Study Area
The Yimuhe Forest Farm is in Ergun City, Inner Mongolia Autonomous Region (Fig. 2). It is an unexplored key state-owned forest farm situated along the Ergun River in the Russian Far East. The geographical coordinates are 120°1′~120°25′E, 50°34′~50°49′N, with an altitude range is 299 ~ 876 meters. The average temperature in the study area is -5.5°C(Fick and Hijmans, 2017), and annual precipitation is 360 ~ 460mm (Fan etal., 2017). The dominant wind direction is northeast-southwest, with a yearly average wind speed exceeding 4.2 m/s and the average annual count of strong wind days is as high as 17–40 days (Liu etal., 2018; Zhao etal., 2021). Additionally, there are more occurrences of strong wind days during spring and autumn, with an average wind speed of 5.1 m/s (Feng, 2012). Consequently, severe forest fires primarily occur in spring and autumn (Cai etal., 2021). The forest coverage rate of the forest farm is exceptionally high, reaching 95.4% (Fig. 3c), and all of it consists of state-owned public welfare natural forest land (Chen etal., 2015; Guo etal., 2020). The primary tree species are Larix gmelinii, supplemented by Betula platyphylla and Populus davidiana Dode (Tian etal., 2011; Zhang etal., 2021c) (Fig. 3b). Most groups of trees fall within age are near-mature and mature age; the landform is the Western Mountainous area of the Greater Khingan Mountains; the soil type is brown coniferous forest soil and ordinary chernozem (Fang etal., 2015).
Since 2006, there have been three forest fires caused by Russian transboundary fires at the border of Yimuhe Forest Farm (Table 1). The number of fires was small, but the burned area was as high as 9478.5ha, all major forest fires and above, resulting in a massive loss of forest resources. According to the State Forestry Administration’s Decision on Further Strengthening the Construction of Firebreaks ([2000] No.222) (National Forestry Administration, 2000), there should be 10m/ha ~ 15m/ha forest firebreaks in the general fire danger zone. The density construction standard of engineered firebreaks shall refer to the planning requirements of the Construction standard for forest fire break system in Northeast and Inner Mongolia border areas (National Forestry and Grassland Administration, 2016): the length density should be 3.8 m/ha when the control area is 11000 ha. However, the existing forest road mileage of the original forest area in the Northern Khingan Mountains is 796 km, and the density of the road network is only 0.84 m/ha (Gao, 2014). The backward forest road network conditions make rescue work more difficult (Li etal., 2012).
Table 1
Details of Sino-Russian border fires in the original forest area of the Northern Greater Khingan Range
Year | Location | Coordinate | Elevation (m) | Burned area(ha) |
2006 | Yimuhe Forest Farm, Wuma Forestry Bureau, Forest class No.108 | 52 °35 ′25 ″ N 120 °09 ′20 ″ E | 730 | 6006 |
2014 | Yimuhe Forest Farm, Wuma Forestry Bureau, Forest class No.115 | 52 °36 ′00 ″ N 120 °05 ′20 ″ E | 468 | 2703 |
2017 | Yimuhe Forest Farm, Wuma Forestry Bureau, Forest class No.124 | 52 °36 ′30 ″ N 120 °14 ′00 ″ E | 652 | 769.5 |
Data Acquisition
The line selection of forest firebreak is based on GIS system, using ArcGIS to analyze and process required data. In this study, the geographic and projection coordinate systems are Xian_1980 and Xian_1980_GK_CM_105E, respectively. Our data sources are: (1) primary data such as the boundary of the study area, vector data of road and river system, and ground features provided by forest staff, (2) meteorological data from Genhe City Meteorological Forecast Bureau, with district station number 50431 and coordinates of 50°28′12″N, 121°18′36″E. We obtained observational data from May 30 to June 4, 2006, April 29 to May 3, 2014, and April 29 to May 2, 2017, (3) terrain data was obtained from Geospatial Data Cloud (http://www.gscloud.cn/), which was digital elevation data with a resolution of 30m, (4) fuel type data was available from the USGS (https://www.usgs.gov/).
Planning Scope of Border Fires
This paper mainly used FARSITE to determine the planning scope of border fires. FARSITE simulation requires topographic data, fire behavior fuel models, and meteorological data (Price and Germino, 2022).. Topographic data includes elevation, slope, aspect, and coverage (Thew etal., 2011). Fire behavior models are composed of several parameters, such as the distribution of fuel types and combustion models, and each parameter affects the results of forest fire spread (Zhang etal., 2021b). Meteorological data consists of wind direction, wind speed, temperature, humidity, and other data (Osorio etal., 2019).
Firstly, we obtained digital elevation model data of the study area through Geospatial Data Cloud and used ArcGIS to extract the raster data of the slope and aspect. Secondly, we considered factors such as climate conditions, fuel types, load, canopy coverage, and topography to conduct a comprehensive analysis combined with historical border fires. Lastly, we used FARSITE to simulate fire spread and determined the planning scope of border fires.
