In the six-week study of Schortemeyer et al. (1999), seedlings of Acacia melanoxylon grown at twice-ambient atmospheric CO2 concentrations displayed photosynthetic rates that were 22% greater than those of ambiently-grown seedlings. In addition, the CO2-enriched seedlings exhibited biomass values that were twice as large as those displayed by control seedlings grown in air of 350 ppm CO2. Likewise, Polley et al. (1999) reported that a doubling of the atmospheric CO2 concentration for three months increased honey mesquite (Prosopis glandulosa) seedling root and shoot biomass by 37 and 46%, respectively.
Several studies have investigated the effects of elevated CO2 on black locust (Robinia pseudoacacia) seedlings. Uselman et al. (2000), grew seedlings for three months at 700 ppm CO2 and reported that this treatment increased the root exudation of organic carbon compounds by 20%, while Uselman et al. (1999) reported no CO2-induced increases in the root exudation of organic nitrogen compounds. Nonetheless, elevated CO2 enhanced total seedling biomass by 14% (Uselman et al., 2000).
In the study of Olesniewicz and Thomas (1999), black locust seedlings grown at twice-ambient CO2 concentrations for two months exhibited a 69% increase in their average rate of nitrogen-fixation when they were not inoculated with an arbuscular mycorrhizal fungal species. It was further determined that the amount of seedling nitrogen derived from nitrogen-fixation increased in CO2-enriched plants by 212 and 90% in non-inoculated and inoculated seedlings, respectively. Ultimately, elevated CO2 enhanced total plant biomass by 180 and 51% in non-inoculated and inoculated seedlings, respectively.
In summary, it would appear that as the CO2 content of the air increases, nitrogen-fixing trees respond by exhibiting enhanced rates of photosynthesis and biomass production, as well as enhanced rates of nitrogen fixation.
For more information on nitrogen-fixing tree growth responses to atmospheric CO2 enrichment see Plant Growth Data: Blackwood (dry weight, photosynthesis), Black Locust (dry weight), and Honey Mesquite (dry weight).
References
Olesniewicz, K.S. and Thomas, R.B. 1999. Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide. New Phytologist 142: 133-140.
Polley, H.W., Tischler, C.R., Johnson, H.B. and Pennington, R.E. 1999. Growth, water relations, and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): responses to CO2 enrichment. Tree Physiology 19: 359-366.
Schortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R. 1999. The impact of elevated atmospheric CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of Acacia melanoxylon. Australian Journal of Plant Physiology 26: 737-774.
Uselman, S.M., Qualls, R.G. and Thomas, R.B. 1999. A test of a potential short cut in the nitrogen cycle: The role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature. Plant and Soil 210: 21-32.
Uselman, S.M., Qualls, R.G. and Thomas, R.B. 2000. Effects of increased atmospheric CO2, temperature, and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.). Plant and Soil 222: 191-202.