Seasonal Response of Grasslands to Climate Change Figure 3. Mean phenological dates and vegetation activity during different growth stages on the Tibetan Plateau, based on NDVI data between 1982 and 2006. doi:10.1371/journal.pone.0049230.g003 until EGS).All vegetation analyses resulted in annual grids for the timing of each vegetation phase, as well as the mean NDVI for each vegetation stage, with a spatial resolution of 8 km. Climate Data Global gridded datasets of mean monthly temperature and precipitation were obtained from the University of Delaware’s 1900–2008 Gridded Monthly Time Series database. This dataset covers the entire terrestrial area of the planet at a spatial resolution of 0.5u [33,34]. Since the climate datasets were coarser than the vegetation data, the latter dataset had to be resampled to the resolution of the climate data, so that meaningful correlation analysis became possible [35]. All vegetation layers were first aggregated into 565 pixel means and then resampled to the resolution of the climate data, using bilinear interpolation. The resulting dataset covered the Tibetan Plateau with 1021 pixels. Following the resampling, vegetation data were combined with temperature and precipitation records for the year of the vegetation observations, as well as the preceding year. Trend Analysis All climate and vegetation data were subjected to trend analysis. For detecting the presence of significant trends, we applied the non-parametric Mann-Kendall test [36], which tests the hypothesis that no trend is present in the data. This test does not require a particular distribution of data points or residuals and should thus produce valid results for all pixels of the study region. Where significant trends were detected, a linear model was fitted PLOS ONE | www.plosone.org 4 November 2012 | Volume 7 | Issue 11 | e49230