Trait-based and non-additive controls of leaf litter flammability in subtropical plantations and managed forests

Leaf litter flammability strongly regulates surface fire behavior in forests and is influenced by both species-specific leaf traits and litter composition. However, trait-based mechanisms underlying flammability of single-species and mixed-species litter remain insufficiently understood. This study examined how interspecific variation in leaf functional traits and physicochemical properties controls single-species litter flammability, and whether mixed-species litter exhibits non-additive effects across mixing ratios. We conducted controlled litter-bed combustion experiments using five subtropical tree species, including the flammable conifer Pinus massoniana and four broadleaved species, and assessed whether mixed-species litter exhibits non-additive effects across a realistic pine-to-broadleaf mass-ratio gradient (3:1, 1:1, 1:3). This ratio-dependent design directly informs stand conversion and green-firebreak planning, moving beyond simple 1:1 screening. Our results indicate that the magnitude and direction of non-additive effects were strongly dependent on mixing ratio. As the proportion of broadleaved litter increased (from 3:1 to 1:3), antagonistic non-additive effects on ignitability-related metrics generally intensified, whereas synergistic non-additive effects on flame-related metrics weakened. This ratio-dependent pattern indicates a progressive dominance of broadleaved litter traits in regulating mixture flammability. These findings support the strategic incorporation of low-flammability broadleaved species, especially C. oleifera and L. formosana, into pine-dominated systems as a potential approach for reducing surface-fuel flammability.