How Does Increased Atmospheric Carbon Dioxide Concentration Affect Plant Competition?

The purpose of this research was to determine effects of elevated levels of atmospheric CO2 on competition between plants for resources. Rising CO2 levels and the resulting climate change will have effects on species competition, but the nature and scope of the effects are not fully understood. In a CO2 enriched atmosphere, plants employing C3 photosynthesis will likely experience increased growth, while plants that employ C4 photosynthesis will likely experience less, if any, increase in growth. Increased availability of CO2 should give a competitive advantage to C3 plants competing with C4 plants and cause a shift in the competitive advantage of species that arrive at a site earlier or that begin growing before competing species (priority effects).The growth of three native plants that coexist in southcentral Texas grasslands were compared: Thelesperma filifolium (C3 forb), Bouteloua curtipendula (C4 grass), and Nassella leucotricha (C3 grass). To test competition, pairs of these species were examined in a replacement series, with a total density of four plants/pot. To test priority effects, differing arrival times were used by establishing one species 45 days (d) before (early), at the same time (coincidental), or 45 days after (late) the competing species. To test the effects of CO2 on competition and priority effects, the experimental setup was replicated in open top growth chambers at ambient and elevated [CO2]. Growth was examined using a nonparametric MANOVA with species, CO2, planting priority, and planting combination as independent variables, and mass/plant, height/plant, number of flower heads/plant, and number of stems/plant as dependent variables. An interaction index was used to more easily compare the intensity of competition. For both T. filifolium and N. leucotricha growth was reduced when grown in mixture, indicating interspecific competition. Bouteloua curtipendula mass was not affected by interspecific competition, but plants in mixture were significantly shorter. Both T. filifolium and B. curtipendula growth in mixture was comparable to monoculture when given a 45 d advantage, indicating positive priority effects. T. filifolium produced significantly more flower heads/plant in the early priority and at elevated [CO2]. Elevated [CO2] did not significantly alter the response of the C3 plants to interspecific competition, priority effects, nor relative interaction intensity and caused only minor changes to plant growth. Contrary to expectations, B. curtipendula growth increased when grown with T. filifolium at elevated [CO2]. Priority effects appear to be a mechanism by which T. filifolium can avoid the negative effects of competition. Nassella leucotricha was affected by interspecific competition and the intensity of competition changed with priority, but its growth was not significantly affected. Bouteloua curtipendula experienced some negative effects from competition when it had a growth disadvantage, but overall experienced no significant reductions in growth from competition. These results support past findings that limited resources or photosynthetic rate limitations mask the fertilizing effect of increased [CO2]. With either of these limiting factors in place, the effect of increased [CO2] on interspecific competition or priority effects becomes radically reduced.