Objective Through field surveys before and after prescribed burning, this study investigated the dynamics of understory fern (Pteridium spp.) fuel in Pinus yunnanensis forests. Combined with laboratory analyses of physicochemical properties, combustion bed experiments, and cone calorimetry tests, it aims to explore the relationship between fern^’s potential fire behavior and its physicochemical properties, as well as its combustion characteristics across different growth stages.

Method Fixed sample plots were established in Pinus yunnanensis forests in Xinping County, Yunnan Province of southwestern China. Field surveys were conducted one day before, five days and three months after prescribed burning to quantify the field characteristics of fern, including population, height and coverage. Laboratory analyses were conducted to determine physicochemical properties, such as ash content, crude fat content, and calorific value for different parts of fern (stem, leaf, and stem-leaf mixture). Combustion bed experiments were used to simulate vertical fire spread, and cone calorimetry was employed to analyze the mass loss rate and mass loss percentage of fern.

Result (1) Field surveys showed that prescribed burning initially reduced the population of fern by 25%, yet increased its relative proportion within the herb layer by 21.3%. Three months after burning, the population surged by 89.36%, making it the dominant herbaceous species; its coverage rebounded by 154.3%, while its height continued to decrease by 10.5%. (2) Physicochemical properties varied among different parts of fern. Ash content ranged from 6.41% to 6.87% (leaf > stem-leaf mixture > stem), crude fat content varied between 0.5% and 2.5% (stem > stem-leaf mixture > leaf), and calorific values were 18 370 ~ 19 191 J/g (stem > stem-leaf mixture > leaf). (3) Combustion bed tests demonstrated that the combustion of surface fuels in Pinus yunnanensis forests produced a flame height of 28.4 cm and a peak temperature of 149.8 ℃. When fern (Pteridium aquilinum) was involved in combustion, the flame height increased by 163.4% to 74.8 cm, and the peak temperature increased by about 299.1% to 597.8 ℃, showing significant enhancement in both flame height and temperature. (4) Cone calorimetry tests revealed that the combustion characteristics of fern were influenced by its moisture state and plant parts. In terms of state, fresh samples exhibited higher peak mass loss rates and greater total mass loss percentages than withered samples. Regarding plant parts, the peak mass loss rate ranked in the order of stem > stem-leaf mixture > leaf, while the total mass loss percentage followed leaf > stem-leaf mixture > stem. The mass loss percentages of fern were overall about 27% to 63% higher than those obtained from combustion bed tests.

Conclusion This study reveals clear links between combustion characteristics of understory fern fuel in Pinus yunnanensis forests across different growth stages and plant parts, and its fire behavior, physicochemical properties, and combustion parameters measured by cone calorimetry. These findings provide new scientific basis for understanding the influence of understory fern on forest fires in this ecosystem.