Could CO₂-induced changes to C₄ grass flammability aggravate savanna woody encroachment?

Abstract

Fire plays an important role in maintaining the savanna tree-grass balance by limiting the recruitment of heat-sensitive tree seedlings. However, fire behaviour may change under increasing CO₂ concentrations, due to altered flammability of the grassy layer. Here, we determined the effect of predicted future CO₂ concentrations, and how it interacts with water-availability, on grass flammability and traits influential to flammability, and uncovered the physiological mechanisms underpinning these responses. Using the widespread C₄ savanna grass, Themeda triandra, as a model, we found that improved water-use efficiency under elevated CO₂ (800 ppm) resulted in a larger (greater aboveground biomass), but wetter (higher moisture content) grass fuel load, that cured at a slower rate under drought conditions. These changes were associated with increased time to ignition, reduced flaming times and reduced predicted rate of spread. We modelled the effect of altered grass flammability on fire behaviour
at a national level (South Africa), finding large-scale reductions in fire spread under elevated CO₂, mitigating the converse effects of predicted increases in aridity, and marginal increases in fireline intensity. CO₂-induced reductions in fire frequency, spread or intensity could have serious implications for savanna vegetation dynamics, possibly exacerbating the woody encroachment already seen in these ecosystems across the world.