Editor’s Choice: CO2-stimulation of savanna tree seedling growth depends on interactions with local drivers

The editor’s choice for our May issue is “CO2-stimulation of savanna tree seedling growth depends on interactions with local drivers” by Raubenheimer & Ripley. Here, Associate Editor Jim Dalling explains the importance of this research. 

Encroachment of grass savannas by woody savanna trees has been widespread over the last century across Africa, Australia, and South America. Some of these changes are likely the result of alterations to land management and climate regimes, but encroachment also been directly attributed to rising atmospheric CO2 concentrations that favour plants with C3 photosynthesis (i.e., trees) over those with C4 photosynthesis (i.e., many savanna grass species). Predicting, and more importantly mitigating, future woody encroachment requires an understanding of how both bottom-up effects of resource availability, and top-down effects of fire and herbivory impact tree seedling recruitment. Raubenheimer and Ripley apply this integrative framework in a series of open-top chamber elevated CO2 experiments to explore how simulated drought and herbivory impact the growth and survival of Vachellia karroo, one of the most prominent woody encroachers in southern African savanna.

Fig. 1 Synthesis of the effects of elevated CO2, resource availability and competition from C4 grasses on the performance of the woody species Vachellia karroo.

Their experiments demonstrated that in the absence competition and herbivory V. karroo seedlings showed a stimulated photosynthetic and growth response to elevated CO2, as expected. Furthermore, this stimulation was maintained under water limitation, reflecting reduced stomatal conductance, greater water use efficiency and increased soil water content relative to plants grown under ambient CO2 conditions. However, follow-up experiments in the same chambers showed that seedling response to CO2 was strongly curtailed either by the application of simulated herbivory, or by competition from tussocks of the C4 grass Themeda triandra, introduced into the same pots (Fig. 2).

Fig. 2. Left: Effects of increasing biomass of the C4 grass Themeda triandra on the growth (final biomass) of seedlings of the woody species V. karroo under ambient (400 ppm) and elevated (800 ppm) CO2. Right: Effects of simulated herbivory (none; moderate – removal of leaflets; intense – clipping of the entire shoot) on V. karoo biomass relative growth rate.

These multi-factorial experiments using open-top chambers represent an important advance in woody encroachment research. They highlight how variation in disturbance regime (potentially including both fire and herbivory) as well as competition for space and soil resources from grasses can all modulate the performance and establishment success of woody plants in savanna systems. Variability in whether growth enhancement is maintained or not can help explain the observed heterogeneity in encroachment rates observed from local to continental scales and also suggests where management efforts might be most effective in minimising the success of C3 trees.

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