抚育间伐对北京西山林场油松林可燃物特征及潜在火行为的影响
Effects of thinning on fuel characteristics and potential fire behavior of Pinus tabuliformis in Xishan Forest Farm of Beijing
[目的]研究间伐后油松林可燃物和潜在火行为特征随时间的动态变化过程，为研究地区的可燃物管理提供依据。[方法]以北京市西山试验林场油松林为研究对象，设置 15%轻度间伐(株数强度)后3年、5年、7年及未间伐油松林样地各 3块，对样地的地表可燃物(1 h时滞、10 h时滞、100 h时滞枯枝及上层枯叶、下层枯叶和灌草可燃物)、冠层可燃物(叶、小枝、大枝)和林分因子(胸径、树高、冠幅等)进行调查，通过单因素方差分析探讨间伐后不同时间油松林地表、冠层可燃物负荷量和冠层容积密度等特征;结合研究区气象条件，设置不同可燃物含水率场景(低、中、高含水率)和 10 m高空风速(0～18 m/s)，利用Behave Plus 5.0.4 软件对间伐后不同时间油松林的地表及冠层潜在火行为指标进行模拟，探讨抚育间伐对树冠火发生的影响。[结果]① 间伐后不同时间的油松林细小可燃物负荷量(1 h和 10 h时滞枯枝、上层枯叶)、冠层可燃物负荷量和冠层容积密度存在显著差异(P<0.05)，但地表可燃物总负荷量差异不显著(P＞0.05)。细小可燃物负荷量最高的是间伐后 3 年油松林(0.64 kg/m2)，各样地冠层可燃物负荷量为 1.80～4.30 kg/m2，间伐后5 年(0.29 kg/m3)、3 年(0.42 kg/m3)油松林的冠层容积密度较对照(0.71 kg/m3)显著(P<0.05)降低。② 在不同含水率条件下，间伐后 3年油松林的潜在火行为指标与其他林分差异较大。低含水率条件下，间伐后 3年油松林地表火最大蔓延速率、地表火线强度、地表火焰长度随风速增大分别变化于3.60～9.91 m/min、4 925.02～13 559.59 kW/m和3.87～6.16 m，单位面积发热量为 82 078.06 kJ/m2，树冠火转化比率随风速增大变化于3.00～8.27，地表火易蔓延发展成树冠火，且树冠潜在火行为指标也较高，树冠火蔓延速率、树冠火线强度、树冠火焰长度随风速增大分别变化于1.31～57.06 m/min、2 546.10～110 966.82 kW/m和 4.96～61.49 m，单位面积冠层热量为 34 611.22 kJ/m2。[结论]间伐后 3年油松林的细小可燃物负荷量和地表潜在火行为指标均高于未间伐及间伐后5年和7年的油松林，且冠层潜在火行为指标值均较高，在风速大于 4 m/s、低可燃物含水率条件下有发生高强度连续型树冠火的可能。建议增加修枝、割灌等林下处理措施，间伐工作后减少枯枝堆积，以降低森林火灾发生的风险。
[Objective] To study the dynamic change process of fuel and potential fire behavior characteristics over time after thinning in Pinus tabuliformis forests, and to provide a basis for fuel management in the study area. [Method] In the Xishan Experimental Forest Farm of Beijing, a total of 12 plots were selected, including 3 plots in each after 7, 5 and 3 years of 15% low-intensity thinning (plant number intensity) and unthinned plots. The stand factors (DBH, height, crown width, etc), surface fuels (1 h, 10h, 100 h, upper dead leaves, lower dead leaves, live herbaceous , live shrub fuel load) and canopy fuels(leaves, twigs and large branches) of the sample plots were investigated. The characteristics of surface,canopy fuel load and canopy bulk density of P. tabuliformis forests at different times after thinning were analyzed by one-way analysis of variance. Combined with the meteorological conditions of the study area, different scenarios of moisture content of fuel (low, medium and high moisture content) and wind speed at 10m altitude (0-18 m/s) were set, and use Behave Plus 5.0.4 to simulate the potential fire behavior indexes of surface and canopy of P. tabuliformis forests at different times after thinning, so as to explore the influence of thinning on canopy fire occurrence.[Result] ① There were significant differences in fine fuel load(include 1 h,10 h dead branch and upper dead leaves), canopy fuel load and canopy bulk density at different thinning times (P<0.05). However, there was no significant difference in total fuel load on the surface (P>0.05). The highest fire fuel load(0.64 kg/m2) was found in the 3-year thinning forest, and the total fuel load of the canopy varied from 1.8-4.30 kg/m2. The canopy bulk density of the 5-year thinning forest (0.29 kg/m3) and 3-year thinning forest (0.42 kg/m3) had a significant decrease (P<0.05) compared with the control forest (0.71 kg/m3). ② Under different scenarios of moisture content, the potential fire behavior indexes of P. tabuliformis forests after thinning for 3 years were significantly different from those of other stands. Under the condition of low moisture, the surface rate of spread(maximum), surface fireline intensity and surface flame length varied with the increase of wind speed of 3.60-9.91 m/min, 4 925.02-13 559.59 kW/m and 3.87-6.16 m, respectively, and the heat per unit area was 82 078.06 kJ/m2. The crown fire transition ratio varied from 3.00 to 8.27 with the increase of wind speed. The surface fire easily spread into crown fire, and the potential crown fire behavior indexes was also high. The crown fire rate of spread, crown fireline intensity and crown flame length varied with the increase of wind speed of 1.31-57.06 m/min, 2 546.10-110 966.82 kW/m and 4.96-61.49 m. The crown heat per unit area was 34 611.22 kJ/m2.[Conclusion] The fine fuel load, and the surface potential fire behavior indexes of the 3-year thinning forests were higher than those of the unthinned forests, the 7-year and the 5-year thinning forests. High intensity continuous canopy fire is possible under the conditions of wind speed greater than 4 m/s and low fuel moisture content. In order to reduce the risk of forest fire, it is suggested to increase the treatment measures such as pruning and cutting irrigation, and reduce the accumulation of dead branches after thinning work.
童大龙;刘晓东;
北京林业大学 森林资源生态系统过程北京市重点实验室,北京 100083##森林草原火灾风险防控应急管理部重点实验室,北京 102202;北京林业大学 森林资源生态系统过程北京市重点实验室,北京 100083##森林草原火灾风险防控应急管理部重点实验室,北京 102202;
Pinus tabuliformis thinning fuel characteristics canopy bulk density potential fire behaviour
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