The Collaborative Mechanism Of Vegetation-Soil Restoration In Burned Area Of Daxing'anling

The Daxing' anling forest is not only an important barrier safeguarding the ecosystem of northern China region,but also a key strategic timber reserve base of China.Forest fire is one of the main factors causing forest degradation and affecting multiple functions of the forest.The restoration of the forest ecosystem and the improvement of its functions after the fire disturbance are the key and hotspot issues of study.In this paper,the burned areas in the northern forest area of Daxing'anling were taken as the research objects to study the vegetation-litter-soil restoration processes and the main driving factors in burned areas with different fire severity by statistical methods of correlation analysis,redundancy analysis,path analysis,and structural equation modeling based on the Landsat series of remote sensing data and field survey data,which further revealed the collaborative mechanism of vegetation-litter-soil restoration,providing theoretical basis and technical support for the vegetation restoration and reconstruction in burned areas.The main research findings are as follows:1.The natural succession process of vegetation on burned areas in the northern forest area of Daxing'anling is mainly as follows:it was dominated by shrubs in the year of moderate fire disturbance,and then became a mixed forest of Larix gmelinii-Birch platyphylla(70%Larix gmelinii and 30%Birch platyphylla)15 years later;in the year experiencing severe fire disturbance,it was dominated by herbaceous,then formed shrubs 8-15 years later,and turned to be a mixed forest of Larix gmelinii-Birch platyphylla(50%Larix gmelinii and 50%Birch platyphylla)30 years later.The mild fire disturbance can promote the vegetation succession,but the burned areas experiencing moderate/severe fire disturbance need 15-30 years for natural restoration.2.Vegetation coverage decreased significantly after different severity of fire disturbances.It firstly underwent a 4-year rapid restoration period,but then tended to be stabilized.With the increase of disturbance intensity,the changes of vegetation coverage showed significant hysteresis and instability:1-10 years after the mild fire interference,it underwent anti-continuous and relatively stable rapid restoration,and then turned to continuous and slow restoration during 10th-30th years;1-10 years after the moderate fire disturbance,it underwent anti-continuous and unstable significant restoration,but then turned to continuous and stable rapid restoration during the period from the 10th to 30th years;and 1-20 years after the severe fire disturbance,it underwent anti-continuous and unstable rapid restoration,but then turned to continuous and stable rapid restoration during the period from the 20th to 30th years.3.After mild fire disturbance,the diameter structure showed a right skewed peak shape,having no significant changes compared to that of the no-disturbance situation;after the moderate/severe fire disturbance,the diameter structure showed a right skewed peak shape,and the proportion of large diameter(14-20cm)trees was 68?100%.30 years later,the renewal trees grew to have 6-10cm diameter with diameter structure of left skewed peak.The undisturbed forest was composed of Larix gmelinii and Birch which were in clustered distribution.But after fire disturbance,their clustering intensity increased significantly.Fire disturbance promoted the renewal.And the renewal seedlings were in clustered distribution.With the increase of disturbance intensity,the renewal speed slowed down.After the fire disturbance,the invasion of light-loving plants,such as Betula fruticosa,Rhododendron dauricum,and Epilobium angustifolium,enriched the diversity of understory vegetation,among which,the Betula fruticosa,Rhododendron dauricum,and Epilobium angustifolium actually dominated in communities of areas experiencing moderate/severe fire disturbance.4.After mild/moderate/severe fire disturbance,the time for the biomass restoration of the herb layer,shrub layer,and tree layer were respectively 2 months,3-8 years,and 30 years.After mild/moderate/severe fire interference,the existing litter fell by 56.58%.87.23%,and 99.37%,respectively,and did not restore within 30 years.5.As for the non-disturbed litter,the contents of TC,TN and TP were 322.56?442.78g/kg,19.37?27.43g/kg,and 0.25-0.51 g/kg respectively.Compared with the control plots,after the mild/medium/severe fire disturbance,the content of TP increased by 10?18%,33?75%,and 50?100%,and the contents of TC and TN decreased by 4?7%,10?16%,30?39%,and 8?11%,15?20%,40?52%respectively.Compared with the control plots,after the mild/moderate/severe fire disturbance,the litter C/P decreased by 5-19%,22-34%,and 36-60%respectively.After mild fire disturbance,the soil TOC,TN,TP,TK,and AP contents increased by 30?33%,1?2%,2?9%,1?4%,and 15?17%respectively,and returned to situation the same as that before the fire within 30 years.After moderate fire disturbance,the contents of soil TOC and TN decreased by 11?18%and 24?28%,while the contents of TP,AP,TK,and AN increased by 6?7%,15?16%,19?23%,and 2?4%respectively.After severe fire disturbance,the contents of soil TOC,TN,and AP decreased by 47?51%,21?24%,and 11?16%respectively,while the contents of TP,TK,and AN increased by 33?35%,41?48%,and 29?34%,among which,the AP content didn't restore within 30 years while other nutrients resumed back to situation the same as that before the fire within 30 years.After the fire disturbance,soil C/N increased by 48?53%,27?32%,and 1?2%,and the C/P decreased by 0?3%,6?8%,and 61?64%.Fire disturbance accelerated the phosphorus release in litter and promoted the mineralization of nitrogen and phosphorus in the soil.6.After the mild/moderate fire disturbance,the bacterial community diversity decreased by 5?6%and 9?10%,while the fungal community diversity decreased by 7?8%and 8?10%;after severe fire disturbance,bacterial community diversity decreased by 9?11%and was dominated by Proteobacteria and chloroflexi species,while the fungal community diversity decreased by 12?14%and was dominated by Archaeorhizomycetales,which can recover within 30 years except for severe fungi.After fire disturbance,soil carbon fixation,lignin degradation,organic nitrogen mineralization,and organic phosphorus hydrolysis were all enhanced,but the denitrification was weakened.Forest fires play positive role in soil carbon,nitrogen,and phosphorus circulations to some certain extent.7.The vegetation-soil showed positive interaction effect:the nutrient element accumulation in the soil is beneficial to the growth of surface vegetation and the accumulation of nutrient elements in litter,thereby promoting the restoration of the soil nutrients;being driven by both the vegetation and the soil,the soil microorganisms jointly participated in nutrient circulation,which promoted the restoration of soil and vegetation.Among the above,the soil TN,TOC,and AP contents are the main driving factors for vegetation restoration,while the soil function restoration is driven by the biomass of trees,shrubs and grasses,the existing amount of litter,and the community structure of soil microorganisms.After the fire disturbance,the restoration of vegetation community is faster than the soil development(succession synergy index>1).8.The habitats and natural succession laws of burned areas with different fire severity were considered comprehensively according to the main driving factors for the restoration of plants and soil functions,based on which,the technical models for fast artificial restoration and function improvement,such as tending and improving of woods,manual promoting of natural renewal,soil regulation,and mycorrhizal inoculum,were proposed in order to promote succession progress.