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Grasses fuel most fires on Earth and strongly influence local fire behaviour through traits that determine how flammable they are. Therefore, grass communities that differ in their species and trait compositions give rise to significant spatial variation in savanna fire regimes across the world, which cannot be otherwise explained. Likewise, fire regimes are continuously modified by alterations to savanna grass community traits, through species introductions and climatic changes. However, current representation of grassy fuels in global fire models misses important variation and therefore limits predictive power. The inclusion of grass trait diversity in models, using remotely sensed trait proxies, for example, will greatly improve our ability to understand and project savanna fires and their roles in the Earth system.

1. Seed mass and growth rate are important dimensions of plant ecological diversity, but their relationship remains unresolved. Negative relationships between RGR and seed mass are well-established. However, RGR is size dependent, so small-seeded species might achieve fast growth simply because they are initially small.\ud \ud \ud \ud 2. Using a dataset of unprecedented size, sampling 382 grass species, we investigated seed mass and growth rate using both RGR and SGR (RGR at a specific size), accounting for diversity in phylogeny, ecology (e.g. life history, photosynthetic pathway) and environment (mean annual temperature and precipitation).\ud \ud \ud \ud 3. RGR and SGR showed contrasting relationships with seed mass, such that large-seeded species had lower RGR but higher SGR than small-seeded species. However, the relationship between SGR and seed mass depended on leaf dry matter content (LDMC), and was only positive in high-LDMC species. When compared at a common size, the fast growth of large-seeded and low-LDMC species was associated with greater biomass allocation to roots in the hot, high-light environment used for our experiment. Photosynthetic pathway and life history contributed to variation in SGR, with C4 annuals having higher SGRs than C3 perennials regardless of seed size.\ud \ud \ud \ud 4. Large seeds therefore afford an intrinsic growth advantage in species with resource-conserving leaf traits, and may provide a competitive edge in resource-poor environments. This work advances the understanding of how seed mass and growth rate co-evolve with other ecological factors.

Plant populations persist under recurrent fire via resprouting from surviving tissues (resprouters) or seedling recruitment (seeders). Woody species are inherently slow maturing, meaning that seeders are confined to infrequent fire regimes. However, for grasses, which mature faster, the relationships between persistence strategy and fire regime remain unknown.\ud \ud \ud Globally, we analysed associations between fire regimes experienced by hundreds of grass species and their persistence strategy, within a phylogenetic context. We also tested whether persistence strategies are associated with morphological and physiological traits.\ud \ud \ud Resprouters were associated with less frequent fire than seeders. Whilst modal fire frequencies were similar (fire return interval of 4–6 yr), seeders were restricted to regions with more frequent fire than resprouters, suggesting that greater competition with long‐lived resprouters restricts seeder recruitment and survival when fire is rare. Resprouting was associated with lower leaf N, higher C:N ratios and the presence of belowground buds, but was unrelated to photosynthetic pathway.\ud \ud \ud Differences between the life histories of grasses and woody species led to a contrasting prevalence of seeders and resprouters in relation to fire frequency. Rapid sexual maturation in grasses means that seeder distributions, relative to fire regime, are determined by competitive ability and recruitment, rather than time to reproductive maturity.

The uptake and deposition of silicon (Si) as silica phytoliths is common among land plants and is associated with a variety of functions. Among these, herbivore defense has received significant attention, particularly with regards to grasses and grasslands. Grasses are well known for their high silica content, a trait which has important implications ranging from defense to global Si cycling. Here, we test the classic hypothesis that C4 grasses evolved stronger mechanical defenses than C3 grasses through increased phytolith deposition, in response to extensive ungulate herbivory (‘C4‐grazer hypothesis’). Despite mixed support, this hypothesis has received broad attention, even outside the realm of plant biology. Because C3 and C4 grasses typically dominate in different climates, with the latter more abundant in hot, dry regions, we also investigated the effects of water availability and temperature on Si deposition. We compiled a large dataset of grasses grown under controlled environmental conditions. Using phylogenetically informed generalized linear mixed models and character evolution models, we evaluated whether photosynthetic pathway or growth condition influenced Si concentration. We found that C4 grasses did not show consistently elevated Si concentrations compared with C3 grasses. High temperature treatments were associated with increased concentration, especially in taxa adapted to warm regions. Although the effect was less pronounced, reduced water treatment also promoted silica deposition, with slightly stronger response in dry habitat species. The evidence presented here rejects the ‘C4‐grazer hypothesis.’ Instead, we propose that the tendency for C4 grasses to outcompete C3 species under hot, dry conditions explains previous observations supporting this hypothesis. These finding also suggest a mechanism via which anthropogenic climate change may influence silica deposition in grasses and, by extension, alter the important ecological and geochemical processes it affects.

