Effects Of Post-disaster Reconstruction On The Landscape Eco-environment Of Lushan County And Its Water Quality Response

As a typical type of human disturbance,reconstruction activities after the“5·12”Wenchuan Earthquake in 2008 and the“4·20”Lushan Earthquake in 2013 have had a great impact on the ecological landscape pattern of Lushan County.The post-disaster reconstruction activities mainly include the following:development of residential community,rehabilitation of arable land,infrastructure building and protection engineering.The post-disaster disturbance ctivities have imposed both positive and negative effects on the ecological restoration of the earthquake-stricken areas in Lushan County,which effects are highlighted on small scales.In addition,the reconstruction is proved to have a lasting impact on plaques,corridors and matrices,the three major elements of landscape.Located in the western margin of the Sichuan Basin,Lushan County borders on Sichuan Basin on the east and Qinghai-Tibetan Plateau on the west.Longmenshan Fault runs through the county,producing large slopes,complicated geological structure in the area.With91.25%of mountainous area,the county has many rivers and enjoys a high vegetation coverage,as well as diversified biological resources.The Lushan River flows through the whole region,and joins the Qingyi River and then reach the Minjiang River Basin.The Lushan River is one of the upstream tributaries of the Yangtze River and serves as an important barrier to the eco-security in the upper reaches of the Yangtze River of China for its great role in the supply of ecological services.In addition,the Min Mountains and Qionglai Mountains,as the key ecological conservation areas,are the world biodiversity hotspots and key region for biodiversity conservation of China.Against such drop,the thesis takes Lushan County as the subject,and is mainly dedicated to（1）analysing correlation between land-use and post-disaster reconstruction,with the aim to determine the effects（positive or negative）of post-disaster reconstruction on the land-use pattern in Lushan County;（2）exploring land-use change pattern under the influence of rebuilding activities in Lushan County by introducing human disturbance degree model and land-use dynamic change degree model（LC）;（3）determining the change pattern of ecological environment quality after rebuilding through a reasonable evaluation of the ecological environment restoration subject to human disturbance with the help of the multiple ecological factor calculation model under the PSR evaluation system;（4）detailing the spatiotemporal response mechanism of water quality in the Lushan River Basin in relation to land-use pattern change before and after the rebuilding activities,in an effort to comprehensively understand the characteristics of the effects of post-disaster reconstruction on Lushan county;（5）giving suggestions on the optimization of the current land-use pattern and spatial expansion of Lushan County based on the spatial distribution situationof ecological resistance through introduction of MCR model to carry out an ecological resistance simulation analysis from the perspective of land-use expansion,soil and water conservation and protection of water body respectively.Findings of the research are as follows:（1）Impacts of post-disaster reconstruction activities on landscape pattern change and driving mechanismLand-use pattern of Lushan county has witnessed the following changes as a result of rebuilding activities after the earthquakes:land-use type pf a lagre amount of land has changed;construction land and arable land see the largest increase while the area of forest land and grassland decrease continually,which are mainly transferred into arable land;the spatial distribution of arable land displays a clear northwards movement;bare mountainous areas are improved and replaced by artificial vegetation to form a new type of landscape;urban development demands are the source power for the change of other landscape patterns.LC of 2008 and 2013 are the highest,and as the warpping of post-disaster construction,LC sees a continual decrease,with the whole dynamic change degree of land-use（LC）takes a“wavy”shape.The bare mountainous area,arable land,construction land and residential land witness the most significant change,and all the aforesaid four types of land-use are positively correlated with human disturbance and other types of land-use negatively.The stronger the human disturbance is,the greater the PD and LPI becomes,and vice versa.The change trend of FN and SHDI is positively correlated with human disturbance,while that of DI,CONNECT,CONTAG and SHEI negatively,indicating a strong relationship between landscape indicaces and human disturbance.From 2008 to 2012,areas that were dominated by severe human disturbance are converted to that dominated by slight human disturbance.From 2013 to 2017,areas that were dominated by extremely human disturbance were converted to that dominated by moderate and slight human disturbance.The change trend of two human disturbances are the same—bad landscapes were gradually“encompassed”and“disintegrated”,which means that human disturbance has positively influenced the ecological restoration in the earthquake-stricken area.However,considering the decrease in the area of forest land and grassland,it is reasonable to conclude that human activities may also impose negative influence on the ecological environment.