Study site and species
The study site was located in a tropical montane rain forest in Jianfengling Nature Reserve in Hainan Island, China (Figure 1a), adjacent to a 60-ha stem-mapping plot established in 2012 (18°53′N, 108°43′E) (Xu et al. 2015). The site receives annual precipitation ranging 1300–3700 mm and has an average annual temperature of 19.8 °C, with summer temperature (June-August) of 27.5°C (Xu et al. 2015). The two tree species selected for this study were O. semicastrata f. litchiifolia , a rhizobia-associated species, carrying coevolved host-specific soil-borne pathogens (Li et al. 2009) and C. patelliormis , an EcM fungi-associated species immune to infestation of soil-borne pathogens. These species were selected because (i ) they were commonly occurring tree species in the region, (ii ) they demonstrated contrasting nutrient acquisition strategies, and (iii ) their seeds could be easily collected and readily germinated.