Soil variables
Thirty-two soil variables were measured for each sampling location, including a number of macro- and micronutrients and variables associated with soil textural properties (Table S1). Details about soil variables measurements are described in Table S2. Preliminary analysis showed that most micronutrients, excepting for soil copper (Cu), had weak associations with fine root trait plasticity and biomass accumulation. Consequently, we selected 13 soil variables previously identified in the literature (Kesanakurti et al. 2011; Valverde-Barrantes et al. 2015a; Luo et al. 2020) to frequently influence root growth and trait variation for the present study, including soil pH, electrical conductivity, total organic carbon, total nitrogen, total phosphorus, total potassium, hydrolysable nitrogen, available phosphorus, available potassium, sand content, silt content, clay content and available Cu.