（2）Improvement and change characteristics of ecological environment quality before and after post-disaster constructionAfter two post-disaster reconstructions,the eco-environment of Lushan County was restored and improved to a satisfactory level.In 2008 and 2013,the areas whose eco-ebvironment rated as good takes the leading role,while that rated as secondary and poor are in the majority,reaching 29.84%and 41.25%respectively,and are distributed along the Lushan River and its tributaries and the two sides of the roads,particularly in such areas as tributary colonies of Lushan River in west Lushan County and vicinity of urban areas in south Lushan County.In 2012 and 2017,the areas whose eco-environment rated as secondary and poor takes up 60.76%and 63.07%respectively.Comparatively,the ecological quality of northern Lushan County is slightly affected.Characteristics of ecological quality changes vary with the change of elevation,slope grade and aspect of slope.On the whole,the water and soil erosion of areas along the two sides of the Lushan River where the slope grade is smaller than 10°is slight with the vegetation coverage rate at 40%-80%;and that of areas where the slope grade is between 10°and 30°see increasingly severe condition,with the vegetation coverage rate at 20%-40%,and when the slope grade is greater than 30°,the water and soil erosion situation is the worst with the vegetation coverage rate at 10%-20%.The vegetation coverage rate for areas where the aspect of slope is west and northwest ward is between 10%and 20%,and that for north and northeast ward is between 20%to 40%,and for east and southeast ward,the number is between 40%and 60%,and that number for south and southwest ward is between 60%and80%.The water and soil erosion situation in the areas 2400-4000 meters above the sea level is the slightest,with vegetation coverage rate at 60%-80%.Areas where the elevation is between 800 meters and 2400 meters,see an increasingly degree of water and soil erosion,with vegetation coverage rate at 20%-40%.In the case of elevation lower than 800 meters,the condition is the worst,with vegetation coverage rate at 10%-20%,and the same of most areas where elevation is higher than 4000 meters above the sea level is lower than 10%.However,the water and soil erosion situation is slight due to the relatively low rainfall in the areas of high elevation area compared with that in the valley plain.The change degree of ecological quality varies as the distance between the subject area and the source of influence changes.The farther away from the river banks near the epicenter,the smaller the bare mountainous area,the better the ecological environment quality.The closer to the river banks near the epicenter,the more exposed the mountainous area,the worse the ecological environment quality;especially in the area within 200 meters away from the river near the epicenter.The closer to the construction land and residential land,the lower the vegetation coverage.Most of land in the aforesaid areas are transferred to arable land and grassland with relatively low forest coverage.The farther away from the construction land and residential land,the higher the vegetation coverage,and the better the ecological environment.The ecological quality increases incrementally as the distance away from the center increases:the constructed urban area increases with each additional 200meters and when the distance is greater than 1000 meters,the ecological quality is the best.Compared with 2008,areas where eco-environment quality rated as good see an increase of 25.02%in 2012,and in 2017 an increase of 27.75%compared with 2013.Generally speaking,ecological restoration after the Lushan earthquake is better than that after the Wenchuan earthquake.Through twice ecological restoration works,the eco-environment of Lushan county has been greatly improved,and the number of secondary disasters is effectively reduced,with the decrease of areas where eco-environment quality is assesed as secondary and poor,and great increase of arable land and improvement of vegetation coverage.