| Gannan region is located in the southern Jiangxi Province,China.It has hilly land resources and its climate conditions are beneficial to plant citrus.With the support and guidance from local government,Gannan region has experienced the boom of citrus planting and has become the largest citrus production region in China over the past decades.Citrus orchard exploitation is a typical land use change process which can impact terrestrial ecosystem services both locally and globally.Despite the economic success,the rapid and extensive citrus orchard expansion has brought great concern about ecological consequences.In this study,satellite-based remote sensing technology was employed to monitor citrus orchard dynamics in both Gannan Region and Xunwu County with a long-time span.Furthermore,some typical landscape index models were applied to quantitatively assess and analysis the fragmentation process for main landscapes in study areas.The main parts of this study includes:(1)Monitoring of citrus orchard dynamics for Gannan RegionThe objective of this part is to map the citrus orchard in 1990,1995,2000,2005,2010 and 2016 in Gannan Region.An image composite method was applied and total2140 tiles of Landsat historical images were employed to generate seasonal images with lowest cloud composite at the pixel level.Random Forest classifier was used to classify multiple dimensional features from spectral,spatial and topographic domains.The image composite and classification were implemented with Google Earth Engine(GEE)platform.Results showed that:(i)GEE can effectively execute complex workflows of remote sensing data processing and information digging.(ii)Lowest cloud composite at the pixel level is a reliable method of producing clear seasonal images for the region influenced by cloud and rain frequently.(iii)Random Forest classifier was suitable for mapping citrus orchard with an average overall accuracy(OA)of 93.15%and a Kappa coefficient of 0.90.(iv)The citrus orchard has expanded extensively with the area from9.77 km~2 in 1990 to 2200.34 km~2 in 2016.Citrus orchard was mainly converted from forest,bush and cropland.(2)Monitoring of citrus orchard dynamics for Xunwu CountyIn this part,using the historical Landsat repository from 1990 to 2016,a framework was developed for mapping annual citrus orchard extent and tracking its long-term dynamics in Xunwu County,China.The results suggested that the average Overall Accuracy(OA)of original annual mapping were 87.73%,and its performance was further improved after the temporal filtering approach(91.46%).Several features(e.g.elevation,slope,NDVI)played more important roles in citrus orchard identification.With the achieved annual mapping layers,we found a rapid citrus orchard expansion trend during the study interval(i.e.,from 22.18 km~2 to 697.21 km~2).Moreover,this expansion process was unevenly distributed in time.Spatially,emerging citrus orchards were primarily transformed from forests and croplands,and mainly distributed in areas with elevations from 200 m to 500 m and slopes range from 5?to20?.(3)Impact assessment on landscape patternNumber of patches,mean patch area,patch density index and patch shape fragmentation index were chosen to quantitatively assess and analysis the fragmentation process for main landscapes in study areas.The results for Gannan Region indicated that the fragmentation process for citrus orchard was extensively aggravated from 1990 to 2016.The variation of fragmentation process for cropland,forest and grass were rarely obvious but at a high level.The results for Xunwu County showed that the fragmentation process for citrus orchard was extensively aggravated from 1990 to 2006 and maintained steady after 2006.The fragmentation for cropland and forest were aggravated to different extent and all with a high level. |
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