关键词: 归一化植被指数, 时空变化, 气候变化, 人类活动, GEE

Abstract:

【Objective】The northeastern China Tiger and Leopard National Park is an important ecological security barrier in the northeast China, due to sensitivity of the vegetation to climate change and anthropogenic activities. This study aimed to explore the spatiotemporal changes in the vegetation of the region to provide a scientific basis for ecological restoration and improvement in the park management system. 【Method】Based on normalized difference vegetation index(MODIS NDVI) data of the growing season (April to October) from 2001 to 2020, Sen+Mann-Kendall trend analysis along with data supplemented by the Google earth engine(GEE) cloud platform, DEM data, meteorological data, land use data, and vegetation type data were used to reveal the spatiotemporal change characteristics of vegetation in the northeast China Tiger and Leopard National Park at different time scales. This considered the time lag effect of different vegetation types on climate change and their differences, as well as partial correlation analysis. Additionally, improved residual and relative role analyses were conducted to quantify the response mechanisms of the vegetation to climate change and anthropogenic activities, and clarify the relative role of climate change and anthropogenic activities in the evolution of vegetation under different conditions.【Result】 (1) Temporally, the growing season NDVI of the park showed a significant increasing trend at a rate of 0.003 2/a in the past 20 a (P<0.05). In different growing seasons, the order of the mean value and rate of increase of NDVI was summer >spring >autumn and spring >autumn >summer, respectively. Spatially, the NDVI trend showed clear seasonal and regional differences from 2001 to 2020; however, the improved area was larger than the overall degraded area. Since the seasonal cycle changes occurred within the growing season, the NDVI improvement area first decreased and then increased, and the main NDVI trend shifted from ‘significant improvement’ to ‘slight improvement’. (2) The spatial heterogeneity of the NDVI of the park in response to climate at different time scales was clear; however, the NDVI was positively correlated with both air temperature and precipitation, and the response of the NDVI to air temperature was stronger than precipitation. The area with a significant positive correlation between the NDVI and air temperature was larger in spring and growing seasons (P<0.05), accounting for 83.558% and 42.241% respectively, while most areas of the park showed no significant positive correlation between the NDVI and precipitation at each time scale (P≥0.05). Different vegetation types had different time lags for the maximum response of the NDVI to air temperature and precipitation, except for cultivated vegetation and meadows, wherein the response lags of other vegetation types were stronger to precipitation than to air temperature. (3) The impact of anthropogenic activity on the NDVI of the park had dual effects, in which the areas that positively promoted and negatively disturbed the NDVI trend accounted for 94.087% and 5.913%, respectively. The implementation of forestry projects was key for increasing NDVI, while the expansion of construction land was the prime factor for increasing NDVI. (4) The NDVI trend of the park was driven by both climate change and anthropogenic activities, but the mean relative role of climate change and anthropogenic activities on the NDVI trend was 32.699% and 67.301%, respectively. The mean relative role of human activities was greater than that of climate change in both the NDVI improved and degraded areas.【Conclusion】 The vegetation status of the northeast China Tiger and Leopard National Park has been generally improved from 2001 to 2020, with significant differences in spatiotemporal trends of vegetation at different time scales. Air temperature was the dominant climatic factor that promoted vegetation growth in the park at each time scale, and the effects of climate change and anthropogenic activities on vegetation changes varied significantly in terms of geography. However, both were dominated by positive effects, with the contribution of anthropogenic activities to the NDVI changes of the park being relatively higher. It is suggested that in the future, in addition to the park vegetation maintenance, efforts should be made to improve the adaptability of vegetation to climate change, along with ensuring continuous implementation of ecological restoration measures and curbing overexploitation of land resources.

Key words: normalized difference vegetation index(NDVI), spatiotemporal change, climate change, anthropogenic activity, Google earth engine(GEE)