Forest understory clover populations in enriched CO 2 and O 3 atmospheres: Interspecific, intraspecific, and indirect effects

Abstract

The effects of CO 2 and tropospheric O 3 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 CO 2 and O 3 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 CO 2 atmospheres, whereas white clover populations showed no response to CO 2. Neither species showed beneficial or detrimental responses to enriched O 3 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 CO 2. We suggest that in enriched CO 2 habitats, increased shading from rapidly growing trees offsets the positive benefits of CO 2 for understory plant performance. Enriched CO 2 and O 3 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.