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Organic N (oN, e.g., amino acids) is an important N-resource for plants in soils replete with oN but not inorganic N (iN; i.e., NH4+ and NO3−), such as cold ecosystems with temperature-limited soil decomposition rates. However, sub-Antarctic literature assumes that plants only acquire iN, potentially underestimating plant-available N. We hypothesised that Marion Island (− 46.90°, 37.75°) grasses (Polypogon magellanicus, Poa cookii, Agrostis stolonifera and Poa annua) acquire oN and that oN relative to iN provision affects plant growth. We investigated oN and iN uptake and growth responses in two hydroponics experiments. In situ N (15N-glycine, 15NO3− and 15NH4+) acquisition was investigated at three field sites with decreasing faunal influence, thus iN input and microbial activity. When plants grown in mire water were supplied with 15N-glycine or 15NO3−, root δ15N enrichment was highest for glycine-supplied plants. In the second hydroponics experiment, plant N-uptake rates (nmol g biomass−1 s−1) were significantly higher for glycine than NO3−, but relative growth rates (g g−1 d−1) lower on glycine. There were species-specific biomass allocation responses to N concentration (4 mM and 0.4 mM) and N-form (glycine and NO3−). Glycine-supplied grasses at the low iN concentration field sites had significantly higher δ15N enrichment relative to those at sites with high iN, suggesting higher oN uptake when iN is limiting. We demonstrate the importance of accounting for oN acquisition in the sub-Antarctic. As a system with high soil oN relative to iN, plants may predominantly meet N-demands through oN rather than iN acquisition. [ABSTRACT FROM AUTHOR]

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]

Extreme changes in temperature, rainfall and wind regimes have been correlated with plant species range expansion upslope on sub‐Antarctic islands. Ongoing climatic changes are expected to continue driving changes in species distributions globally, but niche specialisations may limit the capacity for range shifts.We hypothesised that non‐climatic characteristics of ecological niches of vascular plant species could limit climate induced range shifts. We determined the altitudinal ranges of vascular plant species (n = 13) on sub‐Antarctic Marion Island and measured air temperature, topographic, foliar and soil properties along transects on geologically distinct substrates. Climatic and non‐climatic associations were determined using multiple linear regression and boosted regression tree (BRT) analyses. The degree of niche specialisation was determined using outlying mean index analysis within the range of species on the island.Several species (7 of 13) exhibited niche‐specialisation. Correlation analysis revealed that edaphic properties including soil depth, loss on ignition, the principal component of most soil nutrients (Mg, Cl, K, Ca, Cu, Zn, P, S), Si, Mn and clay dominated the BRT prediction of overall plant cover. Although air temperature was correlated with plant cover in linear models, model simplification dropped temperature in both BRT and linear models. As a consequence, multiple determinants, including temperature, climate, topography and soils control the distribution of vascular plant species on this sub‐Antarctic island.Edaphic and topographic factors is likely to limit range‐shifts of specialised vascular plant species and could limit their survival when climate change drives them beyond the extent of their particular ecological niches. Resilience of such species is likely poorly predicted by distribution models that depend heavily on climate rather than less labile non‐climatic factors. 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]

Questions: Invasive species establish either by possessing traits, or trait trade‐offs similar to native species, suggesting pre‐adaptation to local conditions; or by having a different suite of traits and trait trade‐offs, which allow them to occupy unfilled niches. The trait differences between invasives and non‐invasives can inform on which traits confer invasibility. Here, we ask: (a) are invasive species functionally different or similar to native species? (b) which traits of invasives differ from traits of non‐invasive aliens and thus confer invasibility? and (c) do results from the sub‐Antarctic region, where this study was conducted, differ from findings from other regions? Location: Sub‐Antarctic Marion Island. Methods: We measured 13 traits of all terrestrial native, invasive and non‐invasive alien plant species. Using principal components analysis and phylogenetic generalized least‐squares models, we tested for differences in traits between invasive (widespread alien species) and native species. Bivariate trait relationships between invasive and native species were compared using standardized major axis regressions to test for differences in trait trade‐offs between the two groups. Second, using the same methods, we compared the traits of invasive species to non‐invasive aliens (alien species that have not spread). Results: Between invasive and native species, most traits differed, suggesting that the success of invasive species is mediated by being functionally different to native species. Additionally, most bivariate trait relationships differed either in terms of their y‐intercept or their position on the axes, highlighting that plants are positioned differently along a spectrum of shared trait trade‐offs. Compared to non‐invasive aliens, invasive species had lower plant height, smaller leaf area, lower frost tolerance, and higher specific leaf area, suggesting that these traits are associated with invasiveness. The findings for the sub‐Antarctic corresponded to those of other regions, except lower plant height which provides a competitive advantage to invaders in the windy sub‐Antarctic context. Conclusion: Our findings support the expectation that trait complexes of invasive species are predominantly different to those of coexisting native species, and that high resource acquisition and low defence investment are characteristic of invasive plant species. [ABSTRACT FROM AUTHOR]

