Climatic data availability
To characterize the climatic conditions at each site, we extracted eight
variables from the BIOCLIM database (Hijmans et al. , 2005;
available online): BIO1 (mean annual temperature, °C; AT), BIO4
(temperature seasonality, expressed as the standard deviation of
temperature across months; ST), BIO5 (maximum temperature of the warmest
month, °C; MAXT), BIO6 (minimum temperature of the coldest month, °C;
MINT), BIO12 (mean annual precipitation, mm; AT), BIO13 (precipitation
of the wettest month, mm; WP), BIO14 (precipitation of the driest month,
mm; DP) and BIO15 (precipitation seasonality, expressed as the standard
deviation of precipitation across months; SP). These variables include
annual climatic means, climatic extremes and seasonality aspects.
Soil physicochemical property measurements
To characterize the soil properties at each site, we collected six upper
mineral soil cores (0–10 cm) from random locations within each quadrat
and mixed them thoroughly to produce a composite sample. The soil
samples were then stored at 4℃ for later analysis. Samples were first
sieved through a 2-mm screen to remove stones and visible plant roots,
before air-drying at 25℃. Soil pH and electrical conductivity (mc/cm;
EC) were determined using a 1:5 (w/v) soil-to-water solution. Soil total
carbon (mg/g; Soil C) and total nitrogen (mg/g; Soil N) were measured on
ground soil samples using an elemental analyzer (Elementar vario MACRO
cube, Germany). Soil water content (SWC, %) was determined
gravimetrically by quantifying the weight lost after oven-drying 100 g
of fresh soil at 105℃ for 48 h.