The Optimization of Stand Structure Can Significantly Alleviate the Flammability of Forest Ecosystems

The accurate classification of forest fuels and the evaluation of the flammability of different forest types are crucial for effective forest fire control and classification management. We aimed to evaluate and classify the flammability of surface forest fuels in the subtropical area of China. The surface forest fuels were collected from 12 typical forest types. The flammability of surface forest fuels was assessed by evaluating their drying time, fuel moisture, ignition point, calorific value, combustion duration, and ash content. The principal component analysis (PCA), entropy weight method, k-means clustering algorithm, and Pearson correlation coefficient method were employed for the classification of forest fuels and the evaluation of forest flammability. The results revealed that the flammability of surface living fuels across diverse plant families was significantly different. Rutaceae and Cucurbitaceae plants exhibited relatively high flammability, while Arecaceae plants demonstrated characteristics of low flammability. The surface fuels could be categorized into high, moderate, and low flammability. The high flammability fuels mainly consisted of plant leaves and litter components. The forest humus belongs to the low flammability. The forest flammability was classified into three categories according to the ignition forest fire risk index (IRI) and the burning intensity & severity index (BSI). The highest flammability forest types were EPF: Pinus elliottii pure forest, BMF: broad-leaved mixed forest, CPF: Cunninghamia lanceolata (Lamb.) Hook pure forest, and CBF: coniferous broad-leaved mixed forest. The lowest flammability was in FPF: Liquidambar formosana Hance pure forest, an optimal forest type with a neatly structured environment, few understory weeds, and less dead fuel loading of only 4.32 tons per hectare. The flammability index method presented in this study contains the key elements of flammability, provides a standardized tool for fire managers to assess and mitigate fire risk, and it also applies to other regions.