Table S1. % of the time where measured [CO2] was within the standard deviation (SD), within 50 ppm, and within 100 ppm of the target [CO2].

1. 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 (eCO 2 ) to be realised.2. This research investigated the interactions between the positive effect of eCO 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 eCO 2 (800 ppm) in Open-Top Chambers and exposed to a variety of stresses typical of savanna systems.Photosynthetic, growth and allocation responses to eCO 2 and other treatments were determined.3. Unsurprisingly, we show strong growth and water-saving responses of V. karroo seedlings to eCO 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 eCO 2 effect relative to similarly treated plants grown at ambient [CO 2 ]. 4. Synthesis. We demonstrate that eCO 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 eCO 2 benefits to C 3 tree seedlings are only realised when growth conditions are suitable. Inconsistencies in eCO 2 response will translate into spatial and temporal variation in seedling responses to eCO 2 and CO 2 -driven woody encroachment, explaining some of the variability observed in woody encroachment across geographical regions and resource and herbivore gradients. K E Y W O R D S C 3 -C 4 competition, drought, elevated CO 2 , herbivory, photosynthesis, savanna, woody encroachment | 1091

Elevated carbon dioxide (eCO2) and rising temperatures will have far‐reaching effects on global plant‐insect interactions, yet their implications for future biological control programs are not fully understood. Studies have shown that elevated CO2 will affect insect feeding guilds differently and these responses can be predicted with some confidence. Water hyacinth, Pontederia crassipes Mart. (Pontederiaceae), is a native and representative species of the Del Plata wetlands (Argentina) that invades outside of its native environment. It is considered one of the world's worst aquatic weeds and a target for biological control. In this study, water hyacinth plants were grown under two CO2 concentrations – current (400 p.p.m.) or elevated (800 p.p.m.) –, with and without two biocontrol agents representing different feeding guilds, the leaf‐chewing Cornops aquaticum Brüner (Orthoptera: Acrididae) and the phloem‐feeding Megamelus scutellaris Berg (Hemiptera: Delphacidae). Under eCO2 concentration, photosynthetic rate, total dry weight, and relative growth rate of P. crassipes acclimated to eCO2 conditions and plants showed very little CO2 fertilization response in eutrophic water. Insect herbivory varied depending on feeding guilds at eCO2; however, P. crassipes growth responses increased when exposed to insect herbivory. Chewing herbivory by C. aquaticum was consistent across CO2 conditions, whereas the feeding by M. scutellaris increased substantially at eCO2. These results indicate that successful biological control of P. crassipes under conditions of elevated CO2 might rely on phloem‐feeding insects, with chewers playing a lesser role.

Factors that determine C4 grass distributions have been well documented, with evidence in the literature for C4 photosynthetic subtypes displaying varying levels of drought susceptibility. However, the interactions between C4 photosynthetic subtype and phylogeny add complexity and are relatively under studied. We use species distribution modelling to determine the influence of rainfall on distribution patterns of representative C4 grass families and subtypes. Select C4 grass species, representing different photosynthetic subtypes (NADP‐Me and NAD‐Me) and lineages (Panicoideae and Aristidoideae), were subjected to a progressive 58‐day drought period and recovery phase, to explore drought responses through leaf water relations, gas exchange and chlorophyll fluorescence. We show Panicoideae NADP‐Me species to be more susceptible to drought than both Panicoideae NAD‐Me and Aristidoideae NADP‐Me species due to apparent greater metabolic impairment. The differences between groups were related to how rapidly photosynthesis declines with exposure to drought and the rate of recovery postdrought, rather than the maximum extent of photosynthetic decline. The mechanisms for the relative maintenance of plant water status differed between the Panicoideae NAD‐Me species, which utilized greater stomatal control, and the Aristidoideae NADP‐Me species, which maintained water uptake through osmotic adjustment. Synthesis. We show here that drought susceptibility differs both phylogenetically and according to photosynthetic subtype, but that the role of phylogeny may outweigh physiological control. This research adds novel insight into the physiological differences behind observed rainfall‐related differences in C4 grass distribution patterns. Read the free Plain Language Summary for this article on the Journal blog.

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