北京门头沟区主要林分类型地表火行为模拟研究

Simulation study on surface fire behavior of main forest types in Mentougou District, Beijing

【目的】地表火是最常见的林火类型,直接影响植被更新和生态系统的养分配置与循环。反映林火行为的常见指标有火蔓延速度、单位面积发热量、火强度和火焰高度。根据实际林分和立地条件进行火行为模拟,可以揭示林火发生条件,并有效判断树冠火发生的可能性,为林火的预防和扑救决策工作提供科学依据。【方法】选取北京市门头沟典型林分(刺槐林、油松林、侧柏林)为调查对象,每个林分设置5块样地,共计15块样地。通过野外调查获取可燃物载量(灌木可燃物、草本可燃物、1时滞可燃物、10时滞可燃物、100时滞可燃物)、林分因子(第一活枝下高、第一死枝下高、树高、胸径、郁闭度)和立地因子(海拔、坡度、坡向、坡位)数据,使用BehavePlus6软件,基于气象参数和可燃物参数,模拟不同燃烧条件下不同林分类型的火行为指标,分别为地表火蔓延速度、单位面积发热量、火线强度和火焰长度;使用R语言进行主成分分析,根据贡献率探讨林分因子、立地因子和可燃物因子对火行为的潜在影响。【结果】侧柏林、刺槐林和油松林的可燃物总载量分别为15.35、17.59、15.28 t/hm^(2),其中易燃可燃物载量(即上层枯叶、易燃草本、1时滞可燃物)分别是4.55、4.41、6.18 t/hm^(2),分别占林分总可燃物载量的29.6%、25.1%、40.4%。防火期内门头沟区的平均风速为2.2 m/s(7.9 km/h),地表火蔓延速度油松林>侧柏林>刺槐林,速度分别达11.5、11.1、8.0 m/min。单位面积发热量油松林>侧柏林>刺槐林,分别为23 091、21 155、18 413 kJ/m;火强度油松林>侧柏林>刺槐林,分别为4 426、3 882、2 468kW/m;油松林火焰高度变化范围分别是0.89~3.40 m、1.34~2.91 m、1.78~3.88 m,同等条件下火焰高度油松林>侧柏林>刺槐林;8级大风天气下(风速17.9 m/s,64.4 km/h),地表火行为的模拟结果会略有改变,侧柏林的火蔓延速度更快、火强度更高,油松林、刺槐林次之。根据主成分分析的因子贡献度,分别得出影响3种林分火行为的主成分,侧柏林、刺槐林与油松林的第1与第2主成分分别为可燃物构成(可燃物载量及影响其分布的地形因素)与林分因子、可燃物构成与可燃物含水率、林分因子与可燃物含水率。【结论】(1)易燃可燃物载量是影响林分火行为的关键因子。(2)可燃物含水率对火行为指标的数值起决定作用,可燃物含水率的临界值影响林火发生类型。可燃物湿润时无论风速大小,林地难以起火;可燃物干燥时处于易燃状态,大风天气时容易发生蔓延快、强度高的地表火。(3)可燃物连续性是决定地表火发展成为树冠火的关键因素,火焰高度大于第一活/死枝下高,极有可能从地表火发展成为树冠火,扑救难度极大。建议需定期修枝割灌,清理林下可燃物,降低火险。

[Objective] Surface fire is the most common type of forest fire, which directly affects vegetation regeneration and nutrient allocation and circulation of ecosystem. The common indexes reflecting forest fire behavior are fire spreading speed, calorific value per unit area, fire intensity and flame height. The fire behavior simulation based on the actual stand and site conditions can reveal the conditions of forest fire occurrence, effectively judge the possibility of crown fire occurrence, and provide a scientific basis for forest fire prevention and firefighting decision-making. [Method] Typical forest stands(Robinia pseudoacacia forest, Pinus tabuliformis forest and Platycladus orientalis forest) in Mentougou District of Beijing were selected as the survey objects. 5 sample plots were set for each stand, a total of 15 sample plots. Through field investigation, the data of fuel load(shrub fuel, herb fuel, 1 h fuel load, 10 h fuel load, 100 h fuel load),stand factors(height of the first living branch, height under dead branches, tree height, DBH, canopy density) and site factors(altitude, slope gradient, slope aspect, slope position) were obtained. The BehavePlus6 software was used to simulate the fire behavior indicators of different stand types under varied fuel conditions based on meteorological parameters and fuel parameters, they are the spreading speed of surface fire, the calorific value per unit area, the intensity of fire line and the length of flame. Principal component analysis was conducted with R language, and the potential effects of stand factors, site factors and fuel factors on fire behavior were discussed according to the contribution rate. [Result] The total fuel loads of Platycladus orientalis forest(POF), Robinia pseudoacacia forest(RPF) and Pinus tabuliformis forest(PTF) were respectively 15.35, 17.59, 15.28 t/ha, in which, the inflammable fuel loads(up-layer leaf,flammable herb, 1 h fuel) were respectively 4.55, 4.41, and 6.18 t/ha, accounting for 29.6%, 25.1% and 40.4% of the total fuel load of the stand, respectively. In the period of fire protection, the average wind speed was 2.2 m/s(corresponding to 7.92 km/h), the simulation result of surface fire rate of spread was PTF > POF >RPF, and the numerical values respectively were 11.5, 11.1, 8.0 m/min. The simulation results of fire heat per unit area were PTF > POF > RPF, and the numerical values were 23 091, 21 155, and 18 413 kJ/m~2,respectively. The simulation result of fireline intensity was PTF > POF > RPF, and the numerical values were 4 426, 3 882, 2 468 kW/m. The flame height range of POF, RPF and PTF was respectively 0.89-3.40 m, 1.34-2.91 m, 1.78-3.88 m. Under the same conditions, the flame height was PTF > POF > RPF.According to the contribution rates, the principal component analysis results were different by stands. The first and second principal components in POF, RPF and PTF respectively were fuel material composition and stand factors, fuel material composition and fuel moisture content, stand factors and fuel moisture content. [Conclusion](1) Flammable combustible material load is a key factor affecting forest fire behavior.(2) Fuel moisture rate plays a decisive role in the value of fire behavior indicators, and the critical value of fuel moisture rate affects the type of forest fire. Combustibles are flammable when dry, and surface fires with fast spread and high intensity are easy to occur in windy weather.(3) The continuity of combustibles is the key factor that determines the development of surface fire into crown fire. The flame height greater than the height under first living or dead branches is very likely to develop from surface fire to crown fire, and it is very difficult to put out. It is recommended to prune and mow regularly to clean up the combustibles under the forest and then reduce the fire risk.