Bark and leaf flammability in subtropical semi-humid forest in China, and plant flammable syndromes how to respond to fire frequency

Leaves and trunk bark are the main parts of woody species burned in canopy and surface fires, yet lack of knowledge of flammability of different plant parts and their coordination have impeded the understanding of plant flammable strategies to fire disturbance, and the ecological feedbacks between fire and vegetation.

We sampled 271 common woody species in subtropical semi-humid forest, then measured 10 flammability traits of leaves and bark using the cone calorimeter (including heat release, time to ignition, flame height, smoke production). Bark and leaf flammability showed large difference across woody species. Leaves were generally more ignitable, consumable and combustible, but the sustainability was lower than that of the bark. Moreover, we show that bark flammability and leaf flammability are decoupled because they were driven by size-related traits and economics-related traits, respectively.

According to the decoupling between leaves and bark, we defined plant flammability syndromes based on the different combinations of flammability strategies of leaves and bark, and assessed the impact of fire frequency on different flammability syndromes. Furthermore, the results also indicated that 40.0%, 39.1% and 20.9% woody plant species had hot-, fast-, and low-flammable leaves, respectively; and 28.2%, 35.7% and 36.1% species had hot-, fast- and low- flammable bark. Tree species (47.5%) had a higher percentage of flammability strategy separation between leaves and bark than large shrub (19.7%) and shrub species (18.2%). Community-level evidence showed that species with fast- or hot- flammable leaves and bark may gain a notable advantage with repeated fires. Structural equation models indicated that more frequently burned forests were associated with infertile soil, shrub enrichment and lower species richness, subsequently leading to a favor on flammable plant species.

Therefore, the difference and coordination between leaves and bark flammability, might help to well characterize the flammability strategies of plants. The positive feedback loop would generate between the dominance of flammable species in the plant communities and the fire frequency, fostering the characteristics of fire regimes in the semi-humid evergreen broadleaved forests.