MICROEVOLUTIONARY RESPONSES IN EXPERIMENTAL POPULATIONS OF PLANTS TO CO2-ENRICHED ENVIRONMENTS - PARALLEL RESULTS FROM 2 MODEL SYSTEMS

Despite the critical role that terrestrial vegetation plays in the Ear th's carbon cycle, very little is known about the potential evolutiona ry responses of plants to anthropogenically induced increases in conce ntrations of atmospheric CO2. We present experimental evidence that ri sing CO2 concentration may have a direct impact on the genetic composi tion and diversity of plant populations but is unlikely to result in s election favoring genotypes that exhibit increased productivity in a C O2-enriched atmosphere. Experimental populations of an annual plant (A butilon theophrasti, velvetleaf) and a temperate forest tree (Betula a lleghaniensis, yellow birch) displayed responses to increased CO2 that were both strongly density-dependent and genotype-specific. In compet itive stands, a higher concentration of CO2 resulted in pronounced shi fts in genetic composition, even though overall CO2-induced productivi ty enhancements were small, For the annual species, quantitative estim ates of response to selection under competition were 3 times higher at the elevated CO2 level. However, genotypes that displayed the highest growth responses to CO2 when grown in the absence of competition did not have the highest fitness in competitive stands. We suggest that in creased CO2 intensified interplant competition and that selection favo red genotypes with a greater ability to compete for resources other th an CO2. Thus, while increased CO2 may enhance rates of selection in po pulations of competing plants, it is unlikely to result in the evoluti on of increased CO2 responsiveness or to operate as an important feedb ack in the global carbon cycle, However, the increased intensity of se lection and drift driven by rising CO2 levels may have an impact on th e genetic diversity in plant populations.