<scp> CO <sub>2</sub> </scp> ‐stimulation of savanna tree seedling growth depends on interactions with local drivers
Sarah L. Raubenheimer, Brad S. Ripley
Abstract
Abstract Woody encroachment in southern African savanna has been partly attributed to rising atmospheric [CO 2 ] fertilising the growth of C 3 trees but less so that of competing C 4 grasses. However, growth conditions (resource availability, competition, rooting space and herbivory) must be suitable for the effects of elevated CO 2 ( e CO 2 ) to be realised. This research investigated the interactions between the positive effect of e CO 2 on tree seedling growth and limitations imposed by drought, herbivory and competition with C 4 grasses. Seedlings of the prolific encroaching C 3 tree Vachellia karroo were grown at ambient (400 ppm) or e CO 2 (800 ppm) in Open‐Top Chambers and exposed to a variety of stresses typical of savanna systems. Photosynthetic, growth and allocation responses to e CO 2 and other treatments were determined. Unsurprisingly, we show strong growth and water‐saving responses of V. karroo seedlings to e CO 2 when in the absence of competition and herbivory. However, the addition of either grass competition or simulated herbivory in the first season of growth significantly moderated this stimulation, while neither drought nor shading diminished the e CO 2 effect relative to similarly treated plants grown at ambient [CO 2 ]. Synthesis . We demonstrate that e CO 2 ‐induced C 3 stimulation in encroaching savanna species such as V. karroo will be inconsistent across time and space. This research does not detract from the suggestion that increasing atmospheric CO 2 is implicated in woody encroachment, but rather that e CO 2 benefits to C 3 tree seedlings are only realised when growth conditions are suitable. Inconsistencies in e CO 2 response will translate into spatial and temporal variation in seedling responses to e CO 2 and CO 2 ‐driven woody encroachment, explaining some of the variability observed in woody encroachment across geographical regions and resource and herbivore gradients.