Shoot flammability differences between forest and savanna trees are driven by leaf dry matter content

Abstract

Climate change has led to increased fire risk in many tropical regions, with concerning implications for fire-sensitive forests. Understanding the comparative fire ecology of co-occurring fire-prone and fire sensitive habitats and the drivers of canopy fires is paramount for recognizing fire risk and for selecting plants that may be planted in green firebreaks that could mitigate wildfire damage to these habitats. However, baseline plant flammability data is needed from both habitats. To accomplish this, we compared three flammability measures (maximum temperature, total burn time and burnt biomass) by burning branches of 35 species in a fire-protected forest and fire-prone savanna in tropical northeast Australia. We also measured a set of leaf functional traits (leaf area, leaf mass per unit area and leaf dry matter content) on these species and examined their relationship with shoot flammability. We found that maximum temperature, burnt biomass, and burn time were significantly higher in savanna compared to forest species. Leaf area and leaf mass per unit area did not influence flammability measures, but species with higher leaf dry matter content had a higher percentage of biomass burnt, burnt for longer and hotter. Underpinning these observations, savanna species had significantly higher leaf dry matter content than forest species. Our results enable us to recommend species with low flammability that could be used in the green firebreaks in the study area to mitigate fire risk in sensitive forest habitats. Future studies should look experimentally at the effectiveness of green firebreaks using low flammability species, and examine their post fire recovery.