Coping with temporal variation in fire requires plants to have plasticity in traits that promote persistence, but how plastic responses to current conditions are affected by past fire exposure remains unknown. We investigate phenotypic divergence between populations of four resprouting grasses exposed to differing experimental fire regimes (annually burnt or unburnt for greater than 35 years) and test whether divergence persists after plants are grown in a common environment for 1 year. Traits relating to flowering and biomass allocation were measured before plants were experimentally burnt, and their regrowth was tracked. Genetic differentiation between populations was investigated for a subset of individuals. Historic fire frequency influenced traits relating to flowering and below-ground investment. Previously burnt plants produced more inflorescences and invested proportionally more biomass below ground, suggesting a greater capacity for recruitment and resprouting than unburnt individuals. Tiller-scale regrowth rate did not differ between treatments, but prior fire exposure enhanced total regrown biomass in two species. We found no consistent genetic differences between populations suggesting trait differences arose from developmental plasticity. Grass development is influenced by prior fire exposure, independent of current environmental conditions. This priming response to fire, resulting in adaptive trait changes, may produce communities more resistant to future fire regime changes. [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]

Developmental plasticity enables organisms to cope with environmental heterogeneity, such as seasonal variation in climatic conditions, and is thought to affect a species' capability to adapt to environments with novel seasonal and ecological dynamics. We studied developmental plasticity of the widespread tropical butterfly, Bicyclus safitza, which reaches the southern edge of its distribution in the temperate zone of South Africa. In wet--dry seasonal environments in tropical Africa, adults of Bicyclus butterflies are present all year round and exhibit discrete seasonal forms in alternating generations. We demonstrate that a population that colonized a more temperate climate region has adopted a different strategy to cope with the local environment. No active adults were encountered during the temperate winter. The flight season coincided with a period when evaporation stress was lowest and temperatures were higher in the South African population. Butterflies collected from the field did not express seasonal polyphenism or show full expression of the tropical wet season form phenotype. Reaction norm experiments comparing stocks from South Africa and Uganda indicated that local adaptation of this tropical butterfly to a more temperate climate involved changes in the degree of developmental plasticity, such that a more robust development in response to thermal variation was observed for a broad suite of morphological and life-history traits. Our findings have implications for understanding the mechanisms that facilitate expansion into a novel ecological niche in seasonally variable climatic conditions. [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]

1. Tropical grasses fuel the majority of fires on Earth. In fire-prone landscapes, enhanced flammability may be adaptive for grasses via the maintenance of an open canopy and an increase in spatiotemporal opportunities for recruitment and regeneration. In addition, by burning intensely but briefly, high flammability may protect resprouting buds from lethal temperatures. Despite these potential benefits of high flammability to fire-prone grasses, variation in flammability among grass species, and how trait differences underpin this variation, remains unknown. 2. By burning leaves and plant parts, we experimentally determined how five plant traits (biomass quantity, biomass density, biomass moisture content, leaf surface-area-to-volume ratio and leaf effective heat of combustion) combined to determine the three components of flammability (ignitability, sustainability and combustibility) at the leaf and plant scales in 25 grass species of fire-prone South African grasslands at a time of peak fire occurrence. The influence of evolutionary history on flammability was assessed based on a phylogeny built here for the study species. 3. Grass species differed significantly in all components of flammability. Accounting for evolutionary history helped to explain patterns in leaf-scale combustibility and sustainability. The five measured plant traits predicted components of flammability, particularly leaf ignitability and plant combustibility in which 70% and 58% of variation, respectively, could be explained by a combination of the traits. Total above-ground biomass was a key driver of combustibility and sustainability with high biomass species burning more intensely and for longer, and producing the highest predicted fire spread rates. Moisture content was the main influence on ignitability, where species with higher moisture contents took longer to ignite and once alight burnt at a slower rate. Biomass density, leaf surface-area-to-volume ratio and leaf effective heat of combustion were weaker predictors of flammability components. 4. Synthesis. We demonstrate that grass flammability is predicted from easily measurable plant functional traits and is influenced by evolutionary history with some components showing phylogenetic signal. Grasses are not homogenous fuels to fire. Rather, species differ in functional traits that in turn demonstrably influence flammability. This diversity is consistent with the idea that flammability may be an adaptive trait for grasses of fire-prone ecosystems. [ABSTRACT FROM AUTHOR]

