Study On The Influence Of Snow On The Characteristics Of Grassland Fuel And The Occurrence Of Grassland Fire

Grassland fire is one of the important disturbance elements in grassland ecosystems.Clarifying the influencing elements and influencing mechanisms of grassland fire occurrence has been a key component of grassland fire research.However,grasslands in the middle and high latitudes of the Northern Hemisphere are generally covered by snow for a long time in winter and spring.It is not clear how the snow cover and melting affect the combustible material characteristics and grassland fire occurrence.In this study,we applied field-controlled experiments,remote sensing data processing,geospatial analysis and statistical modeling of data to try to explore how snow affects grassland combustible characteristics,grassland surface temperature and humidity,and grassland fire occurrence within each stage of snowmelt at the micro,meso and macro scales.The main results are as follows:(1)Snow affects the combustion characteristics of grass combustibles,but this effect varies among combustible families.In this study,grassland combustibles were collected before and after snow accumulation in the winters of 2020 and 2021 at the experimental site of Yaojingzi,Changling County,Songyuan City,and the comparative experiments on the combustion characteristics of combustibles before and after snow accumulation were conducted.The results show that the effect of snow made the combustibles more intolerant to burning,consumed less oxygen for combustion,and decreased heat release capacity.Gramineae had an increased ability to produce smoke,become more flammable and produce more harmful gases after snowing,and the effect of snow on combustibles corresponding to combustibility was reversed for both Asteraceae and Leguminosae.This may be related to the differences in changes in the physicochemical properties of combustibles due to the different modes of contact between snow and combustibles.(2)Snow changes soil temperature and humidity,dead combustibles moisture,and some rejuvenation and growth characteristics of new combustibles through snowmelt water.In this study,changes in surface soil temperature and moisture,dead combustibles and fresh combustibles properties were observed for different snow depths at the experimental site during the snow cover and after the snow melt phases in 2021 and 2022.The results show that snow affects soil temperature in the covering stage and in the short term after melting,but not during the melting process and after 2 months of melting;snowmelt makes soil and combustible first wetter and then drier,and snow depth is positively correlated with soil moisture and dead combustible moisture for a certain period of time after melting;snow affects the number of rejuvenated plants of live combustibles,the rate of pre-greening,the time of rejuvenation,and the height of leaves in rejuvenation;snow affects the rate of growth of grasses after they rejuvenate,and affects the dynamics of the amount of combustibles.(3)Snow affects the land surface temperature and humidity for a certain period of time after melting,and snow interacts with some other meteorological factors to affect the land surface temperature and humidity.The study used snow parameters,land surface temperature(LST),and land surface moisture index(LSWI)from 2001 to 2018 on the Mongolian Plateau to analyze the duration of the influence of snowmelt on land surface temperature and humidity.The results show that the influence of snowmelt on LST and LSWI on the Mongolian Plateau is concentrated within 4.61-25.38 days,with a longer duration in the north and a shorter duration in the south,which have a clear latitude zonality.The deeper the snow and the later it melts,the longer the duration of influence of snowmelt on LST and LSWI.The combination of various snow elements and other meteorological elements have interactive effect on LST and LSWI,and this interaction can change the direction and degree of the impact of a single snow or a single other meteorological element on the duration of the influence of snowmelt.(4)Snow interacts with other meteorological factors and surface environmental variables to affect grassland fire density in complex direct and indirect relationships,and snow affects grassland fires in significantly different ways in spring and fall.The structural equation modeling results show that snow significantly affects grassland fire density in the spring by changing spring land surface temperature and humidity,while snow affects fall fires mainly by changing biological growth conditions that affect the density of long-term fire occurrence indirectly,and no longer affects fire density by changing surface temperature and humidity.Future fire densities tended to increase across climate scenarios,with fire densities increasing more rapidly the stronger the anthropogenic influence on climate.The relationship between future snow depth and fire density was significantly positive under all scenarios.(5)The impact of snow on grassland fire density has different forms and degrees of influence at different spatial scales.The direction and degree of influence of the same snow element on fire density are also different in spring and autumn.This study used environmental factor data and fire density data from the Mongolian Plateau from 2001 to 2018,and applied a multi-level model method to explore the similarities and differences of the "snow-fire" relationship at different spatial scales.Compared to the county-level scale,the impact of snow on fire density is more pronounced on relatively large spatial scales such as river basins and climate zones.At the watershed scale,snow affects the surface temperature and humidity environment in which fires occur through snow depth and snow melt rate;at the climate zone scale,snow influences fire occurrence indirectly by controlling the seasonal dynamics of combustible volume through the timing of snowmelt onset.Although some snow elements have a significant impact on fire density,the effect is relatively small compared to climate factors which reflect surface humidity and human factors.This study identified similarities and differences in the effects of snow on combustible characteristics and grassland fire occurrence at different temporal and spatial scales,revealing the mechanisms of snow affects grassland combustibles and grassland fires.Snow affects combustible combustion characteristics at the microscopic scale,influences temporal changes in combustible volume and combustible moisture at the mesoscopic scale,and has different mechanisms of influence on spring and fall grassland fire occurrence in conjunction with other environmental factors at the macroscopic scale.The research results and conclusions have reference value for the relevant research on grassland ecological environment and have certain guiding significance for the management of grassland areas.