Study On Landscape Pattern Dynamics And Driving Forces In Yan'an Urban Forest

Urban forests are complexecosystems of artificial and natural landscapes.Elucidating the urban forest landscape pattern is helpful to understand the ecological processes occurring in this system.Landscape pattern is an important component of urban forest quality,and is the premise to evaluate the quality level of urban forest objectively.In this study,urban forest around Yan'an city was selected as the object for the research.Taking dynamics and their driving forces of landscape pattern as the main line,a landscape-level assessment system for urban forest quality was presented,based on the characteristics and leading functions of urban forest in loess hilly-gully region.Seventeen evaluation indicators from the four aspects of urban forest,i.e.structure,pattern,health and function,were integrated into this assessment system.The landscape-level quality assessment of Yan'an urban forest was conducted by the above assessment system by being used Analytic Hierarchy Process?AHP?.To understand the spatial distributions of soil nutrients and their comprehensive fertility,the soil nutrients were predicated by kriging interpolation,and their comprehensive fertility were evaluated based on the Nemerow fertility index.The mosaic diversity and the variations in forest landscape pattern were uncoveredby affinity analysis and landscape pattern analysis,respectively.For the purpose of characteristics of interannual and seasonal changes in vegetation coverage,ecological quality and forest disturbance,the long-time sequences of normalized vegetation index?NDVI?,vegetation fraction?FV?,remote sensing ecology index?RSEI?,disturbance index?DI?and integrated Z score forest index?IFZ?were analyzed.To reveal landscape connectivity of Yan'an urban forest,a hybrid method of schema theory combined with habitat availability index were used to evaluate the threshold distance,to implement overall assessment,prioritization ranking,and functional type analysis of 1852 natural forest habitat patches,with a total area of 168.18km².The spatial and temporal variations in land use/cover change?LUCC?and land surface temperature?LST?with their relationships were studied by using remote sensing index and landscape pattern index,meanwhile,the relationships of green space cold island intensity?GSCI?between the number of tree species?TSN?,leaf area index?LAI?,the volume of space occupied by the stems and leaves of living plants?LVV?and landscape shape index?LSI?were expounded,the effects of LUCC on LST and the relationships between forest configuration structure of green space and GSCI were also revealed,based on supervised classification,remote sensing quantification,plant community survey and temperature/humidity measurement.According to functional zoning,forest category and dominant tree species,annual average values of the carbon oxygen release,water conservation,soil conservation,nutrient accumulation,purification of atmospheric environment and biodiversity protection were analyzed by 1 km×1 km spatial grids,the spatial differentiations of these main ecological function value of Yan'an urban forest was revealed.The results and conclusions could provide a basis for dynamic management and optimized decision of urban forest.The main conclusions are as follows:?1?The overall quality of Yan'an urban forest was in medium level through comprehensive evaluation.moreover,there is still room for the improvement in the coverage of urban trees and shrubs,noise reduction and SO₂ absorption of urban areas,the area of natural forest and the above-ground biomass of suburb forest.?2?The accuracy of regression kriging?RK?in predicting total nitrogen?0-20 cm and 40-60 cm soil layers?,organic matter?0-40 cm soil layers?and available phosphorus?20-60 cm soil layers?is higher than ordinary kriging?OK?,while the other soil chemical properties are suitable for OK interpolation.The comprehensive fertility of most areas in the study area was poor.Available nitrogen,total nitrogen and organic matter are detected to be the limiting factors in soil comprehensive fertility.?3?The interannual mean values of NDVI,FV,RSEI,DI and IFZ in four seasons showed similar fluctuation rules.On forest disturbance identification,IFZ is better to identify small disturbance because that it is more sensitive than DI.However,both IFZ and DI can identify disturbance enhancement regions,such as,Yan'an city new areas,the areas along the Yanhe river and Duchuanhe river.The significantly enhanced forest disturbance mainly occurred in summer.In each four seasons from 1990 to 2017,the area of moderate and above improvement of NDVI accounted for more than 85%,the northern part of the study area had low vegetation coverage.In the next 30 to 100 years,the improvement of forest ecological quality will be significant,and the improvement of spring and winter is stronger than that of summer and autumn,according to the prediction of remote sensing ecological index time series.?4?The variations in land surface temperature?LST?and land use/cover?LULC?in terms of spatial-temporal variations were synchronous.The high LST value was usually distributed in construction land,agricultural land and unused land with low vegetation coverage.LST is significantly positive correlated with a new index-based built-up index?IBI?,and showed a prominently negative correlation with the normalized difference vegetation index?NDVI?,vegetation coverage?FV?and