Forest understory clover populations in enriched CO₂ and O₃ atmospheres: Interspecific, intraspecific, and indirect effects

The effects of CO₂ and tropospheric O₃ on forest trees are increasingly the subject of experimental evaluation. Little is known, however, about the effects of these gases on understory plant taxa. At the Aspen free-air CO₂ and O₃ enrichment (Aspen FACE) site we assessed colonization and establishment of two common forest understory species, red (Trifolium pratense) and white (Trifolium repens) clover. To better understand these natural patterns in red clover, the more responsive of the two clover species, we also assessed intraspecific variation in growth performance to altered atmospheric conditions. Natural red clover populations were larger in enriched CO₂ atmospheres, whereas white clover populations showed no response to CO₂. Neither species showed beneficial or detrimental responses to enriched O₃ atmospheres. Nine red clover genotypes exhibited similar, but counterintuitive, decreases in shoot and root biomass, and increases in foliar nitrogen concentrations and root nodule numbers, under elevated CO₂. We suggest that in enriched CO₂ habitats, increased shading from rapidly growing trees offsets the positive benefits of CO₂ for understory plant performance. Enriched CO₂ and O₃ atmospheres may have large direct and indirect effects on colonization, establishment, and performance of common understory plants. Such changes may in turn alter forest community and ecosystem dynamics.