2.1 Study sites
A. faxoniana is mainly distributed in the western Sichuan province of China, from 30°N to 35°N in latitude. Our study covered three sites along latitudinal gradients, including the Wolong Nature Reserve (latitude 30°53′N, longitude 102°58′E), Miyaluo Nature Reserve (latitude 31°42′N, longitude 102°46′E), and Wanglang Nature Reserve (latitude 33°00′N, longitude 104°01′E).
Wolong Nature Reserve is located in the western Sichuan Plateau, with subtropical semi-humid climate, characterized by dry, cold winters and wet, cool summers (Li et al. , 2020); the mean annual temperature is 4.06 C, and the mean annual precipitation is about 1062.8 mm. The dominant woody plant species consist of A. faxoniana , Picea purpurea Mast, Betula albosinensisBurkill, Betula platyphylla Suk, Ribes tenue Jancz,Sorbus koehneana Schneid, and Rosa moyesii , etc. The soil type is classified as dark brown soil in coniferous forest according to Chinese Soil Taxonomy (Zhang, 1983), which is developed from the weathering of slate of metamorphic rock (Taylor et al. , 2006).
Miyaluo Nature Reserve is also located in the western Sichuan Plateau, with subtropical semi-humid climate, characterized by dry, cold winters and wet, cool summers (Li et al. , 2013) and mean annual temperature of 1.67 C and mean annual precipitation of 975.2 mm. The dominant tree species consists of A. faxoniana ,Abies fabri (Mast) Craib, Picea purpurea Mast, Picea asperata Mast, Populus davidiana Dode, and Quercus aquifolioides , etc. The main soil type is classified as dark brown soil in coniferous forest according to Chinese Soil Taxonomy, developed from the parental materials of phyllite, slate, and schist (Keyimu et al. , 2020).
Wanglang Nature Reserve is located in the Himalayas -Hengduanshan Mountains, with subtropical semi-humid climate, characterized by dry, cold winters and wet, cool summers (Zhao et al. , 2012) and mean annual temperature of 4.17 C and mean annual precipitation of 1021.8 mm. The dominant tree species consists ofA. faxoniana, Picea purpurea Mast, Sabina saltuaria, Sabina squamata, and Betula albosinensis Burkill, etc. The main soil type is classified as dark brown soil in coniferous forest according to Chinese Soil Taxonomy, with limestone as the main parental material (Taylor et al. , 2006).