Locations of Biological Firebreaks
Biological firebreaks’ positions are determined based on the topography and main wind direction. Changes in topography will cause changes in the fire environment, impacting the fire intensity and severity. The main topographic factors are slope, aspect, valley, and ridge. Extreme weather and interactions of climate and topography exacerbate fire behavior. For example, slope and wind direction alignment can result in higher fire intensity on windward slopes than on leeward slopes when fuel types are similar (Prichard etal., 2020).
We used ArcGIS to conduct Hydrological Analysis on the determined planning area and extract valley lines, ridge lines, slope, and aspects. By obtaining observation data from Genhe Meteorological Forecast Bureau, the main wind direction in the study area was organized and analyzed to create a Wind Rose map. According to the locations of valley lines and ridge lines extracted from the study area, combined with the slop, aspect and main wind direction, the locations of biological firebreak was finally determined.
Locations of Engineered Firebreaks
Frame a Comprehensive Cost Model for Line Selection
When selecting locations for engineered firebreaks, calculating comprehensive cost weight can reflect whether each grid in the planning area is suitable for constructing firebreaks (Tan etal., 2019). The comprehensive cost model is the key to calculating the optimal path of engineered firebreaks (Jha and Schonfeld, 2004). We established a single-factor cost raster dataset using ArcGIS to consider the factors affecting engineered firebreaks, then generated a comprehensive cost model by weighted superposition of each single-factor cost raster dataset. The steps of establishing the comprehensive cost model of line selection in engineered firebreaks: (1) determine influencing factors; (2) determine the weight of each factor through AHP; (3) based on the elevation of the study area, obtain single-factor cost raster dataset; (4) reclassify and assign the influencing factors; (5) the weight of each factor is superimposed with the single-factor cost to obtain comprehensive cost.
The comprehensive cost calculation method is shown in Eq. (1):
$$\:S=\sum\:_{i=1}^{n}{F}_{i}{W}_{i}\left(i=\text{1,2},...,n\right)$$
1
where S is the final cost; F is the cost of each factor; Wi is the weight of each factor; n is the number of factors.
Determine Influencing Factors
The factors involved in the line selection of engineered firebreaks are very complex, and the results of line selection directly affect the fire resistance of the firebreak network (Fang etal., 2015). Therefore, we needed to consider many factors, such as route, direction, surrounding environment, construction difficulty, and association with natural firebreaks. Before planning the location of engineered firebreaks, we considered four aspects: (1) economic status (Li etal., 1995): the construction process and later maintenance of the firebreaks require lots of capital investment in land acquisition, machinery and materials, forest conservation and tending, environmental damage and restoration; (2) environmental impact (Song etal., 2017): understanding natural factors in the study area can ensure that adverse impact on the environment is reduced during the construction process; (3) social conditions (Puri etal., 2020): the construction of firebreaks against border fires involves the stability of foreign relations and border area, and proper planning can reduce social investment; (4) technical indicators (Aricak, 2015): considering natural conditions such as geology and topography, choose a suitable location to construct.
Following the ‘comprehensive consideration, highlighting the critical point’ principle, we selected six influencing factors: elevation, slope, aspect, slope position, fuel crown density, and ground features, which were used to establish a single-factor cost raster dataset.
Calculate Weights
In the AHP analysis process, each element at a different level is quantified and plays a vital role in the analysis results (Saaty, 1988). Therefore, we used the AHP to calculate the weight of the influencing factors. The AHP method has four steps.
(1) Establish a hierarchical structure model, which is divided into the highest, middle, and lowest levels according to the interrelation among issues to be addressed, factors to be considered, and specific measures of the scheme.
(2) Construct judgement matrix A=(aij)n×n to compare relative importance of factors between two layers. The element aij is formed by assigning importance according to scales 1 to 9. Mark the scale and meaning of the judgment matrix in Table 2.
(3) Consistency check. Consistency Ratio (Eq. (2)) can be calculated with the help of a random index scale (Saaty, 1990) (Table 3) and Consistency index (Eq. (3)) obtained from the comparison matrix. If CR value is less than 1.0, then it can be concluded that the matrix’s consistency is correct and acceptable for weight analysis (Das etal., 2022).
$$\:Consistency\:Ratio\:\left(CR\right)=\frac{Consistency\:index\:\left(CI\right)}{Random\:Consistency\:Index\:\left(RI\right)}$$
2
$$\:Consistency\:index\:\left(CI\right)=({\lambda\:}_{max}-n)/n-1$$
3
Where, λmax is the principal Eigen value; n stands for number of considering factors.
Table 2
Description of scales for pair comparison with AHP
Scales | Degree of preferences | Descriptions |
1 | Equally important | The contribution of the two factors is equally important. |
3 | Slightly important | Experiences and judgment slightly tend to certain factor. |
5 | Quite important | Experiences and judgment strongly tend to certain factor. |
7 | Extremely important | Experiences and judgment extremely strongly tend to certain factor. |
9 | Absolutely important | There is sufficient evidence for absolutely tending to certain factor. |
2,4,6,8 | Intermediate values | In between two judgments. |
Table 3
n | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
RI | 0 | 0 | 0.52 | 0.89 | 1.12 | 1.26 | 1.36 | 1.41 |
Reclassification and Assignment of Influencing Factors of Line Selection
According to the characteristics of the six influencing factors, they were divided into five grades, and each grade was assigned a different value. The larger the value, the higher the cost; try to avoid building engineered firebreaks in this area. We used ArcGIS ‘reclassification’ tool to classify the influencing factors and assign values.
