4 DISCUSSION
It is generally uncontroversial that ECM fungi promote the uptake of N and P in plants. While the root N and P nutrition of ECM plants were widely investigated (Franklin et al ., 2014; Almeida et al ., 2019; Zhang et al ., 2019), relatively few studies have attempted to determine the attributes of ECM symbionts to aboveground nutrition in tree species (Michelsen et al. , 1996; Koele et al. , 2012). In this study, we examined the effects of the variations in ECM symbiosis on root and foliar N and P nutrition in A. faxoniana across certain natural environmental gradients. Generally, the ECM in A. faxoniana played more an important role on P uptake than N uptake under both N and P limitations (Figure 2, Figure 3, Figure 6). We found that the root N and P concentrations in A. faxonianawere more strongly related to the ECM traits than the foliar N and P concentrations (Figure 3), and the ECM soil exploration types played different impacts on root and foliar N concentrations or N:P ratio (Figure 5). During the process, soil resources and climatic factors appeared as the primary drivers of the ECM strategies. Our findings allow us to develop a conceptual model on the intervention of ECM symbiosis on root and foliar N and P nutrition using A. faxonianaas a case study (Figure 6). The model illustrates that the ECM strategies strongly affect the root nutrients, and then through the interconnections between roots and aboveground tissues in nutrient transportation and re-allocations, eventually influence the foliar nutrients, with preferential effects on P under both N and P limitations.