【作者】 赵杰；

【导师】 岳超；

【作者基本信息】 西北农林科技大学 ， 土壤学， 2022， 博士

【摘要】 森林野火（林火）作为全球森林生态系统的重要干扰因素,能够通过生物地球化学效应和生物地球物理效应对区域和全球气候产生影响。生物地球化学效应是指林火通过燃烧生物质导致CO₂等温室气体排放,进而影响地球辐射平衡对地表温度产生影响。生物地球物理效应是指由于火后地表状态改变,导致反照率、潜热和感热等地表能量平衡过程的变化而对局部地表温度产生影响。以往研究重点关注了林火的生物地球化学效应,而近期的研究表明,全球尺度上生物地球物理效应导致的地表增温幅度与生物化学效应相当,能够对全球变暖产生显著的正反馈作用。北半球中高纬度地区森林占全球温带和寒带森林的绝大部分（～90%）,该区域林火生物地球物理影响呈现为显著的地表升温作用。气候变暖背景下,全球范围内极端火灾天气发生的频率呈现显著上升趋势,极端大火频率增加,表现为单次林火的过火面积（林火斑块面积,Forest fire size）变大。尽管生物地球物理效应主要为局部作用,目前仍不清楚该效应导致的地表升温是否存在空间尺度效应,即是否林火斑块面积越大,火后地表升温幅度越高?如果证实,这意味着林火的气候反馈强度不仅受到总过火面积（即林火斑块面积的总和）的影响,还会受到平均斑块面积大小的影响。如果火后地表温度变化存在尺度效应,那么导致这一尺度效应的生物地球物理机制为何?不同森林类型的尺度效应是否相同?历史时期火后地表升温趋势如何受到不同森林类型过火面积和平均斑块面积大小变化的影响?针对上述科学问题,本论文以北半球中高纬度地区森林为研究对象,首先基于长时间序列气候数据和历史林火资料分析了森林火灾气象风险和林火面积的长期动态变化。重点分析了研究区域林火气象条件和林火平均斑块面积的关联关系、以及主要国家和区域林火斑块面积的长期变化趋势。其次,进一步分析了林火斑块面积与林火行为和火后森林死亡率间的关系。在此基础上,基于多套地表特征观测数据（叶面积指数、反照率等）、地表温度和辐射通量数据,量化了林火的生物地球物理效应,聚焦于林火斑块面积对火后地表温度变化的影响（即尺度效应）,并分析了其生物地球物理驱动机制。最后,分析了不同森林类型林火生物地球物理作用的尺度效应差异,探讨了通过森林管理减缓极端大火发生和减缓林火气候反馈强度的潜在可能性。本研究得到的主要结果和结论如下:（1）近40年来,北半球中高纬度地区森林野火风险呈显著升高态势。该地区日平均气温及日最高气温呈显著上升趋势;对应的,表层土壤湿度呈显著下降趋势,三个基于不同可燃物湿度的林火气象风险指数均呈显著增加态势。受森林火险增加的影响,过去20至60年间,加拿大、美国以及俄罗斯平均林火斑块面积均呈现为显著上升趋势,上升幅度分别达到72.72%,145.31%以及144.75%。（2）北半球中高纬度地区林火斑块面积与林火行为存在显著的尺度效应。更长的林火持续时间、更快的扩散速度导致更大的林火斑块面积。林火斑块面积越大,以辐射能量释放强度衡量的林火强度越高,并导致更高的火后树木死亡率。（3）北半球中高纬度地区火后地表升温与林火斑块面积呈现显著的尺度效应,夏季火后升温幅度随林火斑块面积增加而显著上升,升温幅度对林火斑块面积的敏感性为0.50±0.02 K[log₁₀（km²）]^-1,即林火斑块面积加倍时,火后地表升温幅度提高0.15±0.01K。这一尺度效应的生物地球物理机制在于林火斑块面积增大时,火行为更加极端,火后森林叶面积下降幅度更大;对应的,林火斑块面积越大,夏季地表反照率下降越明显,吸收的太阳短波辐射随斑块面积增加而增加,但同时叶面积下降导致生态系统蒸散发下降的幅度更大,地表潜热通量下降,感热通量增加,最终导致了火后更高的地表温度。冬季林火生物地球物理效应与夏季基本相反,但是年尺度上的效应受夏季主导。本论文证实了以往研究中所忽视的林火气候反馈尺度效应,表明即使总过火面积维持不变,林火平均斑块面积本身可以强烈的气候正反馈效应。因此,应积极采取措施预防大面积森林野火的爆发。（4）落叶阔叶林和针阔混交林的林火生物地球物理效应随林火斑块面积变化的敏感性低于常绿针叶林和落叶针叶林;并且,阔叶林和混交林的平均林火斑块面积和平均火后地表升温幅度均低于两种针叶林。因为森林类型组成的变化将能够影响大面积野火的爆发和火后地表升温幅度。事实上,2003-2016年间,研究区域森林野火中阔叶林和混交林占比持续下降,导致火后地表升温呈现长期升高的趋势(0.29oC decade^-1)。增加中高纬度地区阔叶树种占比可以作为一种潜在的基于自然的解决方案以缓解大面积林火对气候的影响,并减缓大面积林火的爆发。本研究揭示了斑块面积这一重要的林火情势（fire regime）指标对林火气候反馈的作用。大面积林火的爆发很大程度受气候变暖的驱动,又可以反过来强化气候变暖,因此预防大面积林火应成为林火管理的重要考量。研究结论有助于加深对气候变化与森林野火之间的交互作用、以及林火情势对气候的影响与作用机制方面的科学理解。研究发现增加北半球中高纬度地区阔叶树种占比可以减缓大面积林火的爆发和林火的气候反馈强度,对于政府部门制定科学有效的林火管理策略、减缓全球变暖均有现实意义。更多还原

【Abstract】 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.更多还原