Sample collection and processing
Twenty to thirty leaves were randomly collected per species (S. superba , S. rehderianum , I. chinensis and M. breviflora ; the remaining two species were excluded because of their small biomass with less than 1.1% of the total biomass) for each plot in the dry season and wet seasons from 2013–2019. In 2014, 2015, and 2017–2019, monthly litter samples in each plastic net were collected. In December 2017 and 2018 and June 2018 and 2019, soil samples were collected with five replicates per soil depth (0–10, 10–20, 20–40 cm depth) for each plot using a 3.5-cm-diameter stainless steel soil corer and then were pooled by each plot. Soil bulk density was measured using the cutting-ring method. Soil samples were sieved through a 2-mm mesh and then stored at 4°C. All plant samples were oven-dried at 60 °C for at least 24 h before being weighed. Soil pH was measured at a soil-to-water ratio of 1:2.5.