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.