Environmental variables
At each site, spatial geographical coordinates and altitudes were recorded using a handheld GPS (eTrex Venture, Garmin, Olathe, Kansas, USA). Climate attributes, including mean annual temperature (MAT) and precipitation (MAP) of each sampling site were obtained from the global climate data set WorldClim 2.0 (1-km spatial resolution) (Fick & Hijmans 2017). Total N concentration of soil samples was determined using wet oxidation and a modified Kjeldahl procedure, and total P concentration was measured by colorimetric analysis with ammonium molybdate and persulfate oxidation (Murphy & Riley 1962). Thus, our environmental predictors included MAP, MAT, altitude, soil N, and soil P.