What was done
Rhinanthus alectorolophus, a widely distributed parasitic plant in Central Europe, was grown in isolation and in combinations consisting of one or two host plants (the grass Lolium perenne and the legume Medicago sativa) for two months in pots placed within open-top chambers receiving atmospheric CO2 concentrations of 375 and 590 ppm.  In addition, half of the pots were fertilized to produce an optimal nutrient regime, while the other half remained unfertilized to establish a low nutrient treatment.  Thus, the authors studied the interactive effects of elevated CO2, soil nutrients, and host plant identity on the productivity and growth of this common parasitic species.

What was learned
Nutrient supply had a greater influence on parasite and host biomass than did CO2 concentration.  At low nutrient supply, for example, atmospheric CO2 enrichment decreased mean parasite biomass by 16%, while at high nutrient supply, elevated CO2 increased parasite biomass by an average of 123% over respective control plants.  Similarly, the CO2-induced increase in host plant biomass was greater under high, rather than low, nutrient conditions (29 vs. 18%, respectively).

Besides nutrient supply, CO2-induced changes in parasite biomass were also sensitive to host species.  At nigh nutrient supply, for example, elevated CO2 decreased parasite biomass by 10% when grown with Lolium as its host, but increased its biomass by 170% when grown with Medicago.

The presence of the parasite decreased host biomass under all experimental conditions.  However, the magnitude of this phenomenon was highly dependent upon nutrient condition (73 vs. 19% reductions at low and high nutrients, respectively), but not upon CO2 concentration.  It is interesting to note that elevated CO2 did not significantly influence the effect of the parasite on its hosts.  Moreover, elevated CO2 did not influence the competition between the two different hosts when grown together, either directly or indirectly via its effects on the parasite.

What it means
As the CO2 content of the air continues to rise, it is likely that this parasitic plant will display differential growth responses depending upon its host plant and soil nutrient conditions.  At one end of the spectrum, the parasite could conceivably exhibit growth reductions in low fertility soils when utilizing Lolium as a host, while at the other end of the spectrum, it could display significant increases in biomass in highly fertile soils when utilizing Medicago as a host.  Thus, it is difficult to predict an average growth response for this parasite in response to the rising concentration of atmospheric CO2.

Nevertheless, this short-term study shed some light on the competitive interactions that exist between certain non-parasitic species.  Specifically, as the air's CO2 content rises, it is likely that the leguminous species Medicago will not effectively out-compete and replace Lolium in mixed species habitats, thereby reducing ecosystem biodiversity.  Quite to the contrary, the increasing CO2 content of the air should maintain the competitive balance that exists between these two species, thus preserving ecosystem biodiversity.

Finally, although atmospheric CO2 enrichment did stimulate parasite growth under most of the experimental conditions, it did not alter the effect of the parasite on its hosts, which continued to respond positively to increasing CO2 concentrations.