Seasonal Response of Grasslands to Climate Change Figure 6. Trends in phenological dates (left) and NDVI (right) on the Tibetan Plateau between 1982 and 2006. Gray areas indicate regions, for which no significant trends were detected by the Mann-Kendall test at p,0.1. doi:10.1371/journal.pone.0049230.g006 coefficients indicate that high values for the respective variable are related with an advance in phenology or with lower vegetation activity. Pixel-scale results were mapped to show the spatial variation in the strength and direction of climatic drivers on the various vegetation variables. All procedures were implemented in R programming language [44], drawing heavily from the ‘raster’ package [45] and the ‘pls’ package [46]. Results Vegetation Dynamics on the Tibetan Plateau Based on the means of phenological dates for the green-up phase over 25 years, greening of the Tibetan Plateau typically occurs between April and May in most places. It is followed by the beginning of the growing season in May or June (Fig.3). In most parts of the plateau, plants reached their mature stage in July, and began to senesce in September. The growing season ended in October in most grassland regions of the Tibetan Plateau (Fig. 3). The cooler steppe regions lagged substantially behind the more vigorous meadow vegetation of the eastern part of the plateau. NDVI values were naturally low during green-up at between 0.00 and 0.35 (Fig. 3). In the meadow environment, NDVI peaked during maturity at up to 0.7, while few steppe locations exceeded NDVI values of 0.45. During senescence, NDVI dropped substantially to about half the level of peak NDVI throughout the plateau. The analyses showed a pronounced east-west gradient in mean NDVI during all vegetation phases, with the cooler and dryer steppe environment of the Western Tibetan Plateau showing low vegetation intensity. For most of the northwestern region, mean NDVI during the growing season was less than 0.1, leading to exclusion of these areas from further analysis (Fig. 3). PLOS ONE | www.plosone.org 7 November 2012 | Volume 7 | Issue 11 | e49230