Adaptation to changing environments often requires novel traits, but how such traits directly affect the ecological niche remains poorly understood. Multiple plant lineages have evolved C4 photosynthesis, a combination of anatomical and biochemical novelties predicted to increase productivity in warm and arid conditions. Here, we infer the dispersal history across geographical and environmental space in the only known species with both C4 and non-C4 genotypes, the grass Alloteropsis semialata. While non-C4 individuals remained confined to a limited geographic area and restricted ecological conditions, C4 individuals dispersed across three continents and into an expanded range of environments, encompassing the ancestral one. This first intraspecific investigation of C4 evolutionary ecology shows that, in otherwise similar plants, C4 photosynthesis does not shift the ecological niche, but broadens it, allowing dispersal into diverse conditions and over long distances. Over macroevolutionary timescales, this immediate effect can be blurred by subsequent specialisation towards more extreme niches. [ABSTRACT FROM AUTHOR]

Global climate change is expected to shift regional rainfall patterns, influencing species distributions where they depend on water availability. Comparative studies have demonstrated that C4 grasses inhabit drier habitats than C3 relatives, but that both C3 and C4 photosynthesis are susceptible to drought. However, C4 plants may show advantages in hydraulic performance in dry environments. We investigated the effects of seasonal variation in water availability on leaf physiology, using a common garden experiment in the Eastern Cape of South Africa to compare 12 locally occurring grass species from C4 and C3 sister lineages. Photosynthesis was always higher in the C4 than C3 grasses across every month, but the difference was not statistically significant during the wettest months. Surprisingly, stomatal conductance was typically lower in the C3 than C4 grasses, with the peak monthly average for C3 species being similar to that of C4 leaves. In water-limited, rain-fed plots, the photosynthesis of C4 leaves was between 2.0 and 7.4 μmol m−2 s−1 higher, stomatal conductance almost double, and transpiration 60% higher than for C3 plants. Although C4 average instantaneous water-use efficiencies were higher (2.4-8.1 mmol mol−1) than C3 averages (0.7-6.8 mmol mol−1), differences were not as great as we expected and were statistically significant only as drought became established. Photosynthesis declined earlier during drought among C3 than C4 species, coincident with decreases in stomatal conductance and transpiration. Eventual decreases in photosynthesis among C4 plants were linked with declining midday leaf water potentials. However, during the same phase of drought, C3 species showed significant decreases in hydrodynamic gradients that suggested hydraulic failure. Thus, our results indicate that stomatal and hydraulic behaviour during drought enhances the differences in photosynthesis between C4 and C3 species. We suggest that these drought responses are important for understanding the advantages of C4 photosynthesis under field conditions. [ABSTRACT FROM AUTHOR]