modified normalized difference water index?MNDWI?.Urban green space can alleviate urban surface heat island,and made LST decreased by 0.67⁸.57?.The GSCI was positively correlated with LAI,TSN and LVV,but negatively correlated with LSI,and the GSCI of mixed forest is higher than that of pure forest.?5?From 1990 to 2015,Yan'an urban forest coverage is improved significantly.The following results were at class level of forest patches:The Quercus wutaishanica forest patches,Robinia pseudoacacia forest patches and softwood broad-leaved forest patches are the top 3forest patches ranked by their total area.The total area of the forests in terms of main dominant tree species,including Pinus tabulaeformis,Robinia pseudoacacia,Quercus wutaishanica,and betula platyphlla,were always increasing.Among them,the expanding intensity of Robinia pseudoacacia forest patches was higher than other types.The economic forest patches comprised of Malus pumila forest patches,Zanthoxylum bungeanum forest patches,Armeniaca vulgaris forest patches,and Juglans regia forest patches changed with fluctuation.The Quercus wutaishanica forest patches and Platycladus orientailis forest patches are the top 2 forest types ranked by landscape core area,while their temporal variations are in opposite direction.The edge density of most types of forests showed an upward trend of different degrees,which indicates that the landscape presents of Yan'an urban forest had a fragmentation trend.The connectivity of all types of forest landscape is decreasing.At landscape level,the changes in edge density,area weighted contiguity index,disjunct core area density,interspersion juxtaposition index,Simpson's diversity index all showed a trend of first increase and following decrease,which suggested that the connectivity of forest landscape changed from weak to strong.However,the Shannon's evenness index was continuously decreasing,which indicated that the dominance of landscape was increasing,and forest landscape was dominated by one or more dominant patch types.?6?The species richness of tree layer was low,while shrub layer and herb layer were high in Yan'an urban forests,of which most forest types had a richness of species.The plant species mosaic diversity?m=5.54357?among 20 types of forest patches was above 3,indicating that the vegetation spatial pattern of Yan'an urban forests was complex and there was no significant ecological gradient.As the model sites of forest patches,there are most of the common species in the Betula platyphylla-Populus davidiana mixed forest patches,Betula platyphylla forest forest patches,Pinus tabuliformis forest patches and Acer buergerianum-Populus davidiana mixed forest patches,so these four types of forest patches represented the most environment types in study area.Armeniaca sibirica forest patches,Hippophae rhamnoides shrub patches,Malus pumila forest patches and Quercus wutaishanica-Platycladus orientalis mixed forest patches are outliers.The other 12 types of forest patches were generally distributed in the study area and occupied most of the habitat space.?7?The study showed that the optimal threshold distance for the analysis of natural forest habitat patch composition,patch priority and functional analysis was 300 m.The first component?C1?was identified as the most important component in the whole landscape,in which P5,P9,P11,P57,P67,P84,P139,P337,P352 and P363 were identified as the key patch types to maintain the whole landscape and habitat connectivity.P67 had the best connecting function,P363 had the largest internal habitat area,while P273 had the best fluxing function.Important components and patches,and preferential protection/extension sites were also identified.?8?In 1990-2015,the main changing trajectories of LULC and forest landscape types were:farmland transferred to building land,forest land and grassland,forest land transfered mutually with grassland.Robinia pseudoacacia forest patches transferred mutually with shrub patches,Robinia pseudoacacia forest patches transferred to Populus davidiana forest patches and Platycladus orientalis forest patches transferred to Robinia pseudoacacia forest patches.Forest land was the main part of LULC,while Quercus wutaishanica forest patches,Robinia pseudoacacia forest patches and other shrub patches are the main types of forest patches.?9?The distance to roads had significant impact on the transformation of construction land,farmland,forest land and grassland,especially the railway expansion had a positive impact on the expansion of construction land,farmland and forest land.In 1990-2005,the changing probability of forest patch types had negative correlations with soil fertility in 40-60 cm soil layers,the cosine value of slope directions and the distance to water.In 2005-2015,the change probability of forest patch types had positive correlations with elevation,soil fertility in 0-20cm soil layers and sine value of slope directions,and had negative correlations with population density,GDP and the distance to residents.In addition,there is a significant linear correlation between the change area of 5 land use/cover types?except unused land?and macroeconomic indexes.Farmers'cognition,land control policies,conversion of farmland to forest,development of new areas and strong rainstorms were the qualitative factors that influenced LULC changes.The selection preference of afforestation tree species was the main qualitative factor that influenced forest patch types change.