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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. [ABSTRACT FROM AUTHOR]

CO2‐fertilisation is implicated in the widespread and significant woody encroachment of savannas due to CO2‐stimulated increases in below‐ground reserves that enhance sapling regrowth after fire. However, the effect of CO2 concentration ([CO2]) on tree responses to the other major disturbance in savannas, herbivory, is poorly understood. Herbivory responses cannot be predicted from fire responses, as herbivore effects occur earlier during establishment and are moderated by plant palatability and defence rather than below‐ground carbon accumulation.The relationship between herbivory and [CO2] is explored here using a widespread, strongly encroaching savanna tree, Vachellia karroo. Using greenhouse‐grown seedlings under past—through to predicted future—[CO2] (180–1000 ppm) and field‐grown seedlings under ambient [CO2], we assessed plant survival, growth, defence and palatability.Increasing [CO2] improves the tolerance of greenhouse‐grown seedlings to herbivory by stimulating growth and allowing a critical size threshold associated with survival to be reached earlier, thereby decreasing the probability of fatal herbivory during the vulnerable recruitment phase. Elevated [CO2] also decreases the time taken to reach a second size threshold linked to accelerated recovery of field‐grown seedlings following herbivory. Seedling growth responses to increasing [CO2] are nonlinear, suggesting that historic growth and survival enhancements are smaller than those predicted for the future. Increasing [CO2] is associated with greater resistance to herbivores (more branched shoot architecture) but not leaf palatability (C:N ratio) or defence (leaf tannins and spine density).Increasing V. karroo densities already constitute a major land management problem in southern African savannas. However, encroachment by this species, and likely other savanna tree species, may be greatly exacerbated under future [CO2], as tolerance to herbivory at the recruitment stage is further enhanced. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

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.This research investigated the interactions between the positive effect of eCO2 on tree seedling growth and limitations imposed by drought, herbivory and competition with C4 grasses. Seedlings of the prolific encroaching 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 typical of savanna systems. Photosynthetic, growth and allocation responses to eCO2 and other treatments were determined.Unsurprisingly, we show strong growth and water‐saving responses of V. karroo seedlings to eCO2 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 eCO2 effect relative to similarly treated plants grown at ambient [CO2].Synthesis. We demonstrate that eCO2‐induced C3 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 CO2 is implicated in woody encroachment, but rather that eCO2 benefits to C3 tree seedlings are only realised when growth conditions are suitable. Inconsistencies in eCO2 response will translate into spatial and temporal variation in seedling responses to eCO2 and CO2‐driven woody encroachment, explaining some of the variability observed in woody encroachment across geographical regions and resource and herbivore gradients. [ABSTRACT FROM AUTHOR]

Background and Aims Global warming has large effects on the performance and spatial distribution of plants, and increasingly facilitates the spread of invasive species. Particularly vulnerable is the vegetation of cold environments where indigenous plants selected for cold tolerance can have reduced phenotypic plasticity and associated lower capacity to respond to warming temperatures. In contrast, invasive species can be phenotypically plastic and respond positively to climate change, but at the expense of stress tolerance. Methods We investigate this trade-off in traits, measuring the photosynthetic response to warming, chilling tolerance and specific leaf area (SLA) of Pooid grasses. We compare this between invasive and non-invasive grasses and correlate this to their range expansions on a cold sub-Antarctic island that has warmed significantly in the past five decades. We determined whether these responses remained consistent after temperature acclimation. Key Results Invasive species responded strongly to warming, increasing photosynthetic rates by up to 2-fold, while non-invasive species did not respond. The response was associated with increased stomatal conductance, but not with modified photosynthetic metabolism. Electrolyte leakage and SLA were higher in invasive than in non-invasive species. Acclimation altered the photosynthetic response and invasive species responded to warm temperatures irrespective of acclimation, while non-invasive species responded only after acclimation to warm temperature. Conclusions Traits scaled linearly with rates of range expansion and demonstrate that under sub-Antarctic conditions, anthropogenic warming over the last 50 years may have favoured species with greater capacity to respond photosynthetically to warming to the detriment of species that cannot, and negated the advantage that chilling tolerance would have conferred on endemic species in the past. This suggests that species of cold ecosystems could be particularly vulnerable to warming as selection for stress tolerance has limited their responsiveness to environmental change, while introduced invasive species may have no such limitations. We show mechanistic evidence of the physiology that underpins an apparent trade-off between warming and chilling tolerance traits. [ABSTRACT FROM AUTHOR]