In several taxa, increasing leaf succulence has been associated with decreasing mesophyll conductance (gM) and an increasing dependence on Crassulacean acid metabolism (CAM). However, in succulent Aizoaceae, the photosynthetic tissues are adjacent to the leaf surfaces with an internal achlorophyllous hydrenchyma. It was hypothesized that this arrangement increases gM, obviating a strong dependence on CAM, while the hydrenchyma stores water and nutrients, both of which would only be sporadically available in highly episodic environments. These predictions were tested with species from the Aizoaceae with a 5-fold variation in leaf succulence. It was shown that gM values, derived from the response of photosynthesis to intercellular CO2 concentration (A:Ci), were independent of succulence, and that foliar photosynthate δ13C values were typical of C3, but not CAM photosynthesis. Under water stress, the degree of leaf succulence was positively correlated with an increasing ability to buffer photosynthetic capacity over several hours and to maintain light reaction integrity over several days. This was associated with decreased rates of water loss, rather than tolerance of lower leaf water contents. Additionally, the hydrenchyma contained ~26% of the leaf nitrogen content, possibly providing a nutrient reservoir. Thus the intermittent use of C3 photosynthesis interspersed with periods of no positive carbon assimilation is an alternative strategy to CAM for succulent taxa (Crassulaceae and Aizoaceae) which occur sympatrically in the Cape Floristic Region of South Africa. [ABSTRACT FROM AUTHOR]

During the past 25 Myr, partial pressures of atmospheric CO2 ( Ca) imposed a greater limitation on C3 than C4 photosynthesis. This could have important downstream consequences for plant nitrogen economy and biomass allocation. Here, we report the first phylogenetically controlled comparison of the integrated effects of subambient Ca on photosynthesis, growth and nitrogen allocation patterns, comparing the C3 and C4 subspecies of Alloteropsis semialata. Plant size decreased more in the C3 than C4 subspecies at low Ca, but nitrogen pool sizes were unchanged, and nitrogen concentrations increased across all plant partitions. The C3, but not C4 subspecies, preferentially allocated biomass to leaves and increased specific leaf area at low Ca. In the C3 subspecies, increased leaf nitrogen was linked to photosynthetic acclimation at the interglacial Ca, mediated via higher photosynthetic capacity combined with greater stomatal conductance. Glacial Ca further increased the biochemical acclimation and nitrogen concentrations in the C3 subspecies, but these were insufficient to maintain photosynthetic rates. In contrast, the C4 subspecies maintained photosynthetic rates, nitrogen- and water-use efficiencies and plant biomass at interglacial and glacial Ca with minimal physiological adjustment. At low Ca, the C4 carbon-concentrating mechanism therefore offered a significant advantage over the C3 type for carbon acquisition at the whole-plant scale, apparently mediated via nitrogen economy and water loss. A limiting nutrient supply damped the biomass responses to Ca and increased the C4 advantage across all Ca treatments. Findings highlight the importance of considering leaf responses in the context of the whole plant, and show that carbon limitation may be offset at the expense of greater plant demand for soil resources such as nitrogen and water. Results show that the combined effects of low CO2 and resource limitation benefit C4 plants over C3 plants in glacial-interglacial environments, but that this advantage is lessened under anthropogenic conditions. [ABSTRACT FROM AUTHOR]

Large proportions of the Earth's land surface are covered by biomes dominated by C4 grasses. These C4-dominated biomes originated during the late Miocene, 3-8 million years ago (Ma), but there is evidence that C4 grasses evolved some 20 Ma earlier during the early Miocene/Oligocene. Explanations for this lag between evolution and expansion invoke changes in atmospheric CO2, seasonality of climate and fire. However, there is still no consensus about which of these factors triggered C4 grassland expansion., We use a vegetation model, the adaptive dynamic global vegetation model (aDGVM), to test how CO2, temperature, precipitation, fire and the tolerance of vegetation to fire influence C4 grassland expansion. Simulations are forced with late Miocene climates generated with the Hadley Centre coupled ocean-atmosphere-vegetation general circulation model., We show that physiological differences between the C3 and C4 photosynthetic pathways cannot explain C4 grass invasion into forests, but that fire is a crucial driver. Fire-promoting plant traits serve to expand the climate space in which C4-dominated biomes can persist., We propose that three mechanisms were involved in C4 expansion: the physiological advantage of C4 grasses under low atmospheric CO2 allowed them to invade C3 grasslands; fire allowed grasses to invade forests; and the evolution of fire-resistant savanna trees expanded the climate space that savannas can invade. [ABSTRACT FROM AUTHOR]

