Plant community and soil sampling
We conducted transect sampling during July and August in 2012, during the period with peak aboveground biomass. To ensure that plant samples were collected during the same period of phenology at each site, we sampled sequentially from the west to the east along this transect since this coincides roughly with a decreasing temperature trend (i.e., delayed growing peak period). Study sites were investigated from west to east along the entire transect with an interval of 50~100 km. Sampling sites were generally far from cities, under natural conditions, with little human disturbance, and represent the local natural vegetation (Wang et al. 2014). At each site, ten 1 m × 1 m quadrats were established. For each quadrat, we clipped the aboveground tissues of living plants, sorted these by species, and stored in paper bags. Community data from 10 quadrats were pooled together to represent the local species pool for each site. Soil samples from each quadrat were collected with five soil cores (2.5 cm diameter × 10 cm depth) from the upper 10 cm layer. Five soil cores were collected from four corners 10 cm away from the edge plus one from the centre of the quadrat. Soils from the five soil cores were combined for each quadrat and sieved through a 2.0 mm mesh to remove roots and rocks, homogenized by hand, and preserved for subsequent chemical analysis.