Effects of elevated CO2 on photosynthetic traits of native and invasive C3 and C4 grasses

Abstract

Background

Rising CO₂ is expected to result in changes in plant traits that will increase plant productivity for some functional groups. Differential plant responses to elevated CO₂ are likely to drive changes in competitive outcomes, with consequences for community structure and plant diversity. Many of the traits that are enhanced under elevated CO₂ also confer competitive success to invasive species, and it is widely believed that invasive species will be more successful in high CO₂. However, this is likely to depend on plant functional group, and evidence suggests that C₃ plants tend to respond more strongly to CO₂.

Results

We tested the hypothesis that invasive species would be more productive than noninvasive species under elevated CO₂ and that stronger responses would be seen in C₃ than C₄ plants. We examined responses of 15 grass species (eight C₃, seven C₄), classified as noninvasive or invasive, to three levels of CO₂ (390, 700 and 1000 ppm) in a closed chamber experiment. Elevated CO₂ decreased conductance and %N and increased shoot biomass and C/N ratio across all species. Differences between invasive and noninvasive species depended on photosynthetic mechanism, with more differences for traits of C₃ than C₄ plants. Differences in trait means between invasive and noninvasive species tended to be similar across CO₂ levels for many of the measured responses. However, noninvasive C₃ grasses were more responsive than invasive C₃ grasses in increasing tiller number and root biomass with elevated CO₂, whereas noninvasive C₄ grasses were more responsive than invasive C₄ grasses in increasing shoot and root biomass with elevated CO₂. For C₃ grasses, these differences could be disadvantageous for noninvasive species under light competition, whereas for C₄ grasses, noninvasive species may become better competitors with invasive species under increasing CO₂.

Conclusions

The ecophysiological mechanisms underlying invasion success of C₃ and C₄ grasses may differ. However, given that the direction of trait differences between invasive and noninvasive grasses remained consistent under ambient and elevated CO₂, our results provide evidence that increases in CO₂ are unlikely to change dramatically the competitive hierarchy of grasses in these functional groups.