Burial is one of the major factors influencing plant ecology in deserts and coastal areas. Consequently, many studies have measured the responses of dune plants to sand burial. However, there remains little agreement about the mechanisms and characteristics constituting the burial response of plants. In particular, stimulation of growth has been reported as the most common plant burial response; however, stimulation has not been reported consistently among studies. Here, a literature survey showed that the depth of burial relative to the height of the plant determined whether the growth of a species was stimulated by burial. Growth stimulation was limited to shallow burial depths, while burial depths greater than the height of the plant consistently resulted in reduced growth. As studies used widely differing burial depths or units of growth measurement, the variation in reported stimulation of plant growth can be partly attributed to differences in experimental procedure. The stimulation of growth in many species was accompanied by an increase in photosynthesis over a limited period and by a shift in biomass allocation from root to shoot. Most plants demonstrated stimulated growth (up to 200%) in response to shallow burial indicating that some burial response mechanisms are general to many species. However, a few specialist dune species displayed a much greater ability to respond to burial (up to 700% stimulation of plant mass). Although allocation shifts and increased photosynthesis have been shown to be associated with dune plant burial response, there remains a need for field measurements that focus on the diversity of mechanisms underlying plant response to burial. [ABSTRACT FROM AUTHOR]

•Experimental evidence demonstrates a higher efficiency of water and nitrogen use in C4 compared with C3 plants, which is hypothesized to drive differences in biomass allocation between C3 and C4 species. However, recent work shows that contrasts between C3 and C4 grasses may be misinterpreted without phylogenetic control. •Here, we compared leaf physiology and growth in multiple lineages of C3 and C4 grasses sampled from a monophyletic clade, and asked the following question: which ecophysiological traits differ consistently between photosynthetic types, and which vary among lineages? •C4 species had lower stomatal conductance and water potential deficits, and higher water-use efficiency than C3 species. Photosynthesis and nitrogen-use efficiency were also greater in C4 species, varying markedly between clades. Contrary to previous studies, leaf nitrogen concentration was similar in C4 and C3 types. Canopy mass and area were greater, and root mass smaller, in the tribe Paniceae than in most other lineages. The size of this phylogenetic effect on biomass partitioning was greater in the C4 NADP-me species than in species of other types. •Our results show that the phylogenetic diversity underlying C4 photosynthesis is critical to understanding its functional consequences. Phylogenetic bias is therefore a crucial factor to be considered when comparing the ecophysiology of C3 and C4 species. [ABSTRACT FROM AUTHOR]

The stimulation of dune plant growth in response to burial is a vital attribute allowing survival in areas of mobile sand. Numerous resource-related and physiological mechanisms of growth stimulation have been suggested in the past, but few have been tested comparatively. Manipulation experiments using Scaevola plumieri, an important subtropical coastal dune forming species, demonstrated that physiological shifts were of great importance in determining the nature of the stimulation response to burial. The production of stem length and replacement of leaf area were stimulated by burial, whereas net mass production was similar between buried and unburied treatments. Remobilization of buried leaf resources, seasonal effects, and a shift in biomass allocation to stem production played the greatest role in the compensatory growth response. Other factors, such as increased soil nutrients, changes in photosynthesis, and changes in the costs of producing tissue were of less importance. Thus, the stimulated growth of species adapted to live on mobile dunes is explained by a number of resource-related and physiological mechanisms acting in concert. [ABSTRACT FROM AUTHOR]

The regional abundance of C4 grasses is strongly controlled by temperature, however, the role of precipitation is less clear. Progress in elucidating the direct effects of photosynthetic pathway on these climate relationships is hindered by the significant genetic divergence between major C3 and C4 grass lineages. We addressed this problem by examining seasonal climate responses of photosynthesis in Alloteropsis semialata, a unique grass species with both C3 and C4 subspecies. Experimental manipulation of rainfall in a common garden in South Africa tested the hypotheses that: (1) photosynthesis is greater in the C4 than C3 subspecies under high summer temperatures, but this pattern is reversed at low winter temperatures; and (2) the photosynthetic advantage of C4 plants is enhanced during drought events. Measurements of leaf gas exchange over 2 years showed a significant photosynthetic advantage for the C4 subspecies under irrigated conditions from spring through autumn. However, the C4 leaves were killed by winter frost, while photosynthesis continued in the C3 plants. Unexpectedly, the C4 subspecies also lost its photosynthetic advantage during natural drought events, despite greater water-use efficiency under irrigated conditions. This study highlights previously unrecognized roles for climatic extremes in determining the ecological success of C3 and C4 grasses. [ABSTRACT FROM AUTHOR]

