Determinants of flammability in savanna grass species

Abstract

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.

Does inherent flammability of grass and litter fuels contribute to continental patterns of landscape fire activity?

“…This was similar to other findings that an open litter bed structure with low bulk density burns more rapidly than tightly packed ones (Scarff & Westoby, ; Cornwell et al ., ). Grass flammability is affected by canopy architecture (Simpson et al ., ) and in our experiment, grass fuels were laid horizontally, with no major gaps, unlike the more variable, generally vertical arrangement of grasses in the field. [Other researchers have similarly placed shoots horizontally to measure shoot flammability (Jaureguiberry et al ., ; Wyse et al ., )].…”

Many shades of green: the dynamic tropical forest–savannah transition zones

“…However, grasses are not homogeneous fuels to fire. A recent study on the flammability of grasses from South African fire-prone environments showed different degrees of flammability driven by different functional traits and ultimately by evolutionary history [79]. Finally, regeneration will depend on the ability of the already established vegetation to persist after fire by resprouting, or by favouring new individuals to germinate (e.g.…”

The recent and rapid spread of Themeda triandra

“…Its height and rapid growth enable T. triandra to out compete neighbouring species for light, water and nutrients. Fire also plays an important role in its dominance, with T. triandra being highly flammable relative to co-occurring grass species (Simpson et al 2016) and its persistence reliant on burning (Danckwerts 1993). These characteristics are, however, at least partially shared across the genus (Morales 2014).…”

“…Establishing why savannas are dominated by relatively few species requires considering the origins of the grass species that dominate these habitats, in terms of space, time and ancestral conditions. Among C 4 grass species, variation in flammability is phylogenetically constrained, and the traits of the grassy ground layer have ecosystem-level impacts on fire frequency and intensity (Archibald et al 2013;Simpson et al 2016). A key question is therefore whether grass traits underlying savanna dynamics existed before the colonisation of these ecosystems, or whether they evolved in situ.…”