Fire Behavior And Fuel Spatial Continuity Of Major Forest Types In The Mountainous Area, Beijing

This dissertation studied the fuel spatial distribution and characteristics and structure and potential fire behavior in forest space combustible units using fuel classification, fuel loading survey, statistics analysis, calculation model and other methods, through analyses of fuel loading distributions among tree, shrub, grass and litter/duff in major forests in the mountainous area of Beijing. The relation between the spatial distribution of fuel and crown fire was quantified by using the vertical and horizontal continuity indices and ratings. Based on the continuity models of forest fuel, the hazard of crown fire was evaluated and precautions to prevent the formation and spread of crown fires were proposed for major coniferous forests. The innovation of this paper was fuel continuity model and ratings, fuel loading threshold and fuel quantitative control. The major results are as follow:(1) By studying tree species composition and branches loadings of Pinus tabulaeformis, Platycladus orientalis, and Acer truncatum forests, the difference of fuel loadings for the various forest types and all levels were compared, and the influences of different species composition and fuel loading spatial distribution on the occurrence and spread of crown fire were analyzed. The results showed that fuel loading between forest types varied obviously, Platycladus orientalis forest (61.1t/hm2)> Pinus tabulaeformis forest (47.0t/hm2)> Acer truncatum forest (22.9 t/hm2); the differences of fuel loadings for shrub layer of three forest types were significant, and 90% of shrub layer loading was concentrated on several shrub species including Vitex negundo var. heterophylla, Lespedeza bicolor, Amorpha fruticosa and so on; the existence and the proportion of conifers in forest stands were basic conditions of the occurrence and spread of the crown fire.(2) Through the analysis of physical and chemical properties of different species and fuel categories and the estimates of fire behavior characteristics based on the Rothermel model of fire spread, the reaction intensities and spread speeds for different forest types were compared, and the impacts of fuel loading, moisture content, slope, wind speed on the reaction intensity and spread speed of fires were analyzed. The results indicated that fire reaction intensity was positively correlated with fuel loading; the reaction intensity in Platycladus orientalis forest was highest, Pinus tabulaeformis forest was mediate, and Acer truncatum forest was lowest; under the same moisture content, the order of fire reaction intensities (from high to low) was Pinus tabulaeformis> Platycladus orientalis> Acer truncatum; on the slope, fire spread speed of Pinus tabulaeformis forest faster than that of Acer truncatum and Platycladus orientalis forests; under the same wind speed, the rate of fire spread took the order (from high to low) of Pinus tabulaeformis> Acer truncatum> Platycladus orientalis.(3) Based on Byram's fireline intensity and flame height models, vertical fuel continuity index (C) and the rating system were established based on the flame height under calm condition, the levels of fuel and the height of the base of coniferous crown. The fuel vertical continuities of Platycladus orientalis and Pinus tabulaeformis forests were evaluated using C. The measures of pruning, thinning shrub, scattering dead wood and branches can control fuel vertical continuity and decrease the risk of crown fire occurrence in the coniferous forests.(4) Fuel horizontal continuity index (D) and the rating system were founded according to sampling area, flame horizontal length and the monthly maximum windspeed in fire seasons of Beijing; D was used to evaluate the fuel horizontal continuity of Platycladus orientalis and Pinus tabulaeformis forests. Fuel horizontal continuity in Platycladus orientalis forest was quantitatively primary regulated using the methods such as adjusting the composition proportion of conifers under the influence of the slope and the wind speed.(5) A new concept of the loading threshold (WD) for fuel continuity was proposed and formulated mathematically on the basis of flame height ratings. WD of fuel vertical continuity rating for Platycladus orientalis and Pinus tabulaeformis forests can be computed and used as the basis of fuel vertical continuity regulation in coniferous forests.