1990-2020年西藏天然林分布与景观格局的动态变化

Dynamic Changes in the Distribution of Natural Forests in Tibet from 1990 to 2020 and Related Landscape Pattern Evolution

西藏自治区天然林是我国青藏高原生态屏障的主要组成部分,在维持生态平衡和调节气候等方面发挥着重要作用。掌握该区域天然林的分布状况及景观特征在长时间尺度上的变化尤为必要。已有研究主要集中在天然林和土壤理化性质间的关系、天然林保护现状等方面,受限于大尺度、长时序遥感数据分析处理以及实地调查困难,对西藏自治区天然林景观格局方面的研究尚少。为揭示西藏自治区近30年来天然林分布与景观格局的动态变化规律,本文基于Google Earth Engine(GEE)平台和Landsat数据集,构建归一化植被指数(NDVI)时间序列曲线和优化后分类特征集,参与模型分类,获得最优分类结果,并分析了该区域的天然林分布和景观格局动态变化。结果表明:(1)优化后的光谱特征集、纹理特征集和地形特征集组合的RF模型分类精度最高;(2)西藏自治区天然林主要分布在山南市、林芝市和昌都市,景观优势度整体有所增大,其面积在近30年不断增加,年均增长率约0.94%,其中昌都市天然林面积增幅最大;(3)近30年来,西藏自治区天然林整体破碎化指标值和复杂化程度增大,但连通性和聚集性程度较稳定。其中拉萨市破碎化指标值和复杂化程度增速最快,昌都市破碎化指标值和复杂化程度最大,而林芝市和阿里地区破碎化指标值趋向减小且趋向规则化。上述结果证实了近30年来西藏自治区的天然林保护总体上取得较好的效果,天然林面积明显增加,但由于天然林斑块数量增多导致整体复杂化增大。研究结果可为西藏自治区天然林保护与恢复工作的实施提供科学支撑。

Forests in the Tibet Plateau constitute an essential ecological ba rrier,which plays an important role in ecological balance and climate regulation.The distribution of natural forests and the landscape evolution in the Tibet Plateau had not yet been carefully addressed by scientists and observed on a long-time scale.Past studies concerned about the relationship between natural forests and soil physical and chemical properties,and the situation of natural forests protection,but little attention to the evaluation of landscape pattern,mostly because of difficulties in analyzing long time series remote sensing data or data on large geographical scale,and low accessibility in desolate areas.In this study,it conducted a comprehensive investigation on the distribution of natural forests and the landscape evolution in Tibet based on the Google Earth Engine(GEE)and Landsat dataset.Normalized difference vegetation index(NDVI)time series curve and optimized classification feature set was constructed for model classification and concluded optimal classification.The dynamic changes of natural forests distribution and landscape pattern evolution were analyzed,and the following results are obtained:(1)The classification accuracy of RF model with optimized spectral,textural and terrain features sets was the highest;(2)Natural forests in Tibet were mainly distributed at Shannan,Nyingchi and Qamdo.The landscape dominance degree had increased on the whole,and the area of natural forests has increased continuously for the past 30 years,with an average annual growth rate of about 0.94%.Particularly,the area of natural forests increased at the most occurring at Qamdo.(3)For the past 30 years,the fragmentation index and complexity degree of natural forests has increased in Tibet,but the degree of connectivity and aggregation remained relatively stable.Moreover,the fragmentation index and complexity degree was the largest at Qamdo and increased fastest at Lhasa,respectively,while the fragmentation index at Nyingchi and Ngari tended to decrease and became regular.These results confirm that the protection of natural forests has generally achieved good results in Tibet for the past 30 years,demonstrating by the significant increase in the coverage of natural forests;However,due to the increase in the number of natural forests patches,the overall complexity also increased.These results can provide scientific support for the protection and restoration of natural forests in Tibet Autonomous Region.