Forest And Grassland Coverage Change In The Yellow River-huangshui River Valley In Northeast Qinghai-tibet Plateau In The Qing Dynasty And The Republic Of China

The interaction between the land surface forest,grassland,cultivated land conversion affects the pattern of the surface and the terrestrial ecosystem carbon balance.Forest and grassland vegetation coverage of local and global carbon role play an irreplaceable role.In the past human's land use activities on the original cover change become the key factors to affect the land-carbon flux between the atmosphere and surface albedo.Thus,it was regarded as an important part in the study of global environment change.The Yellow River-Huangshui River Valley(YHV)located in the northeast edge of the Qinghai-Tibet Plateau is a typical farming-pastoral area.Since the Qing Dynasty,the expansion of cultivated land has led to a significant change in forest and grassland cover in this region.Reconstruct the change of the forest and grassland cover,it can not only depict typical regional human-earth relationship,but also can provide basic data of data of global environmental change research.Base on the current vegetation map,terrain factors,and a large amount of historical data in literature,this article reconstructed the potential forest and grassland spatial distribution in the Yellow River-Huangshui River Valley in 300 years ago.Reconstructed the spatial pattern of cropland in the six key point time in the historical period.In the historical period,human reclamation activity must be at the cost of the original cover,so deducted the area of cultivated land form the potential vegetation distribution in each grid and in each period.Combined with the reconstructed change of cropland in historical period,we calculated the change of the forest and grassland coverage.On the basic of this,explored the mechanism between human activity and the changes of the forest and grassland.Finally the following conclusions were obtained:(1)Before cultivated land expansion in the Qing Dynasty the spatial distribution of forest and grassland was consistent with present day vegetation distribution.Thearea of forest was slightly lager,the shrub land was spatial more contiguous,and the distribution of grassland was wider than now.The estimated area of forestland was2797.97km~2,the area of shrubland was 9308.11km~2,and the area of grassland was2.16×10~4 km~2 before reclamation.(2)In the early 18 th century to the middle of the 20 th century,because of cropland reclamation activities,it was estimated that the areas of forestland,shrub and grassland have decreased by 512.8?1550.5?6058.7 km~2.The reducing areas of forest mainly concentrated in Datong river,Huangyuan canyon,forest farm in Huzhu,the junction of Hualong and Jianzha,and the northern area of Tongren.The reduceing areas of shrub mainly distributed in the zone of transition where the boundary of grassland and shrub.The reducing areas of grassland mainly concentrated in the middle reaches of Huangshui river and Datong river,the Guide basin and the Hualong basin.(3)As a general rule,when human reclaims the cropland,they give priority to the lands that are easily reclaimed,watered and utilized and follow the principle of“good first and bad later”.73% of the suitable reclamation grid in Yellow River-Huangshui River Valley was grassland,while the proportion of shrub and forest was lower,so the cost of grassland reclamationwas lowest,followed by shrub and forest.The transformation of land cover type in Yellow River-Huangshui River Valley was mainly the process of grassland,shrub land and forest land changing to cropland.The area of reclaimed grassland was largest(52% of potential grids of grassland became cultivated cropland)and and the intensity of conversion was high(the average reclamation index of grassland grids was 10%),while shrub and forest were converted to cropland relatively weak,the average reclamation index of shrub and forest grids was only 7%~8%.(4)Analysis from the perspective of historical development,human reclamation activities is the main driving factors leading the original cover had changed.Because Yellow River-Huangshui River is located in the farming-pastoral region in our country,the development of agriculture was restricted by the natural and social factors in the historical period.From Han Dynasty to the end of Ming Dynasty,the development of agricultural was very slow,until the early Qing Dynasty in 1726,the watered zone had cultivated as the cropland.The development of agricultural had been on the right track,and it had been formed the land use pattern of agricultural.(5)There are extreme values in the range of different vegetation types by humanreclamation.Whithin this range,the agriculture developed normally.The threshold of forest cultivation was 1260,accounting for 46% of the total grid number of forest,and the average value of cultivating intensity was 7.62%.The threshold of the reclamation area of shrub land was 3313,accounting for about 30% of the total grid number of shrub land and the threshold of grassland cultivation is 11145,accounting for 50% of the grassland grid.