Fire ecology of C₃and C₄grasses depends on evolutionary history and frequency of burning but not photosynthetic type

Abstract

Grasses using the C4 photosynthetic pathway dominate frequently burned savannas, where the pathway is hypothesized to be adaptive. However, independent C4 lineages also sort among different fire environments. Adaptations to fire may thus depend on evolutionary history, which could be as important as the possession of the C4 photosynthetic pathway for life in these environments. Here, using a comparative pot experiment and controlled burn, we examined C3 and C4 grasses belonging to four lineages from the same regional flora, and asked the following questions: Do lineages differ in their responses to fire, are responses consistent between photosynthetic types, and are responses related to fire frequency in natural habitats? We found that in the C4 Andropogoneae lineage, frost killed a large proportion of aboveground biomass and produced a large dry fuel load, which meant that only a small fraction of the living tissue was lost in the fire. C3 species from the Paniceae and Danthonioideae lineages generated smaller fuel loads and lost more living biomass, while species from the C4 lineage Aristida generated the smallest fuel loads and lost the most living tissue. Regrowth after the fire was more rapid and complete in the C4 Andropogoneae and C3 Paniceae, but incomplete and slower in the C3 Danthonioideae and C4 Aristida. Rapid recovery was associated with high photosynthetic rates, high specific leaf area, delayed flowering, and frequent fires in natural habitats. Results demonstrated that phylogenetic lineage was more important than photosynthetic type in determining the fire response of these grasses and that fire responses were related to the frequency that natural habitats burned.

Determinants of flammability in savanna grass species

“…Furthermore, some evidence provides intriguing support for a link between high flammability and ecological success in fire‐prone grassland species (Ripley et al . ). The influence of flammability at the species level on grassland community‐level flammability has not been determined.…”

“…), in contrast to knowledge about interspecific variation in post‐fire response among grass species (Ripley et al . ). A historical belief persists that grasses and other herbaceous plants vary little in their flammability, which has led to the diversity of herbaceous fuels being reduced to one or few fuel classes in fire behaviour modelling (e.g.…”

“…In addition, recent evidence shows that grass traits relating to post‐fire recovery are shaped by fire regime (Forrestel, Donoghue & Smith ; Ripley et al . ), suggesting that traits relating to flammability may be responding in similar ways, resulting in intra‐ and interspecific variation in flammability.…”

Competing consumers: contrasting the patterns and impacts of fire and mammalian herbivory in Africa

“…The ecological effects of a shift from fire-controlled to herbivore-controlled landscapes are far-reaching. These two consumers select for contrasting traits in the woody [94] and herbaceous [70,80] flora, resulting in floristically contrasting savannahs across the African continent. Moreover, the different spatio-temporal patterns and selectivity of fire and grazing (table 1) means that they have differential impacts on woody demographics: intense fires are the most effective way to top-kill large woody individuals [130], although elephants can also achieve significant top-kill rates [131] and the combination of fires and elephants is crippling for large trees [132].…”

“…Here, the interaction becomes competitive: an extensive fire would deplete reserve forage at a critical point in the seasonal cycle [77]. The long-term effect of grazing via grass community composition is negative for fire: flammable grasses tend to grow tall, resprout vigorously from stored reserves and allocate resources to accumulating large above-ground biomass [70].…”