Non-additive effects of leaf and twig mixtures from different tree species on experimental litter-bed flammability

Aims: Tree species can affect litter flammability through leaf size and shape. Larger, simpler-shaped leaf litters form better-ventilated, more flammable litter-beds. However, leaves are generally mixed with twigs in the forest litter layer and together they likely contribute most to surface fire behavior. Here we ask: “Do leaf-twig mixtures have non-additive effects on litter-bed flammability?” Methods: Using laboratory fires, we tested the direction and magnitude of non-additivity of inter- and intra-specific leaf-twig mixtures on litter-bed flammability for four tree species contrasted in leaf size and shape and widespread in fire-prone temperate-boreal forests. Results: Across species, small needles reduced mixture fuel-bed ignitibility through filling the space between twigs and inhibiting ventilation. Within the small broad-leaved species, the thin, frequently branched and open spaced twigs were too loosely packed to be flammable, while in mixtures the small broad leaves connected these twigs to produce flammable fuel-beds. Once ignited, across species flame spread rate in mixtures was driven by leaves, while fire sustainability was predicted by fuel mass. Fuel-bed flammability was driven more by leaves at larger leaf-to-twig ratio. Conclusions: For the first time, we demonstrated the existence and mechanisms of non-additive effects of leaf-twig mixtures on experimental litter-bed flammability.