Effects of prescribed burning on fire behavior of surface fuel in Pinus yunnanensis forest land

  Objective  This paper aims to clarify the status of surface fuel and corresponding fire behavior characteristics of pure Pinus yunnanensis stand under the conditions of periodic (burning cycle was 1 year) prescribed burning, and then explore the impact of prescribed burning on the fire behavior of surface fuel of P. yunnanensis, and reasonably evaluate the ecological significance of prescribed burning, so as to provide scientific basis for forest fire management and fighting.

  Method  Sampling in field investigation and simulating combustion in laboratory, using the SPSS23.0 to process and analyze the data, we explored the influence of fuel types, fuel loads, moisture content and other characteristics of prescribed burning on fire behavior such as the temperature of fire, flame height, thermal radiation, spread rate, burning rate, fire intensity, etc. in Zhaobi Mountain, Xinping County, Yuxi City, Yunnan Province of southwestern China.

  Result  (1) The main fuel on the surface of pure P. yunnanensis stand was the withered pine needles, plus a small amount of withered pine branches and cones and dead ferns, the species was relatively single, and most of the fuel under the forest was fine fuel. (2) From 2018 to 2020, the average moisture content of dead fuel under the pure forest of P. yunnanensis was 9.39%, 8.04% and 9.89%, and the average fuel loads were (0.937 ± 0.303) kg/m², (0.926 ± 0.253) kg/m² and (0.669 ± 0.248) kg/m², respectively. (3) The simulated fire behavior in the laboratory included: the fire intensity was (245.95 ± 130.51) kW/m, the flame height was (0.92 ± 0.22) m, fire ground temperature was (437.5 ± 171.6) ℃, the thermal radiation was (6.3 ± 0.9) kW/m², the spread rate was (1.1 ± 0.3) m/min, the ignition time of the surface fuel was 1 s and burning rate was about 62%. (4) The mortality of P. yunnanensis in natural state was about 1.33%. In the field survey in 2018 and 2020, the mortality of P. yunnanensis under prescribed burning was 0.93% and 1.27%, respectively. P. yunnanensis has certain fire resistance.

  Conclusion  (1) For the pure P. yunnanensis stand under periodic prescribed burning, the canopy density of the standing forest is large, and the combustible vegetation under the forest is less, mainly pine needle litter and grass. (2) Prescribed burning can effectively reduce the surface fuel load, and the average load of surface fuel is less than 1 kg/m². During the fire prevention period, the moisture content of the dead fuel on the surface of P. yunnanensis forest is less than 10%, which is easy to be ignited, but the flame height is within the range of 0.5−1.5 m, and the fire intensity is less than 750 kW/m, which belongs to low intensity fire; the temperature of the fire ground is high, the thermal radiation is strong, the fire spread speed is general, and the fire field can be controlled; the simulated burning rate in the laboratory is high, and the combustion effect is good. (3) It is necessary and feasible to carry out periodic prescribed burning in fire prone habitats and forests with certain adaptability to fire.