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Several studies have recently documented the effects of elevated atmospheric CO2 concentrations on photosynthesis in various tropical and sub-tropical trees. In the relatively short-term study of Lovelock et al. (1999a), for example, seedlings of the tropical tree Copaifera aromatica that were grown for two months at an atmospheric CO2 concentration of 860 ppm exhibited photosynthetic rates that were consistently 50-100% greater than those displayed by control seedlings fumigated with air containing 390 ppm CO2. Similarly, Lovelock et al. (1999b) reported that a10-month 390-ppm increase in the air's CO2 content boosted rates of net photosynthesis in 30-m tall Luehea seemannii trees by 30%. Likewise, in the review paper of Schaffer et al. (1999), it was noted that atmospheric CO2 enrichment had previously been shown to enhance rates of net photosynthesis in a number of tropical and sub-tropical fruit trees, including avocado, banana, citrus, mango and mangosteen. Even at the ecosystem level, Lin et al. (1998) found that a 1700-m2 synthetic rainforest mesocosm displayed a 79% enhancement in net ecosystem carbon exchange rate in response to a 72% increase in the air's CO2 content.
Because elevated CO2 enhances photosynthetic rates in tropical and sub-tropical trees, it should also lead to increased carbohydrate and biomass production in these species. And it does! At a tropical forest research site in Panama, for example, twice-ambient CO2 concentrations enhanced foliar sugar concentrations by up to 30% (Wurth et al., 1998), while doubling the foliar concentrations of starch (Lovelock et al., 1998) in a number of tree species. Also, in the study of Hoffmann et al. (2000), elevated CO2 (700 ppm) enhanced dry weights of an "uncut" Brazilian savannah tree species (Keilmeyera coriacea) by about 50%, while it enhanced the dry weight of the same "cut" species by nearly 300%. Although not specifically quantified, Schaffer et al. (1997) noted that twice-ambient CO2 exposure for one year obviously enhanced dry mass production in two mango ecotypes. Finally, in the six-month study of Sheu et al. (1999), a doubling of the atmospheric CO2 concentration increased seedling dry weight in Schima superba by 14 and 49% when grown at ambient and elevated (5°C above ambient) air temperatures, respectively.
In summary, it is clear that as the air's CO2 content rises, tropical and sub-tropical trees will likely display enhanced rates of photosynthesis and biomass production, even under conditions of herbivory and elevated air temperature. Consequently, greater carbon sequestration will also likely occur within earth's tropical and sub-tropical forests as ever more CO2 accumulates in the atmosphere.
References
Hoffmann, W.A., Bazzaz, F.A., Chatterton, N.J., Harrison, P.A. and Jackson, R.B. 2000. Elevated CO2 enhances resprouting of a tropical savanna tree. Oecologia 123: 312-317.
Lin, G., Marino, B.D.V., Wei, Y., Adams, J., Tubiello, F. and Berry, J.A. 1998. An experimental and modeling study of responses in ecosystems carbon exchanges to increasing CO2 concentrations using a tropical rainforest mesocosm. Australian Journal of Plant Physiology 25: 547-556.
Lovelock, C.E., Posada, J. and Winter, K. 1999a. Effects of elevated CO2 and defoliation on compensatory growth and photosynthesis of seedlings in a tropical tree, Copaifera aromatica. Biotropica 31: 279-287.
Lovelock, C.E., Virgo, A., Popp, M. and Winter, K. 1999b. Effects of elevated CO2 concentrations on photosynthesis, growth and reproduction of branches of the tropical canopy trees species, Luehea seemannii Tr. & Planch. Plant, Cell and Environment 22: 49-59.
Lovelock, C.E., Winter, K., Mersits, R. and Popp, M. 1998. Responses of communities of tropical tree species to elevated CO2 in a forest clearing. Oecologia 116: 207-218.
Schaffer, B., Whiley, A.W. and Searle, C. 1999. Atmospheric CO2 enrichment, root restriction, photosynthesis, and dry-matter partitioning in subtropical and tropical fruit crops. HortScience 34: 1033-1037.
Schaffer, B., Whiley, A.W., Searle, C. and Nissen, R.J. 1997. Leaf gas exchange, dry matter partitioning, and mineral element concentrations in mango as influenced by elevated atmospheric carbon dioxide and root restriction. Journal of the American Society of Horticultural Science 122: 849-855.
Sheu, B.-H. and Lin, C.-K. 1999. Photosynthetic response of seedlings of the sub-tropical tree Schima superba with exposure to elevated carbon dioxide and temperature. Environmental and Experimental Botany 41: 57-65.
Wurth, M.K.R., Winter, K. and Korner, C. 1998. Leaf carbohydrate responses to CO2 enrichment at the top of a tropical forest. Oecologia 116: 18-25.