Non-additive effects of leaf-litter flammability on eight subtropical tree species: Implications for forest species composition and fire susceptibility

The readiness of leaf-litter to burn in the presence of fire differs greatly between species. Thus, forests composed of different species vary in their susceptibility to fire. Fire susceptibility of forests may also differ from the arithmetic means of flammability of their component species, i.e., non-additive effects exist. Here, we assessed nine indices of flammability and five physicochemical properties of the leaf litter of eight common subtropical tree species in China. We then tested the net effects on litter flammability of different mixtures of the eight species. We measured the following variables: time to ignition, combustion time, spread rate, ignition temperature, mass loss, maximum flame height and three temperature indices, moisture, cellulose, lignin and ash contents, and specific leaf area (SLA). Our results show that the flammability of leaf litter: time to ignition, combustion time, ignition temperature, and flame height, varies widely between the eight species. Time to ignition was short (<3 s) for the three conifer species and Quercus variabilis and Q. aliena, but long (3.5-8.8 s) for Q. fabri, Q. glauca, and Liquidambar formosana. The five species with a short time to ignition all have high cellulose and lignin contents, and SLA, and are highly flammable. In contrast, the three species with long time to ignition have low cellulose and lignin contents, and SLA, and high ash content. Cellulose and lignin contents, and SLA are the major drivers of litter flammability, and ash and moisture contents are important negative drivers. Mixed litters containing species with high cellulose and lignin contents and SLA have positive non-additive effects (synergistic) on overall flammability whereas those containing species with low cellulose and lignin contents, and SLA have negative non-additive effects (antagonistic) on flammability. These results are essential for assessing forest fire-risks and assisting species selection in plantations or fire-break forests as a part of a forest fire-management strategy.