1.    Ectomycorrhizal (ECM) symbiosis is an evolutionary biological trait of higher plants for effective nutrient uptakes. However, little is known that how the formation and morphological differentiations of ECM roots mediate the nutrients of below- and aboveground plant tissues and the balance among nutrient elements across environmental gradients. Here we investigated the effects of ECM foraging strategies on root and foliar N and P concentrations and N:P ratio Abies faxoniana under variations of climate and soil conditions.

2.    The ECM symbionts preferentially mediated P uptake under both N and P limitations. The uptake efficiency of N and P was primarily associated with the ECM root traits, e.g., ECM root tip density, superficial area of ECM root tips, the ratio of living to dead root tips, and was affected by the ECM proliferations and morphological differentiations. The tissue N and P concentrations were positively associated with the abundance of the contact exploration type, and negatively with that of the short-distance exploration type.

3.    Our findings indicate that the nutritional status of both below- and aboveground plant tissues can be strongly affected by ECM symbiosis in natural environments. Variations in the ECM strategies in response to varying environmental conditions significantly influence plant nutrient uptakes and trade-offs.

Root and soil samples were collected during June-August, 2018, at five elevations (2850, 3000, 3194, 3413, and 3593 m asl.) in Wolong Nature Reserve, and two elevations in Miyaluo Nature Reserve (3077 and 3612 m asl.) and Wanglang Nature Reserve (3070 and 3150 m asl.), respectively, using point-centered quarter sampling method with randomly selected mature A. faxoniana trees (n = 8 focal trees per site) as center points. We collected fully developed, current year leaves from each target tree evenly at northern, eastern, southern, and western directions and among the four target trees at each center point. we identified key soil variables and ECM traits by principal component analysis (PCA), and the first and second principal components of soil variables (named Soil-PC1, Soil-PC2 respectively) and ECM traits (named ECM-PC1, ECM-PC2 respectively) were selected for SEMs analyses. 

The gridded daily climate dataset (CN05.1), with a spatial resolution of 0.25^°×0.25^° constructed by the “anomaly approach” during the interpolation with more 2400 station observations in China, was employed to obtain the meteorological data for the study sites during 1997-2016. The mean annual temperature (MAT, ℃) for each elevation on each site was derived through topographic correction with the lapse rate of air temperature set at 0.65^◦C (100 m)^-1. The same value of the mean annual precipitation (MAP, mm) was assumed along the elevational gradient on each site.