What was done
The authors collected samples of red clover (Trifolium pratense L.) and white clover (T. repens L.) from the understoreys of trembling aspen (Populus tremuloides Michx.) and sugar maple (Acer saccharum Marsh.) communities that had been exposed since 1998 to ambient air, elevated CO2, elevated O3 or elevated CO2 + O3 at the Aspen Free-Air CO2 and O3 Enrichment (FACE) site located near Rhinelander, Wisconsin, USA, after which they analyzed the plant samples for concentrations of nitrogen, total cell wall constituents, lignin and soluble phenolics, along with in vitro dry-matter digestibility (IVDMD) and in vitro cell-wall digestibility (IVCWD).

What was learned
Muntifering et al. report that "exposure to elevated CO2 alone did not affect chemical composition or in vitro digestibility" in either of the clovers. However, they say that "exposure to elevated O3 resulted in increased concentrations of lignin and decreased IVDMD and IVCWD compared with exposure to ambient air," and that "the response was similar regardless of whether the plants had been co-exposed to elevated CO2."

What it means
Although the deleterious effects of O3 exposure on biomass production and yields of agricultural crops may often be totally ameliorated by co-exposure to elevated CO2 (see Ozone (Effects on Plants) in our Subject Index), the extra CO2 supplied to the clover plants in this study could not reverse the O3-induced reductions in the digestibility and nutritive quality of red and white clover. As beneficent a substance as CO2 is, therefore, it is clear that it cannot resolve all of the environmental problems facing mankind; other remedial measures must be employed as well.