1. Photosynthetic pathway is an important cause of growth rate variation between species, such that the enhanced carbon uptake of C4 species leads to faster growth than their C3 counterparts. Leaf traits that promote rapid resource acquisition may further enhance the growth capacity of C4 species. However, how root economic traits interact with leaf traits, and the different growth strategies adopted by plants with C3 and C4 photosynthetic pathways is unclear. Plant economic traits could interact with, or act independently of, photosynthetic pathway in influencing growth rate, or C3 and C4 species could segregate out along a common growth rate‐trait relationship.\ud \ud 2. We measured leaf and root traits on 100+ grass species grown from seeds in a controlled, common environment to compare with relative growth rates (RGR) during the initial phase of rapid growth, controlling for phylogeny and allometric effects.\ud \ud 3. Photosynthetic pathway acts independently to leaf and root functional traits in causing fast growth. Using C4 photosynthesis, plants can achieve faster growth than their C3 counterparts (by an average 0.04 g g‐1 day‐1) for a given suite of functional trait values, with lower investments of leaf and root nitrogen. Leaf and root traits had an additive effect on RGR, with plants achieving fast growth by possessing resource‐acquisitive leaf traits (high specific leaf area and low leaf dry matter content) or root traits (high specific root length and area, and low root diameter), but having both leads to an even faster growth rate (by up to 0.06 g g‐1 day‐1). C4 photosynthesis can provide a greater relative increase in RGR for plants with a ‘slow’ ecological strategy than in those with fast growth. However, aboveground and belowground strategies are not coordinated, so that species can have any combination of ‘slow’ or ‘fast’ leaf and root traits.\ud \ud 4. Synthesis: C4 photosynthesis increases growth rate for a given combination of economic traits, and significantly alters plant nitrogen economy in the leaves and roots. However, leaf and root economic traits act independently to further enhance growth. The fast growth of C4 grasses promotes a competitive advantage under hot, sunny conditions.

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.

Tropical savannas cover over 20% of land surface. They sustain a high diversity of mammalian herbivores and promote frequent fires, both of which are dependent on the underlying grass composition. These habitats are typically dominated by relatively few taxa, and the evolutionary origins of the dominant grass species are largely unknown. Here, we trace the origins of the genus Themeda, which contains a number of widespread grass species dominating tropical savannas. Complete chloroplast genomes were assembled for seven samples and supplemented with chloroplast and nuclear ITS markers for 71 samples representing 18 of the 27 Themeda species. Phylogenetic analysis supports a South Asian origin for both the genus and the widespread dominant T. triandra. This species emerged ~1.5 Ma from a group that had lived in the savannas of Asia for several million years. It migrated to Australia ~1.3 Ma and to mainland Africa ~0.5 Ma, where it rapidly spread in pre-existing savannas and displaced other species. Themeda quadrivalvis, the second most widespread Themeda species, is nested within T. triandra based on whole chloroplast genomes, and may represent a recent evolution of an annual growth form that is otherwise almost indistinguishable from T. triandra. The recent spread and modern-day dominance of T. triandra highlight the dynamism of tropical grassy biomes over millennial time-scales that has not been appreciated, with dramatic shifts in species dominance in recent evolutionary times. The ensuing species replacements likely had profound effects on fire and herbivore regimes across tropical savannas.

