基于MODIS序列的北京市土地利用变化对净初级生产力的影响

了解土地利用变化对区域净初级生产力(NPP)的影响,对于综合理解区域植被的固碳能力及其与土地利用变化的关系,以及维持区域生态安全均具有重要意义。以北京市为例,基于2000—2012年MODIS NPP遥感影像和土地利用类型数据,利用同期Landsat TM影像(分辨率30 m)对MODIS NPP数据进行降尺度计算的基础上,分析北京市区域NPP的时空分布特征,探讨土地利用变化对植被固碳能力的影响。结果表明:研究期内Landsat NDVI与NPP数据的线性关系显著(R~2为0.22—0.68,P0.01),基于该线性关系可实现对MODIS NPP的降尺度重计算。土地利用面积分配和景观格局的变化均显著影响NPP。13年间,北京市林地和草地面积增加而耕地面积迅速减少,NPP总量由123万t C增长至190万t C,其中66%的NPP由林地贡献,其次为耕地(26%),草地最低(8%)。2000—2006年,面积变化促进了NPP的增加,贡献率为34%;而2006—2012年,面积变化则抑制区域NPP的增加,贡献率降低至27%。同时,研究区域内斑块聚集度的降低,斑块密度及丰富度的增加均有利于其NPP的增加。提高研究区植被固碳能力,需要在维持区域当前植被数量和质量的基础上,增加区域景观的异质性和多样性。

Effects of land-use change on net primary productivity in Beijing based on the MODIS series

Understanding the impact of land-use change on Net Primary Productivity (NPP) is important in the elucidation of the relationship between the carbon assimilation ability of vegetation and land-use changes at the regional scale, which is also critical to the maintenance of regional ecological safety. Using Beijing as a case study, we analyzed the spatiotemporal distribution of NPP, and the impact of land-use changes based on land-use maps and MODIS data downscaled by 30-m resolution Landsat TM images from 2000 to 2012. The results revealed significant linear relationships between Landsat NDVI and MODIS NPP (R² values between 0.22 and 0.68, P < 0.01) and these linear functions were used to downscale the NPP data for this region. The NPP levels were also significantly affected by the area allocation and distribution of different land-use types. From 2000 to 2012, areas of woodland and grassland increased rapidly, whereas the area of farmland declined. Overall, the NPP of Beijing increased from 1.23 million t C to 1.9 million t C, and 66% of NPP was contributed by woodlands, followed by farmland (26%) and grassland (8%). For 2000 to 2006, land area variations benefited NPP growth and contributed 34% to its change, whereas the variations from 2006 to 2012 impeded NPP growth and contributed 27% to the change. Moreover, reduction of patch aggregation, and increment in patch density and patch richness density contributed to the enhancement of NPP. The results suggest that maintaining the current vegetation quantity and quality, and increasing local landscape heterogeneity and diversity would improve NPP in Beijing.