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Effects of Elevated CO2 on Plant-Fungal-Soil Relationships in Birch
Rouhier, H. and Read, D.  1999.  Plant and fungal responses to elevated atmospheric CO2 in mycorrhizal seedlings of Betula pendulaEnvironmental and Experimental Botany 42: 231-241.

What was done
Seedlings of birch (Betula pendula) were inoculated, or not inoculated, with a common ectomycorrhizal fungus (Paxillus involutus) and grown for three months in small Plexiglas containers placed within growth chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm.  The purpose of this study was to investigate the short-term effects of elevated CO2 and fungal inoculation on plant biomass accumulation and carbon distribution within the different parts of this plant-fungus-soil system.

What was learned
In contrast with the results of other studies on birch (see references listed below, which are all reviewed on our website), seedlings grown in elevated CO2 accumulated less biomass than those grown in ambient CO2 concentrations, regardless of fungal inoculation.  However, fungal inoculation always led to greater plant biomass at both atmospheric CO2 concentrations, demonstrating that fungal mycelia have the ability to increase soil nutrient acquisition for accommodating increased plant growth.

By using radioactive 14CO2 as a tracer, the authors were able to determine how carbon was being distributed in their experimental systems.  Elevated CO2 and fungal inoculation did not significantly affect carbon allocation in plant shoots and roots, nor did they affect its allocation in fungal mycelia.  Soil carbon, however, was significantly greater in CO2-enriched systems than it was in ambient systems, indicating that elevated CO2 was enhancing soil carbon inputs through increased root turnover or rhizodeposition of organic compounds.  Moreover, the greater carbon contents of CO2-enriched soils facilitated the growth and development of fungal mycelia to a greater extent than in the ambient CO2 treatment, as evidenced by the association of the mycelia of the CO2-enriched plants' roots with 30% more soil area than that established by mycelia produced in soils exposed to ambient CO2 concentrations.

What it means
Even though plants grown in elevated CO2 did not produce the greatest biomass, their associated soils contained the greatest amounts of carbon and fungal mycelia.  Thus, it is logical to predict that had this experiment lasted longer, it likely would have yielded different results, with CO2-enriched birch seedlings accumulating more biomass than the seedlings exposed to ambient air, due to the enhanced nutrient acquisition resulting from the association of the CO2-enriched seedlings' roots with a more robust network of fungal mycelia.  Thus, it is likely that over the long term, birch trees will exhibit increases in biomass as the atmospheric CO2 concentration continues to rise, as demonstrated by many other studies.

Berntson, G.M. and Bazzaz, F.A.  1998.  Regenerating temperate forest mesocosms in elevated CO2: belowground growth and nitrogen cycling.  Oecologia 113: 115-125.

Catovsky, S. and Bazzaz, F.A.  1999.  Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure.  Global Change Biology 5: 507-518.

Godbold, D.L., Berntson, G.M. and Bazzaz, F.A.  1997.  Growth and mycorrhizal colonization of three North American tress species under elevated atmospheric CO2New Phytologist 137: 433-440.

Wang, Y.-P., Rey, A and Jarvis, P.G.  1998.  Carbon balance of young birch trees grown in ambient and elevated atmospheric CO2 concentrations.  Global Change Biology 4: 797-807.

Wayne, P.M., Reekie, E.G. and Bazzaz, F.A.  1998.  Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts.  Oecologia 114: 335-342.

Reviewed 1 May 2000