Effects of elevated atmospheric CO2 and/or O3 on intra- and interspecific competitive ability of aspen

Three model communities of trembling aspen (mono-culture, and mixed with either paper birch or sugar maple) were grown for seven years in elevated atmospheric CO2 and O-3 using Free Air CO2 Enrichment (FACE) technology. We utilized trends in species' importance, calculated as an index of volume growth and survival, as indications of shifting community composition. For the pure aspen communities, different clones emerged as having the highest change in relative importance values depending on the pollutant exposure. In the control and elevated CO2 treatments, clone 42E was rapidly becoming the most successful clone while under elevated O-3, clone 8L emerged as the dominant clone. In fact, growth of clone 8L was greater in the elevated O-3 treatment compared to controls. For the mixed aspen-birch community, importance of aspen and birch changed by -16% and +62%, respectively, in the controls. in the treatments, however, importance of aspen and birch changed by -27% and +87%, respectively, in elevated O-3, and by -10% and +45%, respectively, in elevated CO2. Thus, the presence of elevated O-3 hastened conversion of stands to paper birch, whereas the presence of elevated CO2 delayed it. Relative importance of aspen and maple changed by -2% and +3%, respectively, after seven years in the control treatments. But in elevated O-3, relative importance of aspen and maple changed by -2% and +5%, respectively, and in elevated CO2 by +9 and -20%, respectively. Thus, elevated O-3 slightly increases the rate of conversion of aspen stands to sugar maple, but maple is placed at a competitive disadvantage to aspen under elevated CO2.