Forest Wildland Fire Dynamics And Biophysical Impacts In Middle And High Latitudes Of The Northern Hemisphere

As an important disturbance to global forest ecosystems,forest wildfires can have a significant impact on regional and global climate through both biogeochemical and biogeophysical effects.The biogeochemical effect refers to the emission of CO₂ and other greenhouse gases caused by forest fire through biomass burning,which then affects the earth’s radiative balance and influences the near-surface temperature.The biogeophysical effect refers to the impact on land surface temperature due to changes in land surface characteristics that affect surface energy processes such as surface albedo,latent heat and sensible heat fluxes.Previous studies have focused on the biogeochemical effects of forest fires;however,recent studies have shown that biogeophysical effects on a global scale lead to surface warming of comparable magnitude caused by the biochemical effects,and thus can have significant positive feedback effects on global warming.Forests in the mid-and high-latitude regions of the Northern Hemisphere account for the vast majority（～90%）of the world’s temperate and boreal forests.The biogeophysical effects of forest fires in this region exhibit a significant surface warming effect.In the context of climate warming,the frequency of extreme fire weather show a significant global increasing trend over past few decades,with increasing occurrences of extreme fires,namely,the average area of forest fire patch（fire size）has been reported to increase over several regions.Although the biogeophysical effect mainly occurs at a local scale,it remains unclear whether postfire surface warming scales with fire size,i.e.whether larger forest fires lead to greater postfire surface warming.If this is true,then the overall surface climate impacts of forest fires will depend not only on the total area burned,but also the average fire size.This the scale dependency indeed exists,then what are the biogeophysical mechanisms driving such dependency?Is the strength of scale dependency the same among different forest types?How the historical trend of postfire surface warming has been influenced by the those in average fire size and the changes in forest type composition of burned area?The present thesis addressed the listed science questions above.The long-term dynamics of forest fire weather states,forest fire size and total burned area were analyzed based on long-term time series of climate data and historical forest fire statistics.The analysis focused on the correlation between forest fire weather conditions and the mean forest fire size in the study region,as well as the long-term trends in the mean forest fire size in major countries and sub-regions.Secondly,the relationships between forest fire size and forest fire behaviour and post-fire forest mortality were further analyzed.The biogeophysical effects of forest fires were then quantified based on multiple datasets of land surface characteristics（leaf area index,surface albedo,etc.）,land surface temperature and radiation fluxes,focusing on the effects of forest fire size on post-fire surface temperature changes（i.e.the scale-dependency effect）and the underlying biogeophysical mechanisms.Finally,potential differences in the sensitivities of postfire biogeophysical effects to forest fire size were analysed based.The potential for forest management to mitigate the occurrence of large fires and their climate feedback were then explored.The main results and conclusions obtained from this thesis are:（1）Over the past 40 years,forest wildfire risk has shown a significant increasing trend in the mid-and high-latitude regions of the Northern Hemisphere.With significant increases in the average daily temperature and maximum daily temperature,surface soil moisture has shown a corresponding significant decreasing trend,driving significant increases in three different forest fire weather indexes that integrate information of fuel moisture.Driven by the increase in forest fire risk,the average forest fire size in Canada,the United States,and Russia showed a significant increase of 72.72%,145.31%and 144.75%over the past 20 to 60 years,respectively.（2）Significant scaling relationships were found between forest fire size and forest fire behavior in the study region.Longer forest fire duration and faster spread rate lead to larger forest fire size.Larger fires have a greater fire intensity as indicated by radiative energy released during active combustion,and a higher post-fire tree mortality.（3）Postfire surface warming was found to scale logarithmically with fire size.Postfire surface warming（ΔΤ）in summer increased with fire size,with a quantified sensitivity as0.50±0.02 K[log₁₀（km²）]^-1,or withΔΤincreasing by 0.15±0.01 K every time fire size doubles.The scale-dependency was driven by systematic changes in land surface energy processes with fire size.Larger fires have more extreme fire bebaviors and lead to greater reduction in postfire leaf area index.Correspondingly,postfire surface albedo decrease scaled with fire size,resulting in greater amount of shortwave radiation being absorbed by the land surface.However,ecosystem evapotranspiration decreased more with increasing fire size,leading to reduced latent heat flux and increased sensible heat flux,ultimately leading to increased postfire surface warming with fire size.Fire size-dependent changes in postfire surface energy fluxes in winter were largely inverse to those in summer.The postfire surface temperature change on the annual time scale,however,remains a warming effect that scales with fire size.These results revealed the previously overlooked fire-climate feedback effect exerted by fire size,suggesting that sheer changes in fire size can render the climate impacts of forest fires going beyond the effects of changes in burned area per se.Therefore,it is imperative to take actions to prevent the occurrence of large forest fires.（4）The sensitivity of warming to fire size was found lower in deciduous broadleaf and mixed forests in contrast to deciduous and evergreen coniferous forests.Broadleaf and mixed forests also have lower strengths of postfire surface warming and smaller fire size on average than coniferous forests.Changes in forest type composition can hence greatly influence the occurrence of large fires and the magnitude of postfire surface warming.In fact,between 2003and 2016,postfire surface warming in the study domain increased by an average of 0.29oC decade-1,mainly driven by an increase in the burned area of coniferous forests.Increasing broadleaf species in northern forests could serve as a nature-based solution to mitigate the climate impacts of forest fires and to prevent the occurrence of large fires.This thesis has elucidated on the role of fire size,which is an important aspect of fire regimes,on the climate feedback of forest fires.Climate warming strongly drives the occurrence of large fires,which further enhances warming by postfire biogeophysical impacts,hence making the prevention of large fires an important aspect of forest management.The findings can help improve our understandings on the interactions between climate change and wildfire dynamics and on the mechanisms of forest fire regime impacts on climate system.The climate mitigation potential by increasing broad-leaved tree species in the northern hemisphere’s mid and high latitudes,as inferred from the findings,has practical values for government departments to develop scientific and effective modern forest fire management strategies in the context of global warming.