The species richness of C4 grasses is strongly correlated with temperature, with C4 species dominating subtropical ecosystems and C3 types predominating in cooler climates. Here, the effects of low temperatures on C4 and C3 grasses are compared, controlling for phylogenetic effects by using Alloteropsis semialata, a unique species with C4 and C3 subspecies. Controlled environment and common garden experiments tested the hypotheses that: (i) photosynthesis and growth are greater in the C4 than the C3 subspecies at high temperatures, but this advantage is reversed below 20 °C; and (ii) chilling-induced photoinhibition and light-mediated freezing injury of leaves occur at higher temperature thresholds in the C4 than the C3 plants. Measurements of leaf growth and photosynthesis showed the expected advantages of the C4 pathway over the C3 type at high temperatures. These declined with temperature, but were not completely lost until 15 °C, and there was no evidence of a reversal to give a C3 advantage. Chronic chilling (5–15 °C) or acute freezing events induced a comparable degree of photodamage in illuminated leaves of both subspecies. Similarly, freezing caused high rates of mortality in the unhardened leaves of both subtypes. However, a 2-week chilling treatment prior to these freezing events halved injury in the C3 but not the C4 subspecies, suggesting that C4 leaves lacked the capacity for cold acclimation. These results therefore suggest that C3 members of this subtropical species may gain an advantage over their C4 counterparts at low temperatures via protection from freezing injury rather than higher photosynthetic rates. [ABSTRACT FROM PUBLISHER]

C4 plants dominate the world's subtropical grasslands, but investigations of their ecology typically focus on climatic variation, ignoring correlated changes in soil nutrient concentration. The hypothesis that higher photosynthetic nitrogen use efficiency (PNUE) in C4 than in C3 species allows greater flexibility in the partitioning of growth, especially under nutrient-deficient conditions, is tested here. Our experiment applied three levels of N supply to the subtropical grass Alloteropsis semialata, a unique model system with C3 and C4 subspecies. Photosynthesis was significantly higher for the same investment of leaf N in the C4 than C3 subspecies, and was unaffected by N treatments. The C4 plants produced more biomass than the C3 plants at high N levels, diverting a greater fraction of growth into inflorescences and corms, but less into roots and leaves. However, N-limitation of biomass production caused size-dependent shifts in the partitioning of growth. Root production was higher in small than large plants, and associated with decreasing leaf biomass in the C3, and inflorescence production in the C4 plants. Higher PNUE in the C4 than C3 subspecies was therefore linked with greater investment in sexual reproduction and storage, and the avoidance of N-limitations on leaf growth, suggesting advantages of the C4 pathway in disturbed and infertile ecosystems. [ABSTRACT FROM PUBLISHER]

The synchronous origin of agriculture in at least four independent climatic regions at the end of the last glacial period (c10 kyr bp) points to a global limitation for crop domestication. One hypothesis proposes that a rapid carbon dioxide (CO2) increase from 18 Pa to ∼27 Pa during deglaciation caused significant increases in the growth rates of wild crop progenitors, thereby removing a productivity barrier to their successful domestication. However, early C4 crops present a challenge to this hypothesis, because they were among the first domesticates, but have a carbon-concentrating mechanism that offsets the limitation of photosynthesis by CO2. We investigated the CO2-limitation hypothesis using the wild progenitors of five C4 founder crops from four independent centres of domestication. Plants were grown in controlled environment chambers at glacial (18 Pa), postglacial (28 Pa) and current ambient (38 Pa) CO2 levels, and photosynthesis, transpiration and biomass were measured. An increase in CO2 from glacial to postglacial levels caused a significant gain in vegetative biomass of up to 40%, but the equivalent rise in CO2 from postglacial to modern levels generally had no effect on biomass. Investigation into the underlying mechanisms showed C4 photosynthesis to be limited more by glacial than postglacial CO2 levels, matching theoretical expectations. Moreover, the increase in CO2 from glacial to postglacial levels caused a reduction in the transpiration rate via decreases in stomatal conductance of ∼35%. In combination, these physiological changes conferred a large improvement in water-use efficiency at the postglacial CO2 partial pressure compared with the glacial level. Our data, therefore, provide experimental support for the CO2-limitation hypothesis. [ABSTRACT FROM AUTHOR]

