As the CO2 content of the air increases, nearly all of earth's plants will exhibit increases in their photosynthetic rates. Additional photosynthetic sugars can then be utilized to increase plant growth and development, or they can be exuded belowground to enhance the growth and development of symbiotic fungal organisms that live in close association with plant roots. This latter phenomenon is extremely important, for fungi form intricate mycelial networks that act to increase nutrient and water harvesting from the soil and the subsequent transfer of these important resources to plants. In this summary, we review how elevated CO2 impacts these several processes in the case of woody species, i.e., trees and shrubs.
Researchers often report that atmospheric CO2 enrichment increases the percentage of the root tip that is colonized by the hyphae of ectomycorrhizal fungi (Staddon and Fitter, 1998). Rygiewicz et al. (2000), for example, observed a 95% increase in this parameter for Douglas fir seedlings exposed to an atmospheric CO2 concentration of 550 ppm for four years. Similarly, Godbold et al. (1997) noted that the percentage root tip colonized by ectomycorrhizal fungal hyphae in paper birch and Eastern white pine were enhanced by 13 and 38%, respectively, when the atmospheric CO2 concentration was doubled for nearly ten months. In addition, other scientists have documented this positive influence of elevated CO2 on the percentage root tip colonized by fungal hyphae in yellow birch (+71%, Berntson and Bazzaz, 1998), white birch (+45%, Berntson and Bazzaz, 1998), and Eastern hemlock (+47%, Godbold et al., 1997).
Although the percentage increase in root colonization by fungal hyphae is by far the most commonly reported response of fungi to atmospheric CO2 enrichment, it is not the only way in which elevated levels of atmospheric CO2 enhanced fungal-root relationships.. In the study of Rillig and Allen (1998), for example, elevated CO2 (750 ppm) did not significantly impact the percentage root colonized by fungal hyphae in Gutierrezia sarothrae shrubs, but it did significantly increase its percentage colonization by arbuscules and vesicles by 14- and 2.5-fold, respectively. Similarly, Klironomos et al. (1998) reported that twice-ambient levels of atmospheric CO2 increased the percentage root colonization in sagebrush by arbuscules by 70%. In addition, Rillig et al. (2000) documented a 4-fold linear increase in the percentage root colonization by arbuscular mycorrhizal fungi in native vegetation growing for 20 years along a natural CO2 gradient (370 to 670 ppm) near a CO2 spring in New Zealand.
Elevated CO2 has also been noted to significantly enhance other fungal properties. In the study of Walker et al. (1998), an atmospheric CO2 concentration of 700 ppm increased the total number of ectomycorrhizal hyphae on ponderosa pine seedling roots by 85% relative to what was observed on roots of seedlings exposed to ambient air. In the more complex study of Wiemken et al. (2001), the biomass of ectomycorrhizal fungi associated with roots of beech and spruce seedlings was about 118% greater at high soil nitrogen and elevated CO2 (560 ppm) than it was at low soil nitrogen and ambient CO2.
Finally, atmospheric CO2 enrichment also impacts certain fungal properties that can act to enhance soil stability. In the previously mentioned study of Rillig et al. (2000), for example, it was reported that total soil glomalin - a protein secreted by fungal hyphae that increases soil aggregation and stability - increased nearly 5-fold along the natural 300-ppm CO2 gradient. In addition, Rouhier and Reed (1998) noted that the mycorrhizal network constructed around roots of Scots pine seedlings grown at 700 ppm CO2 occupied 444% more soil area than that occupied by networks constructed around roots of seedlings exposed to ambient air. Rouhier and Read (1999) documented the same phenomenon for mycorrhizal networks established around roots of CO2-enriched beech seedlings, albeit the response was much smaller in magnitude (30% increase).
In summary, as the air's CO2 content continues to rise, most of earth's woody species should respond by exhibiting enhanced rates of photosynthesis that will lead to the stimulation of belowground fungal development (increased fungal hyphal length and biomass) and activities (greater penetration of the soil, enhanced soil stability), leading ultimately to enhanced root uptake of important soil minerals and water.
References
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Godbold, D.L., Berntson, G.M. and Bazzaz, F.A. 1997. Growth and mycorrhizal colonization of three North American tress species under elevated atmospheric CO2. New Phytologist 137: 433-440.
Klironomos, J.N., Ursic, M., Rillig, M. and Allen, M.F. 1998. Interspecific differences in the response of arbuscular mycorrhizal fungi to Artemisia tridentata grown under elevated atmospheric CO2. New Phytologist 138: 599-605.
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Rygiewicz, P.T., Martin, K.J. and Tuininga, A.R. 2000. Morphotype community structure of ectomycorrhizas on Douglas fir (Pseudotsuga menziesii Mirb. Franco) seedlings grown under elevated atmospheric CO2 and temperature. Oecologia 124: 299-308.
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Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T. 1998. Growth and ectomycorrhizal colonization of ponderosa pine seedlings supplied different levels of atmospheric CO2 and soil N and P. Forest Ecology and Management 109: 9-20.
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