Effects of elevated CO(2) on photosynthetic traits of native and invasive C(3) and C(4) grasses

BACKGROUND: Rising CO(2) is expected to result in changes in plant traits that will increase plant productivity for some functional groups. Differential plant responses to elevated CO(2) 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 CO(2) also confer competitive success to invasive species, and it is widely believed that invasive species will be more successful in high CO(2). However, this is likely to depend on plant functional group, and evidence suggests that C(3) plants tend to respond more strongly to CO(2). RESULTS: We tested the hypothesis that invasive species would be more productive than noninvasive species under elevated CO(2) and that stronger responses would be seen in C(3) than C(4) plants. We examined responses of 15 grass species (eight C(3), seven C(4)), classified as noninvasive or invasive, to three levels of CO(2) (390, 700 and 1000 ppm) in a closed chamber experiment. Elevated CO(2) 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 C(3) than C(4) plants. Differences in trait means between invasive and noninvasive species tended to be similar across CO(2) levels for many of the measured responses. However, noninvasive C(3) grasses were more responsive than invasive C(3) grasses in increasing tiller number and root biomass with elevated CO(2), whereas noninvasive C(4) grasses were more responsive than invasive C(4) grasses in increasing shoot and root biomass with elevated CO(2). For C(3) grasses, these differences could be disadvantageous for noninvasive species under light competition, whereas for C(4) grasses, noninvasive species may become better competitors with invasive species under increasing CO(2). CONCLUSIONS: The ecophysiological mechanisms underlying invasion success of C(3) and C(4) grasses may differ. However, given that the direction of trait differences between invasive and noninvasive grasses remained consistent under ambient and elevated CO(2), our results provide evidence that increases in CO(2) are unlikely to change dramatically the competitive hierarchy of grasses in these functional groups. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12898-016-0082-z) contains supplementary material, which is available to authorized users.