Cost Path Analysis
Cost Path Analysis (Fig. 4) is a method that uses Dijkstra algorithm to find the path with the lowest cost from ‘starting place’ to ‘destination’ based on determining comprehensive cost weight (Yildirim and Bediroglu, 2019). According to the principle from right to left and from top to bottom, search and calculate the cumulative cost of the eight adjacent cells at the starting point, and finally, find the cell with the lowest cost. In the model, for different path moving directions, the calculation formulas of path cost are as follows: (1) adjacent node cost (Eq. (4)): cost of moving from Cell 1 to adjacent position Cell 2 (horizontal or vertical directions of Grid 1); (2) diagonal node cost (Eq. (5)): cost of moving from Cell 1 to Cell 2 along the diagonal; (3) cumulative cost (Eq. (6)): cost of moving two units of Cell 1 horizontally or vertically continuously to Cell 3; (4) cumulative cost (Eq. (7)): cost of moving two units of Cell 1 to Cell 3 along the diagonal.
$$\:{a}_{1}=({cost}_{1}+{cost}_{2})/2$$
4
$$\:{a}_{2}=\sqrt{2}\ast\:({cost}_{1}+{cost}_{2})/2$$
5
$$\:{accum\_cost}_{1}={a}_{1}+({cost}_{2}+{cost}_{3})/2$$
6
$$\:{accum\_cost}_{2}={a}_{2}+\sqrt{2}\ast\:({cost}_{2}+{cost}_{3})/2$$
7
Where, cost1 is the cost of Cell 1; cost2 is the cost of Cell 2; cost3 is the cost of Cell 3; a1 is the total cost of moving from Cell 1 to Cell 2 in horizontal or vertical direction; a2 is the total cost of moving from Cell 1 to Cell 2 along the diagonal; accum_cost1 and accum_cost2 are the total cost of moving from Cell 1 to Cell 3 in two directions respectively.
The Spatial Analysis of ArcGIS uses the weighting function of cost distance and shortest path function to analyze the optimal path on raster data (Tan etal., 2019). After analyzing the cost-weighted distance, two cost files can be output: girds of cost distance and backtracking link, representing cost-based and direction data, respectively (Chang et al. 2015). Using the method, it is possible to analyze the reasonableness of locations of firebreaks according to the weight values of different influencing factors to determine the optimal locations of engineered firebreaks.
Validation of FARSITE model and firebreak network
Evaluation of FARSITE Model
The data required for FARSITE model simulation is inaccurate or missing data will cause errors between the fire simulation results and the actual fire (Benali etal., 2017). To evaluate the simulation result and prove that the simulation of FARSITE is effective, this study selected the fires that occurred in the study area in 2014 and 2017 to compare with the simulation results. Simulation accuracy is calculated by using the SC (Sørensen’s coefficient) (Sorensen, 1948) (Eq. (8))comparing the similarity of two samples. The value of SC is between 0 ~ 1, and the closer it is to 1, the higher the degree of conformity between the simulated range and the actual.
$$\:SC=\frac{2A}{2A+B+C}$$
8
Where, A is the intersection of the actual and simulated range; B is the simulated range that does not intersect the actual fire range; C is the actual fire range that does not intersect the simulated range.
Simulation of Random Fire
Since the designed firebreak network is based on historical transboundary fires in the study area, we conducted random fire simulations to assess its effectiveness in other fire scenarios. The fire intensity will change at any time with the combustibles, temperature, humidity, and wind speed, which will cause the fluctuation of the heat flow and then make the angle of the convective column tilt. The heat flow can transport flying fire particle, and the maximum lifting height of flying fire particle is related to the intensity of heat flow (Albini, 1976).
$$\:H=0.173{E}^{\frac{1}{2}}$$
9
$$\:E=If\left(u\right)$$
10
$$\:f\left(u\right)=A{\left(0.295u\right)}^{B}$$
11
$$\:S=1.3\ast\:{10}^{-3}uℎ\left[0.362+{\left(\frac{H}{ℎ}\right)}^{2}\frac{1}{2}\text{ln}\frac{H}{ℎ}\right]+5.03\ast\:{10}^{-4}u{H}^{0.643}$$
12
Where, h is the maximum lifting height of flying fire particle, m; E is the intensity of heat flow, kJ/m; I is the average of fire intensity, kW/m; f(u) is the function of mean wind speed; A and B are parameters related to the type of combustibles; A is parameter; B is the dimension number; u is the wind speed, m/s; S is the distance of fire, m; H is the average of tree height, m; h is the lifting height of flying fire particle, m.