Abstract:
Greenhouse-vegetable production serves as a sort of farming, in which vegetable crops grow in built structures, such as wood, plastic, metal and net. Recently, the greenhouse-vegetable farming has become an ideal way to meet the increasing demand of residents for vegetable consumption, particularly on Lhasa in the cold regions. However, a clear understanding for the changing process is still lacking in the greenhouse pattern. This study aims to clarify the change characteristics in the spatial and temporal pattern of the greenhouse-vegetable land in Lhasa from 2008 to 2018, particularly on land use, soil texture and vegetable yield. 11 high-resolution remote sensing images of greenhouse were captured from Lhasa in the northwestern China, from 2008 to 2018. Combined with field research, the barycenter shift and geostatistical techniques were used to determine the total area of greenhouse-vegetable land in various districts or counties. The movement of barycenter position was related to the direction of tracking position, including the altitude gradient and slope. Super-resolution images were obtained for the layout of greenhouses facility in alpine regions, as well the early built greenhouses-vegetable land. The results show that, 1) there was an upward trend in the area of greenhouse-vegetable land in Lhasa from 2008 to 2018, with an average annual growth rate of 6.93%. Three stages were observed for the change features of greenhouse-vegetable land in the study period, including developing, adjustment, and stability. The annual average areas of greenhouse-vegetable land in each stage were 1 050 hm2, 1 413 hm2 and 1 668 hm2, respectively, while the average annual change rates were 11.08%, -2.13%, 0.77%, respectively. 2) In the past ten years, the proportion of greenhouse-vegetable land in Chengguan and Doilungdêqên of Lhasa decreased by 56.2%, while the proportion in Dagze and Quxu increased by 51.58%. 3) The newly developed greenhouse-vegetable lands were transferring to high-altitude and high-slope regions, far away from urban or industrial areas. In the altitude range of 3 675-3 800 m, the areas of greenhouse-vegetable lands increased from 22.05% to 30.41%, while that in the 6°-10° slop regions increased by 5.92%. 4) The spatial change of greenhouse-vegetable lands in Lhasa revealed that the newly added greenhouse-vegetable land much more than farmland, indicating the expansion of construction land. A basic driving force for the growth of greenhouse vegetable lands can be attributed to the large demand for vegetables and the high yield of the greenhouse. Regional land use can also be another important driving force for the distribution of greenhouse-vegetable land. In greenhouse-vegetable land operation, there was no obvious effect on soil texture, indicating the particle size of soil texture changed a little at different elevation gradients. The overall output of vegetables in Lhasa was increasing, with an average annual growth rate of 9.57%. The increase in the greenhouse area has effectively promoted the production capacity of vegetables, thereby to meet the demand of residents for vegetable consumption in Lhasa in the cold regions.