Abstract:
Rising atmospheric CO2 (Ca) concentrations have generated concern among scientists, mainly because of CO2’s role as a greenhouse gas and its influence on plant growth and development. Previous research has suggested that future CO2 enriched atmospheres may enhance the success of invasive aliens. Chromolaena odorata is an example of an invasive alien proving to be a serious threat to indigenous vegetation in South Africa, and effective control measures are desperately needed to curb infestations in the future. The current study aimed at assessing the response of C. odorata to elevated Ca and interactive factors, and was divided into two trials. During PART A, C. odorata was grown in competition with 2 grass species: Eragrostis curvula and Themeda triandra (selected for their differential preferences to nutrient availability). All three species were potted in a greenhouse at the University of KwaZulu-Natal (Howard College). There were 16 pots in total, and each pot contained four C. odorata plants, four T. triandra seedlings, and four E. curvula seedlings. Eight pots were exposed to elevated Ca (~700ppm), and eight pots were exposed to ambient Ca (~370ppm). The pots at each Ca treatment were further divided: four received high nutrient treatments (3L per addition), while the other four received low nutrient treatments (300 ml per addition). Studies on growth (e.g. plant height, dry weight, etc.), as well as physiology (e.g. Jmax), were undertaken. Results showed that generally, plants responded positively to high nutrient treatments. In contrast, elevated Ca did not affect growth or any of photosynthetic parameters of C. odorata significantly, but did reduce stomatal limitations. During PART B, C. odorata plants were grown monospecifically to assess whether there was a “chamber effect” associated with planting density. Pots at both Ca treatments contained either four C. odorata or two C. odorata seedlings. Growth and physiology were assessed. The fact that elevated Ca did not affect any of the photosynthetic parameters studied, suggests that photosynthetic down-regulation did not occur. This, together with the fact that no increase in stomatal limitations were observed in elevated Ca, implies that enhancement of photosynthetic assimilation could have occurred in C. odorata plants exposed to CO2 enrichment. Results from this study (PART A and PART B), when compared to previous research on this species, suggests that CO2 enrichment may enhance the success of monoculture populations of C. odorata. However, other species may gain competitive advantages over C. odorata occurring in mixed communites, under CO2 enriched environments. In addition, results of this study support the prediction that increasing Ca will reduce the importance of carbon as an external limiting resource, and that the extent of a plant’s response to Ca enrichment will depend on resources other than CO2. If increases in temperature caused by elevated Ca increases nutrient availability in the soil, then Ca could indirectly enhance the success of C. odorata occurring in mixed communities.
