Growth and photosynthetic responses of encroaching tree seedlings to CO2 and stress interactions

1. Woody encroachment in southern African savanna has been partly attributed to rising atmospheric CO2 fertilising the growth of C3 trees but less so that of competing C4 grasses. However, growth conditions (resource availability, competition, rooting space, and herbivory) must be suitable for the effects of elevated CO2 (eCO2) to be realised.
2. This research investigated the interactions between the positive effect of eCO2 on tree seedling growth and limitations imposed by drought, disturbance, and competition with C4 grasses. Seedlings of the prolific encroacher C3 tree Vachellia karroo were grown at ambient (400 ppm) or eCO2 (800 ppm) in Open-Top Chambers and exposed to a variety of stresses and disturbances typical of savanna systems. Photosynthetic, growth and allocation responses to eCO2 and other treatments were determined.
3. Unsurprisingly, we show strong growth and water-saving responses of V. karroo seedlings to eCO2 when in the absence of competition and herbivory. However,...