Effects of thinning on canopy characteristics and potential crown fire behavior of Platycladus orientalis in Xishan Forest Farm of Beijing

  Objective  This paper aims to explore the effects of different thinning intensities on canopy fuel characteristics and potential crown fire behavior of Platycladus orientalis in Xishan Experimental Forest Farm in Beijing, so as to provide reference for the regulation of canopy fuel and the prevention of high-energy active crown fire.

  Method  Taking Platycladus orientalis in Xishan Experimental Forest Farm in Beijing as the research object, three thinning intensities (low, 15%; moderate, 35%; high, 50%) and the control were set up, and three repeated sample plots were set for each thinning intensity. Based on the canopy fuel parameters (branch number, length, base diameter) obtained by the standard branch method, a linear regression model was established for canopy fuel load, and the canopy bulk density was further calculated. The effects of different thinning intensities on canopy fuel characteristics (canopy fuel load and canopy bulk density) were discussed by single factor analysis of variance (ANOVA). Using BehavePlus 5.0 software, according to the fuel moisture content and meteorological conditions of the sample plots, the humidity condition and 10 m high altitude wind speed (0 − 18 m/s) were set to explore the effects of different thinning intensities on the potential crown fire behavior indexes (crown fire spread rate, fire line intensity, flame length, calorific value per unit area, etc.). According to the crown fire transformation model, the effect of thinning on the occurrence of active crown fire was further studied.

  Result  (1) The characteristics of canopy fuel in different thinning intensities were different and the difference was the most significant between moderate thinning and control. Canopy fuel load and canopy bulk density decreased with the increase of thinning intensity, the former decreased from 3.280 to 0.540 kg/m² and the latter decreased from 0.478 to 0.056 kg/m³. (2) After thinning with different intensities, the canopy fuel load and canopy bulk density distribution increased at first and then decreased with the increase of tree height. (3) The index of potential crown fire behavior under moderate thinning intensity was significantly different from control, in which the fire line intensity, flame length and calorific value per unit area decreased with the increase of thinning intensity. The canopy fire spread rate after thinning was lower than that of control, but did not change with the change of thinning intensity, and the critical crown fire spread rate increased with the increase of thinning intensity. Active crown fire will occur in the sample plot without thinning and low thinning intensity, and will not occur in the sample plot with moderate and high thinning intensity. With the increase of thinning intensity, the required 10 m high-altitude wind speed for active crown fire gradually increased from 6 to 8 m/s, the fire line intensity decreased from 6 930 to 5 829 kW/m, the flame length decreased from 9.7 to 8.6 m, the calorific value per unit area decreased from 47 817 to 40 667 kJ/m², and the crown fire spread rate increased from 8.7 to 8.9 m/s.

  Conclusion  Thinning affects canopy fuel characteristics and potential crown fire behavior indicators. Moderate thinning intensity has significant influence on canopy fuel characteristics and potential crown fire behavior indexes. By reducing canopy bulk density, the potential crown fire behavior can be effectively reduced and the occurrence of active canopy fire can be reduced. For Platycladus orientalis, moderate thinning is recommended in the regulation of canopy fuel considering both economic and ecological benefits.