Impacts Of Drought And Forest Fire On Forest Dynamics In The Southern Edge Of The Boreal Forest

The southern edge of the boreal forest is located in the marginal zone of the forest vegetation belt,where forests are highly sensitive to global climate change.Under the dramatic climate warming,forest dynamics in this region have profound effects on regional carbon sinks,climate feedbacks and ecosystem services.In this study,we performed comprehensive analyses and addressed the climate aridification characteristics of the southern edge of boreal forest and the impact of drought and forest fires on regional forest dynamics,based on the relatively complete tree ring data,vegetation survey data and climate data collected in the region.A comprehensive understanding of the forest dynamic under climate change in this region will serve regional ecological construction and ecological security prevention more effectively.In this study,we focus on four scientific issues:(1)What is the historical climate change trend in the semi-arid area of the southern edge of the boreal forest?(2)How do tree growth and forest regeneration respond to drought stress and extreme drought events in the southern edge of the boreal forest?(3)What is the impact factors of tree growth and vegetation recovery after forest fire in the southern edge of the boreal forest?According to the above scientific issues,we choose an semi-arid area and an arid sub-humid area in the southern edge of the boreal forest as our research area to discuss the historical climate change trend.At the same time,we explore the tree growth,forest regeneration and tree resistance in semi-arid areas under the influence of drought,and analysis of the factors affecting tree growth and vegetation recovery after a forest fire.The main results are summarized as follows:(1)In the past three centuries,the southern edge of boreal forest has been in a state of drought for a long time,and the relatively humid year(the year of PDSI>0)only accounts for 10.1%of the entire reconstruction years.Extremely dry years frequently occur in the late 19th and early 20th centuries,and drought changes in the region are affected by multiple climate models(e.g.,East Asian monsoon,El Nino,etc.).The significant cycles(2.1-3.3 Years,5.9-6 years and 85-92 years)of our reconstructed summer PDSI series have been detected by our study of the southern edge of boreal forest.(2)In the southern edge of the boreal forest,The arid timberline forest has a relatively stable radial growth trend and can be recruitment in time after the forest dead,while arid treeline forest are subjected to more drought stress and the tree growth is declining.And the forest lacks effective renewal potential.It is necessary to consider the combined effects of factors such as climate factors and landscape patterns when discussing the tree growth and forest regeneration under the drought stress.These factors also change the trees resilience components(resistance,recovery,resilience and relative resilience).Large patch forests have higher resistance,small patch forests have higher resilience,but small patch forests also face a higher risk of death.The difficulty of regeneration caused by drought makes small patch forests show the community structure of young trees and lack of saplings and seedlings.It is predicted that small patch forests may die-off under the future drying trend in the boreal forest.(3)The surface permafrost conditions are important factors limiting tree growth in dry sub-humid on the southern edge of the boreal forest.The growth of trees on lack surface permafrost area is significantly higher than that of trees on the surface permafrost area.The release rate of tree growth on the slope sites after a forest fire is lower than valley sites where there was surface permafrost.Given that the higher moisture availability in lowlands after forest fire could increase water availability would contribute to faster growth rates compared with slopes.Therefore,there is a stronger growth release phenomenon.The interaction between surface permafrost and forest fire has long-term effects on tree growth after a fire.The increase of temperature may be one of the main reasons for the divergence of Larix gmelinii radial growth on two types of terrain.Forest fires in permafrost areas modified soil moisture conditions,leading to failure of tree regeneration and prompted some species tolerant of saturated soils(e.g.,Betula fruticosa Pall.)have become dominant.This effect of post-fire permafrost degradation leads to the divergence of vegetation recovery patterns in the valley and slope topographies.In summary,forests in southern edge of the boreal forest face multiple effects by drought and forest fires and have a higher potential risk of mortality,which is likely to increase under the future climate change.The cessation of forest regeneration and weakening of regeneration potential in arid timberline,as well as the failure of regeneration of burned area effect by permafrost degradation in the arid sub-humid region,are all direct manifestations of forest degradation under the influence of climate change.