基于土地覆盖和NDVI变化的拉萨河流域生境质量评估

气候变化和人类活动导致的土地覆盖和植被变化都会对生境质量产生影响。青藏高原是众多珍稀高原动植物的栖息地,具有重要的生物多样性维持价值。拉萨河流域是青藏高原经济最发达、人口最密集的核心地区,人类活动对生境质量带来的胁迫和压力持续增加。为揭示近些年来土地覆盖和植被变化对拉萨河流域生境质量的影响,选择生长季NDVI作为植被变化的指示因子,通过对不同植被类型各年份的生境适宜度进行修正,利用In VEST模型评估了拉萨河流域1990—2015年的生境质量时空变化。研究结果表明,1990—2015年拉萨河流域土地覆盖变化整体相对较小,其中人工表面和湿地面积增幅相对较大,分别为82.65%和32.40%;土地覆盖变化的转移方向主要为稀疏草地转化为草原和草甸、耕地转化为人工表面以及冰川/积雪转化为荒地。植被变化方面,1990—2000年,除流域中上游的裸岩、裸土地区和念青唐古拉山地区外,流域NDVI整体有较显著上升;而2000年以后略有下降。从生境质量的空间分布来看,高质量生境主要分布在流域下游、念青唐古拉山南侧河谷地区以及拉萨河源头等地区,低质量生境主要分布在拉萨市市辖区及周边、林周县县城及周边,以及流域中上游的荒地等地区。从时间变化上来看,1990—2000年,拉萨河流域整体生境质量指数从0.51上升到0.57; 2010年和2015年整体生境质量指数分别为0.56和0.55,较2000年略有下降。相比于土地覆盖变化,NDVI对生境质量变化的影响更为显著。

Assessment of habitat quality on the basis of land cover and NDVI changes in Lhasa River Basin

Changes in land cover and vegetation driven by climate change and human activities can both affect habitat quality. Qinghai-Tibet Plateau has very high biodiversity and provides a habitat for numerous wildlife species. The Lhasa River Basin, located in the southern part of the Tibetan Plateau, is where human activities are very intense, increasing the threat and pressure on habitat quality. To analyze the effects of land cover and vegetation changes on the habitat quality of Lhasa River Basin, we selected growing season (from June to October) Normalized Difference Vegetation Index (NDVI) as the index for vegetation condition to adjust the suitability index of different vegetation types and then assessed habitat quality change in Lhasa River Basin from 1990 to 2015 by using the InVEST model. Results showed that the overall change in land cover in Lhasa River Basin was relatively small, except for the sharp increase in artificial surface and wetland (increased by 82.65% and 32.40%, respectively). The main land-cover transfer directions were from sparse grassland to meadow and steppe, farmland to artificial surface, and ice/snow to barren land. Except for the barren land in the upper and middle regions of the basin as well as Nyenchen Tanglha Mountain, overall NDVI showed a significant increase from 1990 to 2000, and then a slight decrease thereafter. From the perspective of spatial distribution, the high habitat quality areas were distributed throughout the valley of the Nyenchen Tanglha Mountain as well as downstream and headstream of Lhasa River. The low habitat quality areas were mainly located at the construction sites of Lhasa City and Linzhou County and their surrounding areas, along with the barren land located in the upper and middle regions of the basin. From the perspective of temporal changes, the average habitat quality index increased significantly from 0.51 in 1990 to 0.57 in 2000; the average habitat quality indices of 2010 and 2015 were 0.56 and 0.55, respectively, and they showed a slight decrease when compared with the value in 2000. When compared with land cover change, the NDVI change had more significant effects on habitat quality in the study region.