 4.1 it can be concluded that plants typically found on nutrient-poor tropical soils may be relatively slow-growing, at least by the standards of the moist tropics. And also given that many species associated with these forests do not appear to be able to respond significantly to phosphorus fertilization (Sec. 4.1) it might also be argued that these species may not be able to significantly increase their growth in response to increases in ambient [CO,] (Poorter, 1993, 1998). On the other hand, Lloyd and Farquhar (1996) have argued that tropical plants typically have high respiratory costs. Any increase in photosynthesis in response to increased [C02] should therefore result in a greater than average increase in growth (Lloyd and Farquhar, 2000).

Unfortunately, there are only scant experimental data with which to make a judgment on this. The [C02] growth responses of potted tropical trees have also been investigated by Oberbauer et al. (1985), Reekie and Bazzaz (1989), Ziska et al. (1991), Lovelock et al. (1996), Winter and Lovelock (1999), and Carswell et al. (2000). With the exception of Reekie and Bazzaz (1989) and Carswell et al. (2000), substantial increases in the rate of plant biomass accumulation have been observed, with Ziska et al. (1991) observing a massive dry-weight increase of 164% at harvest for Tabebuia rosea, a canopy tree species native to Panama. Substantial growth enhancement stimulations in response to C02 enrichment have also been observed for another Panamanian tree species, Beilschmiedia pendula (Lovelock et al., 1996). Lovelock et al. (1997) also showed that the presence of mycorrhizae serves to significantly increase rates of P uptake per plant and maintain high photosynthetic capacities of B. pendula when grown under C02 enrichment. Hogan et al. (1991) have suggested that the lack of positive growth enhancement observed by Reekie and Bazzaz (1989) was a consequence of the use of very small pots in that experiment. Working with Cedrel odorata from Costa Rica, Carswell et al. (2000) observed substantial stimulations of leaf photosynthetic rates for high [CO,]-grown plants at both high and low nutrient supply rates. Although they observed a trend toward increased growth at higher [C02] at both nutrient supply rates, this effect was not statistically significant. This was considered to be mostly a consequence of an unusually high within-treatment variation in plant growth rates.

The idea that a strong growth stimulation to elevated CO, concentrations should be seen in understory plants in tropical forests which typically grow close to their light compensation points was tested by Würth et al. (1998). Investigating a range of species, growth enhancements ranging from 25 to 76% in response to a doubling of [C02j were observed.

A second approach has been the use of "model" tropical rain forest communities (Körner and Arnone, 1992; Arnone and Körner, 1995). In their first experiment plants were exposed to reasonably well-fertilized soil but without any attempt to ensure adequate mvcorrhizal infection, and only a modest difference of 11% was observed in final harvest biomass between ambient [C02] and 2 X [CO,] treatments (Körner and Arnone, 1992). After accounting for the significant biomass of both communities before the instigation of treatments, this does, however, suggest an overall growth stimulation of about 20% in response to CO,, not greatly different from the enhancements typically associated with woody C3 species (Poorter, 1993). In a follow-up experiment, where nutrients where purposely kept low, this growth stimulation was much reduced, though still significant (Arnone and Körner, 1995). But the "soil" used in this circumstance was a C-free quartz sand. It is thus hard to relate the results of such an experiment to rain forests in situ, where significant amounts of inorganic and organic phosphorus are available in various forms. Lovelock et al. (1998) did, however, observe that for communities of tropical forest tree seedlings grown at ambient and elevated [C02] in open-top chambers at the edge of a forest in Panama, no enhancements in plant biomass occurred under elevated [C02], either for the whole communities or in the growth of individual species. But, particularly as several different successional types were present, the extent to which the intense interplant competition in that experiment was actually representative of a typical tropical forest regeneration pattern is unclear.

Such methodological concerns with "model communities" aside, the tendency seems to be for potted tropical plants to show significant growth responses to elevated [C02], but for model communities these responses are much reduced or absent. Clearly, much more work is required to elucidate the basis of these contradictory results. Nevertheless in what follows, we assume that moist tropical forest trees increase their productivity in a manner typical of C3 plants, investigating the extent to which changes in phosphorus availability might modify that response.

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