Responses Of Spatial Patterns Burn Severity In The Great Xing’an Mountains To Climatic Change

Fire is a major driver of forest landscape change in boreal forests.Burn severity is one of the main indexes for measuring the damage degree of fire on forest ecosystems.Spatial patterns of burn severity affect numerous ecological processes(e.g.,species composition,seed dispersal,and vegetation restoration).Explaining spatial patterns of burn severity is conducive to reveal the formation mechanism of forest landscape patterns after fire,which is of great significance for predicting spatial patterns of burn severity in the future and formulating scientific fire management strategies.Based on Landsat TM/ETM remote sensing images,we mapped the burn severity of 42 fires that occurred between 2000 and 2018 in Huzhong forest region of the Great Xing’an Mountains by calculating the post-fire Normalized Burn Ratio index(NBR)and classified the fires into unburned,low,moderate and high severity classes.For each fire,we calculated five landscape metrics to quantitatively describe spatial patterns of burn severity at the class level using the FRAGSTATS program.The landscape pattern metrics were percentage of landscape(PLAND),area-weighted mean patch size(AREA_AM),area-weighted mean fractal dimension index(FRAC_AM),perimeter–area ratio(PARA_AM),and patch density(PD).Using Random Forest models,we analyzed the relative importance and marginal effects of weather,topography,and vegetation variables on determining spatial patterns of burn severity.Finally,the potential spatial patterns of burn severity under different fire weather conditions were predicted and compared according to historical meteorological data.The results showed that:1)The proportion of low-severity patches in the landscape was small and the overall shape was relatively regular(to the rectangular),but the edges were complex(the boundary fluctuation was obvious);The moderate-severity patches accounted for a large proportion in the landscape and the overall shape was irregular as well as the edges were complex;The proportion of high-severity patches in the landscape was the largest;the overall shape was relatively regular and the edges were not complex.Compared with unburned patches,low-,moderate-,and the high-severity patches were more larger and simpler in shape.2)Vegetation coverage and relative humidity were the most important factors affecting the spatial pattern of the low-severity patches.When vegetation coverage was greater than 0.6,the proportion of burned patches in the landscape increased;Relative humidity and wind speed were the most important factors affecting the spatial pattern of moderate-severity patches.When the relative humidity was less than 60%,the marginal effect value of the proportion and the area-weighted average patch area increased significantly.Elevation played an important role in shaping spatial patterns of high-severity patches,followed by aspect,slope,vegetation coverage,relative humidity,and temperature variables.With the increase in elevation,the marginal effect curve of area-weighted mean patch area and area-weighted mean patch fractal dimension showed an obvious increasing trend,whereas area-weighted perimeter-area ratio and patch density exhibit a decreasing trend.3)As fire weather rating increases,the spatial pattern of low-severity patches was insensitive to the response of climate change.The spatial pattern of moderate-severity patches was sensitive to the change of high fire risk weather.Under the condition of high risk of fire,the size and shape of the moderate patches increased,and the number of patches decreased;Under the condition of high fire risk,the number and the edge of high-severity patches increase,but the edge-area ratio and the area-weighted average patch area reduced.(4)In general,the high-severity patches differed significantly from unburned,low-and moderate-severity patches for five spatial pattern metrics.Topography and vegetation were more important in shaping the spatial pattern of high-severity patches than climate.Therefore,it would be urgent to implement forest fuel treatment in high-severity areas.It is necessary to allocate different forest patches reasonably from the landscape level,then to reduce the risk of high-severity forest large fires.