Summary• Experimental evidence demonstrates a higher efficiency of water and nitrogen use in C 4 compared with C 3 plants, which is hypothesized to drive differences in biomass allocation between C 3 and C 4 species. However, recent work shows that contrasts between C 3 and C 4 grasses may be misinterpreted without phylogenetic control.• Here, we compared leaf physiology and growth in multiple lineages of C 3 and C 4 grasses sampled from a monophyletic clade, and asked the following question: which ecophysiological traits differ consistently between photosynthetic types, and which vary among lineages?• C 4 species had lower stomatal conductance and water potential deficits, and higher water-use efficiency than C 3 species. Photosynthesis and nitrogen-use efficiency were also greater in C 4 species, varying markedly between clades. Contrary to previous studies, leaf nitrogen concentration was similar in C 4 and C 3 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 C 4 NADP-me species than in species of other types.• Our results show that the phylogenetic diversity underlying C 4 photosynthesis is critical to understanding its functional consequences. Phylogenetic bias is therefore a crucial factor to be considered when comparing the ecophysiology of C 3 and C 4 species.

Summary 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.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.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. 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.

Roughly 3% of the Earth's land surface burns annually, representing a critical exchange of energy and matter between the land and atmosphere via combustion. Fires range from slow smouldering peat fires, to low-intensity surface fires, to intense crown fires, depending on vegetation structure, fuel moisture, prevailing climate, and weather conditions. While the links between biogeochemistry, climate and fire are widely studied within Earth system science, these relationships are also mediated by fuels-namely plants and their litter-that are the product of evolutionary and ecological processes. Fire is a powerful selective force and, over their evolutionary history, plants have evolved traits that both tolerate and promote fire numerous times and across diverse clades. Here we outline a conceptual framework of how plant traits determine the flammability of ecosystems and interact with climate and weather to influence fire regimes. We explore how these evolutionary and ecological processes scale to impact biogeochemical and Earth system processes. Finally, we outline several research challenges that, when resolved, will improve our understanding of the role of plant evolution in mediating the fire feedbacks driving Earth system processes. Understanding current patterns of fire and vegetation, as well as patterns of fire over geological time, requires research that incorporates evolutionary biology, ecology, biogeography, and the biogeosciences.

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.

Summary• Large proportions of the Earth's land surface are covered by biomes dominated by C 4 grasses. These C 4 -dominated biomes originated during the late Miocene, 3-8 million years ago (Ma), but there is evidence that C 4 grasses evolved some 20 Ma earlier during the early Miocene ⁄ Oligocene. Explanations for this lag between evolution and expansion invoke changes in atmospheric CO 2 , seasonality of climate and fire. However, there is still no consensus about which of these factors triggered C 4 grassland expansion.• We use a vegetation model, the adaptive dynamic global vegetation model (aDGVM), to test how CO 2 , temperature, precipitation, fire and the tolerance of vegetation to fire influence C 4 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 C 3 and C 4 photosynthetic pathways cannot explain C 4 grass invasion into forests, but that fire is a crucial driver. Fire-promoting plant traits serve to expand the climate space in which C 4 -dominated biomes can persist.• We propose that three mechanisms were involved in C 4 expansion: the physiological advantage of C 4 grasses under low atmospheric CO 2 allowed them to invade C 3 grasslands; fire allowed grasses to invade forests; and the evolution of fire-resistant savanna trees expanded the climate space that savannas can invade.