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.

火通过限制对热敏感的树苗的补充，在维持稀树草原树草平衡方面发挥着重要作用。然而，由于草层的可燃性改变，火灾行为可能会随着 CO _{2浓度的增加而改变。}在这里，我们确定了预测的未来 CO ₂浓度的影响，以及它如何与水的有效性、草的可燃性和对可燃性有影响的性状相互作用，并揭示了支持这些反应的生理机制。使用广泛存在的 C ₄稀树草原草（Theda triandra ）作为模型，我们发现在 CO ₂升高的情况下提高了用水效率（800 ppm）导致更大（更大的地上生物量）但更潮湿（更高水分含量）的草燃料负荷，在干旱条件下以较慢的速度治愈。这些变化与点火时间增加、燃烧时间减少和预测的传播速度降低有关。_{我们在国家层面（南非）模拟了草可燃性改变对火灾行为的影响，发现在 CO 2}升高的情况下火势蔓延大规模减少，减轻了预测的干旱增加的相反影响，以及火线强度的边际增加。二氧化碳₂- 火灾频率、蔓延或强度的降低可能对稀树草原植被动态产生严重影响，可能会加剧世界各地这些生态系统中已经出现的木质侵蚀。