Plant and soil sampling and climate monitoring
In early August 2020, we harvested all the plant aboveground parts at ground level and sorted them to species at the plot level. We then dried plants at 65 ℃ for 48 hours to constant mass and weighed them to 0.01 g; we then summed the dry biomass of each species from each plot to quantify aboveground biomass (hereafter ‘AGB ’, see Table 1 for abbreviations). We also collected four soil cores (5 cm in diameter and 10 cm in depth) in the four corners of each plot and pooled them to form one sample per plot. We flushed all fine roots collected from each sample with water and then dried them to measure plant belowground biomass (BGB ). We measured soil moisture content (W; %) gravimetrically after 5 h of desiccation at 120 ℃ and used a pH analyzer and a conductivity analyzer to measure the soil pH (pH) and soil conductivity (C; ms/s), respectively. We extracted 5 g of fresh soil with 50 ml 0.2 M KCl for 1 h at 60 rev s-1 using a shaker, then measured nitrate-nitrogen (NO3-; mg/kg) and ammonium-nitrogen (NH4+; mg/kg) using an auto-analyzer (AA3, Bran-Luebbe, Germany).
We placed a Temperature-Humidity Recorder Cos-03-0 (Renke Control Technology Co., Ltd., Jinan, Shandong, China) at each elevation, which continuously monitored ambient temperature and humidity every 60 seconds for 72 hours in early August 2020. We then calculated the mean daily temperature (MDT ) and mean daily humidity (MDH ) for each elevation. Given the relatively short-term temperature and humidity may not represent the environmental conditions that were present during pathogens development, we only test the associations between elevation with MDT andMDH (thereby confirming whether the elevational gradient can serve as a temperature and humidity gradient), but we did not included these measurements in other analyses.