（3）Response mechanism of water quality in relation to land-use pattern change in Lushan County under the influence of post-disaster reconstruction activitiesLand-use pattern in the Lushan River Basin has a significant impact on the water quality.The“sink”landscape represented by forest land,wetland and grassland is negatively correlated with pollutant concentration indexes of water,while the“source”landscape represented by arable land and construction land and LC are negatively correlated with the water quality indexes:the higher the proportion of forest land,grassland and wetland area,the lower the pollutant concentration,and the larger the proportion of construction land and arable land,and the higher the pollutant concentration indexes.The increase of forest land,wet land and grass land,as well as appropriate application of other types of land greatly improves the water quality of Lushan River,and the increase of arable land and construction land negatively impacts the water quality,with area 4#,5#and 6#in the downstream of the Lushan River Basin delivering the greatest correlation.The landscape pattern of the Lushan River Basin has a significant impact on the water quality.From the perspective of landscape pattern,the more the number of plaques,the greater the fragmentation and diversity of landscape,the higher the concentration of pollutants in the water:it is found that PD,FN and SHDI are positively correlated with water quality indexes,and LPI and CONTAG negatively.In addition,area#3,4#,5#and 6#are all identified with coexistence of positive correlation and negative correlation in terms of relations between SHDI and CONTAG and water quality indexes.River water quality in Lushan River Basin differs greatly with the change of time and place.Land-use pattern has a greater impact on water quality in the dry and wet seasons than in the normal water season:the land-use pattern changes in the area 4#,5#and 6#and other downstream areas of the river significantly influence the water quality;the land-use pattern of area 5#and 6#in the dry season is highly correlated with water quality,and in the normal water season,the land-use pattern of the above-mentioned areas is significantly correlated with water quality only in the year of 2008 and 2013.From 2008 to 2017,significant correlation is identified between land-use pattern and water quality in the west season,and less significant in the dry season,and least significant in the normal water season.Among all pollutant concentration indexes,COD_Mn,NH₃-N and TP are the ones that demonstrate with the most significant correlation with the land-use pattern in the wet season.The correlation between the spatial location of area 1#,2#and 3#in the upstream of the river and water quality is clearly weaker than that between area 4#,5#and 6#in the downstream of the river and water quality,especially in the year of 2013 and 2017 with the area#5 and#6 displaying the most significant correlation,and the greater the human disturbance,the worse the downstream water quality of area 4#,5#and 6#in the Lushan River Basin.In the other words,human disturbance is positively correlated with deterioration of water quality,and the increase of land area of construction land and arable land has a negative impact on the improvement of water quality.（4）Optimization and spatial expansion of landscape pattern in Lushan CountyBased on effects brought about by post-disaster rebuilding on the eco-environment,landform and topography,vegetation coverage,land-use type,water body and other ecological factors are categorized by ArcGIS10.2,and valued by AHP for further analysis of minimum ecological resistance from the perspective of land-use expansion,water and soil conservation and protection of water body under the MCR model.According to analysis,it is concluded that the land expansion is mainly hindered by landform and topography as well as potential geological disasters;the water and soil conservation works are mainly held back in the central-western and southern regions of Lushan County while protection works of water body in the urban and suburban areas of southern part of Lushan County.Based on aforesaid findings,suggestions are put forward to help optimize landscape pattern and future spatial expansion and development direction of Lushan County.This study,by exploring how the human disturbance of reconstruction actibities has improved and destroyed the landscape of Lushan county,and examining the characteristics of land-use change pattern subject to the influence of post-disaster reconstructio,and eco-environment response mechanism,can help solve ecological problems in the post-disaster reconstruction,and effectively avoid and reduce negative impact of human disturbance.For the eathquake-striken area,the study can provide a scientific basis for resource allocation,ecological optimization,and sustainable economic and social development,and is of guiding significance for ecological civilization development,and reconstruction works of other earthquake-striken areas in the future.Therefore,the study is not only of typical importance in the case of Lushan county,but also of theoretical and practical significance.