2.2 Field sampling and processing
Root and soil samples were collected during June-August, 2018, at five elevations (i.e. 2850, 3000, 3194, 3413, and 3593 m asl. ) in Wolong Nature Reserve, and two elevations in Miyaluo Nature Reserve (i.e. 3077 and 3612 m asl. ) and Wanglang Nature Reserve (3070 and 3150 m asl. ), respectively, using point-centered quarter sampling method (Mitchell, 2007) with randomly selected mature A. faxoniana trees (n = 8 focal trees per site) as center points. Four trees with DBH of 35-60 cm were sampled at each center point; these trees were all within 10 m distance from the focal tree. A 10 × 10 × 10 cm soil block was collected near the lateral root at 1 m away from each target tree after clearing the surface litter. Fine roots (diameter < 2 mm) were carefully separated from the soil, and then the soil samples from the same target tree were mixed to form a single composite sample, with a total of eight soil samples at each elevation on each site for chemical analysis. The field-collected root and soil samples were immediately placed in zip-lock plastic bags and stored in a cooler before being transported to laboratory for later processing. In laboratory, two random root samples for each center point were gently cleaned-washed with deionized water and stored in 5% glycerin at -20C for ECM identification following the method of Köhle et al. (2018), and the other two samples were oven-dried at 48°C to constant weight and used for biomass measurement and chemical analysis. Fresh soil samples were frozen-stored at -20 °C until further processing and analysis.
We collected fully developed, current year leaves from each target tree evenly at northern, eastern, southern, and western directions and among the four target trees at each center point. The leaf samples for each center point were mixed and placed in envelope, and then stored in a cooler before being transported to laboratory for later processing. After clean-wash with deionized water, the foliar samples were oven-dried at 48°C until constant weight for chemical analysis.