[Performance evaluation of GIMMS NDVI based on MODIS NDVI and SPOT NDVI data]. / 基于MODIS NDVI、SPOT NDVI数据的GIMMS NDVI性能评价.

The study evaluated GIMMS NDVI based on MODIS NDVI and SPOT NDVI over the same period from 2000 to 2015. We assessed their absolute values, dynamics, trends and cross-relationships between any two of the NDVIs for the national scale, as well as four separate land use types, i.e., paddy field, dry land, forest, and grassland. GIMMS NDVI was numerically greater than MODIS NDVI and SPOT NDVI. The three NDVIs exhibited equal capability of capturing monthly phenological variations. During the study period, the three NDVIs showed increasing trends in most regions, with GIMMS NDVI showing the smallest increment. Pronounced differences were identified in trends between GIMMS NDVI and MODIS NDVI or SPOT NDVI in the northwest, northeast, south-central China, Tibetan Plateau and Yunnan-Guizhou Plateau, implying that GIMMS NDVI trends in these regions should be interpreted with caution. High correlations existed between the three datasets. MODIS NDVI and SPOT NDVI showed stronger correlations at national scale. The GIMMS NDVI and MODIS NDVI were in highest accordance for dry land, while MODIS NDVI and SPOT NDVI were in higher accordance for the paddy field, forest, and grassland than dry lands.

[Effects of precipitation changes on the precipitation use efficiency and aboveground productivity of alpine steppe-meadow on northern Tibetan Plateau, China.] / 降水变化对藏北高寒草原化草甸降水利用效率及地上生产力的影响.

Total amount, distribution pattern and occurrence time of precipitation determine the water condition of alpine vegetation growth on the Tibetan Plateau, China. Precipitation use efficiency (the ratio of aboveground productivity to precipitation, PUE) is an effective indicator for the relationship between precipitation and vegetation productivity. In this study, we calculated aboveground net primary productivity (ANPP) of an alpine steppe-meadow in northern Tibetan Plateau from 2000 to 2016 based on the correlation model between long-term monitoring biomass data and enhanced vegetation index (EVI) in Damxung. Meteorological data during the same period was also analyzed. Growing season precipitation (GSP) represented the amount of precipitation, improved precipitation concentrated index (PCI) indicated their distribution pattern in the growing season, and precipitation centroid (PC) represented the time of precipitation occurrence. Structural equation model was used to explore the effects of climatic factors on PUE and ANPP, with the consideration of growing season temperature (GST). Results showed that ANPP of this alpine steppe-meadow was mainly controlled by precipitation during the growing season. GSP and ANPP showed significantly positive correlation, whereas the correlation of GST with PUE and ANPP was not significant. PCI and PUE showed significantly positive correlation, which indicated that concentrated distribution of precipitation was beneficial to the increase of PUE. Indirect coefficient between PCI and ANPP was greater than direct coefficient, indicating that PCI influenced ANPP through PUE. Changes of precipitation concentrated time did not show significant effect on PUE and ANPP. Our results suggested that under the rapid climate warming on the Tibetan Plateau, future changes of precipitation and their concentration would have important impacts on aboveground productivity of the alpine grassland.

[Responses of normalized difference vegetation index (NDVI) to precipitation changes on the grassland of Tibetan Plateau from 2000 to 2015.] / 2000­2015年青藏高原草地归一化植被指数对降水变化的响应.

Precipitation change is an important factor in the inter-annual variation of grassland growth on the Tibetan Plateau. The total amount, distribution pattern and concentration time are three basic characteristics of precipitation change. The temporal and spatial characteristics of precipitation change were analyzed based on climate data of 145 meteorological stations on the Tibetan Plateau and nearby areas from 2000 to 2015. The total precipitation amount was characterized by annual precipitation, distribution pattern of precipitation during the year was characterized by improved precipitation concentration index (PCI), and precipitation centroid (PC) was defined to indicate the change in precipitation concentrated time. To better illustrate the response of grassland to precipitation change, vegetation growth status was characterized by the maximum value of normalized difference vegetation index (NDVImax). Results indicated that the annual precipitation and PCI had an apparent gradient across the whole plateau and the latest PC occurred in the southern plateau. NDVImax of alpine shrub grassland was significantly correlated with the change of PCI,increased with even distribution of precipitation during growth period, and limited by the total annual precipitation. Alpine meadow did not show significantly correlations with these three indices. The inter-annual variability of NDVImax of steppe was controlled by both PCI and PC. NDVImaxof alpine desert grassland was mainly controlled by annual precipitation. In addition to annual total amount of precipitation, the distribution characteristics of precipitation should be further considered when the influence of precipitation change on different types of vegetation on the Qinghai Tibet Plateau was studied.