C4 photosynthesis is a complex trait that boosts productivity in warm environments. Paradoxically, it evolved independently in numerous plant lineages, despite requiring specialised leaf anatomy. The anatomical modifications underlying C4 evolution have previously been evaluated through interspecific comparisons, which capture numerous changes besides those needed for C4 functionality. Here, we quantify the anatomical changes accompanying the transition between non‐C4 and C4 phenotypes by sampling widely across the continuum of leaf anatomical traits in the grass Alloteropsis semialata. Within this species, the only trait that is shared among and specific to C4 individuals is an increase in vein density, driven specifically by minor vein development that yields multiple secondary effects facilitating C4 function. For species with the necessary anatomical preconditions, developmental proliferation of veins can therefore be sufficient to produce a functional C4 leaf anatomy, creating an evolutionary entry point to complex C4 syndromes that can become more specialised. [ABSTRACT FROM AUTHOR]

C4 photosynthesis is a complex trait resulting from a series of anatomical and biochemical modifications to the ancestral C3 pathway. It is thought to evolve in a stepwise manner, creating intermediates with different combinations of C4-like components. Determining the adaptive value of these components is key to understanding how C4 photosynthesis can gradually assemble through natural selection. Here, we decompose the photosynthetic phenotypes of numerous individuals of the grass Alloteropsis semialata, the only species known to include both C3 and C4 genotypes. Analyses of δ13C, physiology and leaf anatomy demonstrate for the first time the existence of physiological C3-C4 intermediate individuals in the species. Based on previous phylogenetic analyses, the C3-C4 individuals are not hybrids between the C3 and C4 genotypes analysed, but instead belong to a distinct genetic lineage, and might have given rise to C4 descendants. C3 A. semialata, present in colder climates, likely represents a reversal from a C3-C4 intermediate state, indicating that, unlike C4 photosynthesis, evolution of the C3-C4 phenotype is not irreversible. [ABSTRACT FROM AUTHOR]

The Miocene radiation of C4 grasses under high-temperature and low ambient CO2 levels occurred alongside the transformation of a largely forested landscape into savanna. This inevitably changed the host plant regime of herbivores, and the simultaneous diversification of many consumer lineages, including Bicyclus butterflies in Africa, suggests that the radiations of grasses and grazers may be evolutionary linked. We examined mechanisms for this plant-herbivore interaction with the grass-feeding Bicyclus safitza in South Africa. In a controlled environment, we tested oviposition preference and hatchling performance on local grasses with C3 or C4 photosynthetic pathways that grow either in open or shaded habitats. We predicted preference for C3 plants due to a hypothesized lower processing cost and higher palatability to herbivores. In contrast, we found that females preferred C4 shade grasses rather than either C4 grasses from open habitats or C3 grasses. The oviposition preference broadly followed hatchling performance, although hatchling survival was equally good on C4 or C3 shade grasses. This finding was explained by leaf toughness; shade grasses were softer than grasses from open habitats. Field monitoring revealed a preference of adults for shaded habitats, and stable isotope analysis of field-sampled individuals confirmed their preference for C4 grasses as host plants. Our findings suggest that plant-herbivore interactions can influence the direction of selection in a grass-feeding butterfly. Based on this work, we postulate future research to test whether these interactions more generally contribute to radiations in herbivorous insects via expansions into new, unexploited ecological niches. [ABSTRACT FROM AUTHOR]