What was done
The authors measured total biomass and a number of physiological processes of N2-fixing Alnus glutinosa (L.) Gaertn. (common alder) trees that were grown for three years in open-top chambers in either ambient or elevated (ambient + 350 ppm) concentrations of atmospheric CO2 and two soil nitrogen regimes (full nutrient solution or no fertilizer), while they measured nitrogen fixation by Frankia spp. in the root nodules of the trees by the acetylene reduction method.

What was learned
In the words of the authors, the "net photosynthetic rate of Alnus glutinosa grown in elevated CO2 was stimulated throughout the 1996 growing season, but significant CO2 treatment differences in biomass were found only in September 1994 and June 1995."  With respect to this finding, they report that "Mooney et al. (1999) concluded that large CO2 treatment differences in biomass accumulation occur during dry years when productivity is low," noting that "the combination of a dry year and fungal attack may have accentuated CO2 treatment differences in 1995 compared with 1996."

Nitrogenase activity was found to be consistently higher in the elevated CO2 treatment in both 1995 and 1996.  In addition, the authors report that "in October 1996, elevated CO2 had a significant effect on total nodule dry mass, and there was a trend toward heavier nodules in the elevated CO2 treatment than in the ambient CO2 treatment."  With respect to these findings, they note that "most single-species studies on the effect of elevated CO2 on N2-fixing species have reported stimulation of growth, nodule mass and nitrogenase activity (Norby, 1987; Arnone and Gordon, 1990; Hibbs et al., 1995; Vogel and Curtis, 1995; Tissue et al., 1997; Vogel et al., 1997; Thomas et al., 2000)," which is similar to what they observed.

Last of all, the authors report that "Alnus glutinosa trees growing under low soil nutrient conditions exhibit[ed] similar growth enhancement as fertilized trees," confirming their original hypothesis.

What it means
In view of the several positive effects of atmospheric CO2 enrichment on the growth and nitrogen-fixing attributes of the common alder trees observed in this study, we concur with Temperton et al. that, as they conclude in the final sentence of their paper, "Alnus glutinosa trees growing in natural, nutrient-poor ecosystems should benefit from exposure to elevated CO2."

References
Arnone, J.A. and Gordon, J.C.  1990.  Effect of nodulation, nitrogen fixation and CO2 enrichment on the physiology, growth and dry mass allocation of seedlings of Alnus rubra Bong.  New Phytologist 116: 55-66.

Hibbs, D.E., Chan, S.S., Castellano, M. and Niu, C.-H.  1995.  Response of red alder seedlings to CO2 enrichment and water stress.  New Phytologist 129: 569-577.

Mooney, H.A., Canadell, J., Chapin III, F.S., Ehleringer, J., Korner, C., McMurtrie, R., Parton, W.J., Pitelka, L. and Schulze, E.-D.  1999.  Ecosystem physiology responses to global change.  In: Implications of Global Change for Natural and Managed Systems.  Walker, B.H., Steffen, W.L., Canadell, J. and Ingram, J.S.I.  (Eds.).  International Geosphere-Biosphere Programme Book Series 4.  Cambridge University Press, Cambridge, UK, pp. 141-189.

Norby, R.J.  1987.  Nodulation and nitrogenase activity in nitrogen fixing woody plants stimulated by CO2 enrichment of the atmosphere.  Physiologia Plantarum 71: 77-82.

Thomas, R.B., Bashkin, M.A. and Richter, D.D.  2000.  Nitrogen inhibition of nodulation and N2 fixation of a tropical N2-fixing tree (Gliricida sepium) grown in elevated atmospheric CO2.  New Phytologist 145: 233-243.

Tissue, D.T., Thomas, R.B. and Strain, B.R.  1997.  Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4-year experiment in the field.  Plant, Cell and Environment 20: 1123-1134.

Vogel, C.S. and Curtis, P.S.  1995.  Leaf gas exchange and nitrogen dynamics of N2-fixing field-grown Alnus glutinosa under elevated atmospheric CO2.  Global Change Biology 1: 55-61.

Vogel, C.S., Curtis, P.S. and Thomas, R.B.  1997.  Growth and nitrogen accretion of dinitrogen-fixing Alnus glutinosa (L.) Gaertn. under elevated carbon dioxide.  Plant Ecology 130: 63-70.