Functional Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society. Plant phenotypes reflect trade-offs between competing resource-intensive physiological processes. A shift in resource allocation, away from anti-herbivore defences and towards growth and reproduction, is predicted through plant domestication, such that crops are faster growing and higher yielding than their wild ancestors. These changes are hypothesized to have come at the cost of defence investment, leaving crops “disarmed by domestication”. Silicon is the principal anti-herbivore defence in grasses, including many of our most important staple cereal crops, but the impact of domestication on silicon-based defences is unknown. We measured the effects of both domestication and modern agronomic selection on growth rate and a suite of anti-herbivore defences, specifically leaf toughness, silicon and phenolic concentrations. Our comparison of wild, landrace and modern cultivated cereals spanned multiple cereal species, including wheat, barley and maize, sampling eight independent domestication events and five examples of modern agronomic selection. Leaf silicon concentration showed a small, but significant, 10% reduction through domestication, but there was no effect of modern agronomic selection, and phenolic concentration was not affected by either factor. Silicon concentration correlated positively with leaf tensile strength, but negatively with foliar phenolic concentrations, suggesting a trade-off between chemical and physical defences. Size-standardized growth rate was independent of domestication status, and did not trade-off with silicon or phenolic defences. However, modelling showed that relative growth rate slowed more with increasing size in plants with higher silicon levels, so that they reached a smaller final size, implying a cost of silicon-based defence. We found the opposite pattern for phenolic-based defence, with increasing phenolic concentrations associated with a greater plant size at maturity, and faster maximum relative growth rates. Silicon-based defences have been reduced in cereals through domestication, consistent with our predicted costs of these defences to growth. However, modern agronomic selection has not influenced silicon defences in cereal crops and the small decrease in silicon concentration associated with domestication is unlikely to have a major effect on the ability of cereals to withstand a range of abiotic and biotic stresses. These findings have broad implications for crop protection and our understanding of plant trade-offs. \ud \ud A plain language summary is available for this article. (http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12935/suppinfo).

1. Tropical grasses fuel the majority of fires on Earth. In fire-prone landscapes, enhanced flammabil-ity may be adaptive for grasses via the maintenance of an open canopy and an increase in spa-tiotemporal opportunities for recruitment and regeneration. In addit ion, by burning intensely butbriefly, high flammability may protect resprouting buds from lethal temperatures. Despite thesepotential benefits of high flammability to fire-prone grasses, variation in flammability among grassspecies, and how trait differences underpin this variation, remains unknown.2. By burning leaves and plant parts, we experimentally determined how five plant traits (biomassquantity, biomass density, biomass moisture content, leaf surface-area-to-volume ratio and leaf effec-tive 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-pr one SouthAfrican grasslands at a time of peak fire occurrence. The influence of evolutionary history onflammability was assessed based on a phylogeny built here for the study species.3. Grass speci es differed significantly in all components of flammability. Accounting for evolution-ary history helped to explain patterns in leaf-scale combustibility and sustainability. The five mea-sured plant traits predicted components of flammability, particularly leaf ignitability and plantcombustibility in which 70% and 58% of variation, respectively, could be explained by a combina-tion of the traits. Total above-ground biomass was a key drive r o f combustibility and sustainabi litywith high biomass species burning more intensely and for longer, and producing the highest pre-dicted fire spread rates. Moisture content was the main influence on ignitability, where speci es withhigher moisture conten ts took longer to ignite and once alight burnt at a slower rate. Bioma ss den-sity, leaf surface-area-to-volume ratio and leaf effective heat of combustion were weaker predictorsof flammability components.4. Synthesis. We demonstrate that grass flammability is predicted from easily measurable plant func-tional traits and is influenced by evolutionary history with some components showing phylogeneticsignal. Grasses are not homogenous fuels to fire. Rather, species differ in functional traits that inturn demonstrably influence flammability. This diver sity is consistent with the idea that flammabilitymay be an adaptive trait for grasses of fire-prone ecosystems.