The C4 photosynthetic pathway uses water more efficiently than the C3 type, yet biogeographical analyses show a decline in C4 species relative to C3 species with decreasing rainfall. To investigate this paradox, the hypothesis that the C4 advantage over C3 photosynthesis is diminished by drought was tested, and the underlying stomatal and metabolic mechanisms of this response determined. The effects of drought and high evaporative demand on leaf gas exchange and photosynthetic electron sinks in C3 and C4 subspecies of the grass Alloteropsis semialata were examined. Plant responses to climatic variation and soil drought were investigated using a common garden experiment with well-watered and natural rainfall treatments, and underlying mechanisms analysed using controlled drying pot experiments. Photosynthetic rates were significantly higher in the C4 than the C3 subspecies in the garden experiment under well-watered conditions, but this advantage was completely lost during a rainless period when unwatered plants experienced severe drought. Controlled drying experiments showed that this loss was caused by a greater increase in metabolic, rather than stomatal, limitations in C4 than in the C3 leaves. Decreases in CO2 assimilation resulted in lower electron transport rates and decreased photochemical efficiency under drought conditions, rather than increased electron transport to alternative sinks. These findings suggest that the high metabolic sensitivity of photosynthesis to severe drought seen previously in several C4 grass species may be an inherent characteristic of the C4 pathway. The mechanism may explain the paradox of why C4 species decline in arid environments despite high water-use efficiency. [ABSTRACT FROM PUBLISHER]

Water status in relation to standing biomass and leaf area indices (LAI) of the subtropical foredune species Arctotheca populifolia, Ipomoea pes-caprae and Scaevola plumieri were studied in the Eastern Cape, South Africa. The plants showed little evidence of water stress, never developing leaf water potentials more negative than -1.55 MPa, a value which is typical of mesophytes rather than xerophytes. The plants showed no seasonal changes in osmotic potential, an indication that they did not need to osmoregulate, nor were there significant alterations in tissue elasticity. Turgor potential for the most part remained positive throughout the day or recovered positive values at night, a condition suitable for the maintenance of growth that may be essential to cope with sand accretion. All three species show relatively high transpiration rates and only I. pes-caprae showed any evidence of strong limitations of transpiration rate through reductions in midday stomatal conductance. All three species had relatively high instantaneous water use efficiencies as a result of high assimilation rates rather than low transpiration rates. Simple water budgets, accounting for losses by transpiration and inputs from rainfall, suggest that the water stored in the dune sands is sufficient to meet the requirements of the plants, although water budgets calculated for I. pes-caprae suggest that this species may on occasion be water limited. The results suggest that it is the low biomass and LAI that lead to these favourable water relations. [ABSTRACT FROM AUTHOR]

Scaevola plumieri is an important pioneer on many tropical and subtropical sand dunes, forming a large perennial subterranean plant with only the tips of the branches emerging above accreting sand. In South Africa it is the dominant pioneer on sandy beaches along the east coast, less abundant on the south coast and absent from the southwest and west coasts. Transpiration rates (E )of S. plumieri are predictably related to atmospheric vapour pressure deficit under a wide range of conditions and can therefore be predicted from measurement of ambient temperature and relative humidity. Scaling measurements of E at the leaf level to the canopy level has been demonstrated previously. Using a geographic information system, digital maps of regional climatic variables were used to calculate digital maps of potential transpiration from mean monthly temperature and relative humidity values, effectively scaling canopy level transpiration rates to a regional level. Monthly potential transpiration was subtracted from the monthly median rainfall to produce a map of mean monthly water balance. Seasonal growth was correlated with seasonal water balance. Localities along the coast with water deficits in summer corresponded with the recorded absence of S.plumieri, which grows and reproduces most actively in the summer months. This suggests that reduced water availability during the summer growth period limits the distribution of S. plumieri along the southwest coast, where water deficits develop in summer. Temperature is also important in limiting the distribution of S. plumieri on the southwest coast of South Africa through its effects on the growth and phenology of the plant. [ABSTRACT FROM AUTHOR]