Landscape Dynamics And Its Causes Of Shivang River Basin

Landscape dynamics is the important part of environments changing analysis. As one ofthe three inland river basin in Hexi corridor, Shiyang river basin was experiencing drouth,salination, vegetation degeneration and obvious landscape change caused by natural and humandisturbers. Therefore landscape dynamics research could contribute to reflect the relationshipbetween disturbers and environments change. On the basis of landscape classificationaccording to the vegetation type,3periods landscape map from1974to2006of Shiyang riverbasin and6periods landscape map from1959to2006of Minqin oasis were obtained using RSand GIS technology.On completion of landscape classification and cartography, dynamicss andcauses of landscape pattern of Shiyang river basin since1959were analyzed using landscapeecological method.(1)Along with the increasing altitude, grassland, natural forest, sub alpine shrub and colddesert were distributed in the up reaches. Similar to up reaches, artificial oasis, salt vegetationand artificial forest together, gravel and lithoid desert and sandy desert were orderly distributedin Oasis Desert System with large area of saline desert and salt meadow distributed in the lowarea of the edge of sandy desert also. Gravel and lithoid desert,grassland, sandy desert andartificial oasis were the main landscape types of Shiyang river basin. Except natural forest,landscape types composed by local vegetation had well connectivity, whereas landscape typescomposed by alien vegetation only salt meadow and saline desert were well connected.(2)Landscape dynamics and pattern changes were different by regions. Compared withthe whole basin, up reaches was more fragmented, whereas middle reaches and lower reach'sstructure were simpler then the hole basin. From the east reaches to west reaches, salinationand landscape fragmentation aggravated, and that enhanced downwards the stream with theconnectivity of sandy desert and gravel and lithoid desert increased. Since1974, the landscapestructure tended to be complicated because of obviously increased fragmentation in the middle and lower reaches, and that enhanced from the middle to lower reaches; while the landscapestructure of the upper reaches tended to be simple, but with changing speed accelerated.Artificial oasis's area percentage increased5.96%, and middle reaches exceed lowerreaches. Gravel and lithoid desert decreased most obvious by2.32%, especially in middlereaches. Artificial forest decreased obviously by1.45%, especially in lower reaches. Temperatedeciduous shrub, artificial forest, salt desert and secondary salt land, the most severely losslandscape types, were reduced to the original0.02,0.58,0.64and0.68, with salt desert mostobvious in middle reaches and others in lower reaches. Due to mass of small patches increasedin expending of artificial oasis and reducing of gravel and lithoid desert, artificial forest, saltdesert and secondary salt land, their landscape pattern complicated while that of otherssimplified.(3)Landscape dynamics and pattern changed by stage. During1974-1995, grasslandconversed to dry farmland in up reaches; gravel and lithoid desert, dry farmland and artificialforest conversed to artificial oasis in middle reaches; in lower reaches, salt meadow, temperatedeciduous shrub and salt desert conversed to gravel and lithoid desert, salt meadow and graveland lithoid desert conversed to, artificial oasis and artificial forest, artificial forest conversed toartificial oasis, artificial oasis conversed to secondary salt land. Eventually, the area percentageof grassland decreased1.76%in up reaches; gravel and lithoid desert, dry farmland andartificial forest decreased4.24%,1.99%and81%and artificial oasis increased8.02%inmiddle reaches; in lower reaches, artificial oasis, secondary salt and artificial forest increased2.23%,0.02%and0.85%, and gravel and lithoid desert decreased1.74%.During1974-1995, dry farmland conversed to grassland in up reaches; gravel and lithoiddesert and salt desert conversed to artificial oasis in middle reaches; in lower reaches, artificialforest, secondary salt land and gravel and lithoid desert conversed to artificial oasis, artificialforest conversed to gravel and lithoid desert. Eventually, the area percentage of dry farmlanddecreased0.99%in up reaches; artificial oasis increased4.25%in middle reaches; in lower reaches, artificial oasis and artificial forest increased3.07%and1.43%, artificial forest andsecondary salt land decreased0.45%and3.31%.(4)Landscape dynamics changes was concentrated in oasis. Same as the lower reaches,water, salt meadow, temperate deciduous shrub, salt desert and gravel and lithoiddesert(before1995) were decreased obviously but artificial forest and secondary salt land wereincreased in Huqu, Quanshanqu and Baqu before1987, while artificial forest and secondarysalt land were decreased but artificial oasis and gravel and lithoid desert were increased after1987. Until2007, water and temperate deciduous shrub were disappeared and Secondary saltland, salt meadow and salt desert remained few in Baqu and Quanshanqu; gravel and lithoiddesert decreased3.46%and artificial oasis increased17.87%in Bqqu; gravel and lithoid desertdecreased1.26and artificial oasis increased12.395in Quanshanqu; secondary salt land, graveland lithoid desert and sandy land increased3.20%(1/2times)3.53%and1.40%but artificialoasis decreased0.11%and artificial forest decreased2.62%(1/2times) compare to1974.The landscape structure of artificial oasis, artificial forest and gravel and lithoid deserttended to be complicated in, Baqu and Quanshanqu; salt desert fragmentation aggravated andonly few salt meadow patches were left in Baqu; only few salt desert patches were left inQuanshanqu; in Huqu, artificial forest and salt desert fragmentation aggravated and thelandscape structure of salt meadow and secondary salt land tended to be simple due to lossmass of small patches, but the connectivity of gravel and lithoid desert was increased.(5)The causes of landscape dynamics and landscape pattern changes were studied byanalysis Influence factors change. Climate tended to be dry and warm since50`s, although therainfall increased but the temperature was raised simultaneity since70`s led to water resourcesdecreased which had negative affect to vegetation, therefore, salt vegetation and natural forestdecreased, but artificial oasis increased proved that human disturbances were the main reasonof landscape dynamics. On account of building Hongyashan reservoir in50`s, salt vegetation inMinqin oasis decreased quickly before1987. The expansion of artificial oasis in middlereaches led to the shortage of water resources which resulted in secondary salinization and artificial forest's degradation in lower reaches subsequently. But the artificial oasis increasedsince80`s and gravel and lithoid desert increased in lower reaches since90`s owing tosalinization weakening, groundwater exploitation and planting of high price, salt and droughttolerable economic crops in lower reaches.