Vegetation Succession And Forest Fire History Over The Past 1680 Years In The Lake Haixihai Catchment Of Northwest Yunnan

During the past 2,000 years,human activities intensified signigicantly,and thus environmental changes driven by nurural forcings converted into those driven by both natural and anthropogrniv fovings.At the same time,studying the relationship between human activities and past climate and environmental changes can provide important evidence for explaining the impact of human activities on vegetation and forest fires and predicting future environmental changes.Lake Haixihai is located in northwest Yunnan,where the vegetation,climate,environment and other environmental elements are strongly affected by the South Asian monsoon,and the ecological environments of the lake catchment is thus sensitive to environmental changes.In this study,three AMS¹⁴C dating data from the 294 cm HXH-1-1 sediment core in Lake Haixihai were used to establish the Bayesian depth-age model of core HXH-1-1.The high-resolution pollen and charcoal analyses of core HXH-1-1 were conducted to reconstruct the vegetation succession and forest fire history and to reveal their driving mechnisms in the Lake Haixihai catchment over the past 1680 years were reconstructed.The results showed that the vegetation and forest fires in the Lake Haixihai catchment underwent the following six-stage of changes over the past 1680 years:The first stage（294-265 cm;340-560 AD）:The vegetation is mainly composed of coniferous forests（Pinus and Tsuga）and evergreen/deciduous broadleaved mixed forests（Quercus（E）,Cyclobalanopsis,Rosaceae,Alnus）,with high vegetation coverage as shown by its proxy,i.e.,total pollen influx value.The average charcoal influx values of this stage were at the minima of the study period,indicatingthe lowest frequency and intensity of forest fires over the past 1680 years.This may be related to cold and wet climate conditions.The second stage（265-185 cm;560-1050 AD）:The range of coniferous forests increased significantly,and the range of deciduous broadleaved forests and evergreen broadleaved forests shrank.The contents of Pinus and Tsuga in coniferous forests increased significantly;among deciduous broadleaved trees,Rosaceae,Alnus,and Betula decreased markedly,and the other species were still common;among evergreen broadleaved trees,Quercus（E）,Cyclobalanopsis,and Lithocarpus/Castanopsis exhibited a significant decerease.Overall,the vegetation amount of this stage increased significantly,and the vegetation coverage was the densest in the past 1680 years.At the same time,the influx values of various charcoal particle sizes in this stage increased significantly,suggesting that the frequency and intensity of forest fires enhanced significantly.This may be assiciated with changes in the natural environments and the strengthening of human activities.The third stage（185-99 cm;1050-1600 AD）:Coniferous forests shrank significantly,as suggested by the decrease of Pinus and Tsuga trees;deciduous broadleaved forests increased slightly,as indicated by a significant increase of Alnus trees and a small increase in Rosaceae trees and shrubs;evergreen broadleaved forests dominated by Quercus（E）,Cyclobalanopsis,and Lithocarpus/Castanopsis were as common as those in the previous stage.However,the total pollen influx values at this stage decreased sharply to the minima over the past 1680 years,indicating that the vegetation coverages reduced to the least density.We believe that a vegetation landscape similar to today was formed around 1050 AD.The charcoal influx valuesof various particle sizes also greatly reduced,implying a great decrease in the frequency and intensity of forest fires probably due to huge reduction in combustible materials.Again,a large reduction in vegetation coverages forest fires were possibly the result of the combination of natural and anthropogrnic forcings.The fourth stage（99-65 cm;1600-1770 AD）:Coniferous forests continued to shrink,and deciduous broadleaved forests increased as indicated bythe increase of Alnus and Betula trees.Compared with the previous stage,the vegetation coverages rebounded to a certain extent,especially Poaceae plants and secondary Alnus forest.Since both of them have of great significance to human activities,the increase of their vegetation coverages implied significantly enhanced human activities.The charcoal influx of various particle sizes also rebounded slightly,but still at a low level,indicating that the frequency and intensity of forest fires at this time were still low.Vegetation succession and forest fires at this stage were driven by the combined influences of human activities and natural changes.The fifth stage（65-23 cm;1770-1950 AD）:Pinus forests gradually decreased,while evergreen broadleaved forests and deciduous broadleaved forests increased significantly.As suggested by the total pollen influx values,the biomass and vegetation coverages decreased slightly from the previous stage.Among them,the expansion of deciduous and evergreen broadleaved forests were characterized by a significant increase in Alnus and Betula as well as Quercus（E）,Cyclobalanopsis,and Lithocarpus/Castanopsis,respectively.At this stage,the charcoal influx values of each particle size showed a significant increase,even exceeding those at the second stage,implying that the frequency and intensity of forest fires increased significantly.It is evident that both vegetation succession and forest fires at this stage were mainly affected by human activities instead of natural forcings.The sixth stage（above 23 cm;1950 AD to the present）:Pinus forests continued to decline significantly,whereas evergreen and deciduous broadleaved forests increased significantly,which were marked by the expansion of Quercus（E）,Cyclobalanopsis,Lithocarpus/Castanopsis,Alnus,and Betula.The slight increase in biomass occurred in this stage.the total charcoal influx values were at the maxima of the study period,indicating the occurrence of the most frequent and intense forest fires.At this stage,human activities played a major role in the processes of vegetation dynamics and forest fires.In conclusion,the driving factors of vegetation succession and forest fires in the Lake Haixihai catchment gradually changed from climatic factors to human activities during the past 1680 years.Among them,vegetation succession and forest fires in 340-560 AD were mainly affected by climatic factors;560-1770 AD was the period when climatic factors and human activities worked together,and the role of human activities gradually increased;after 1770 AD,human activities became the main driving force of vegetation succession and forest fires.