Choices of ectomycorrhizal foraging strategy as an important mechanism of environmental adaptation in Faxon fir (Abies fargesii var. faxoniana)

Abstract

Faxon fir (Abies fargesii var. faxoniana) is endemic to the cool and moist subalpine region of Southwest China, where the occurrence of warmer and drier climate may impose uncertain environmental constraints to its population stability and community scuccession. Here we determine the role of root-mycorrhizal symbiosis in the adaptability of Faxon fir to varying environmental conditions. The traits and foraging behaviors of the ectomycorrhizal (ECM) associations in Faxon fir were investigated along elevational gradient in three representative natural distributional areas of the species. Soil and Faxon fir root samples were collected from varying elevations in three study areas, namely Wolong Nature Reserve, Miyaluo Nature Reserve, and Wanglang Nature Reserve, in Sichuan province, Southwest China, using point-centered quarter sampling method, with 32 samples for each elevational site. Measurements were made on fine root biomass, ECM morphological diversity and soil exploration types, density and superficial area of ECM root tips, and soil chemistry. Values for selective climatic variables were derived from the national grid-datasets of meteorology of China. Principle component analysis (PCA), redundancy analysis (RDA) and linear regressions were performed to determine the inter-relationships of ECM root traits with climatic and soil factors, and the variations of the ECM root traits with elevation. We found that the biomass of absorptive fine roots and ECM morphological diversity decreased with rising elevations and increasing soil water content or precipitation and soil C:N ratio. The foraging ability of ECM roots, inferred by the soil exploration type of ECM symbionts, and density and superficial area of ECM root tips, were weakened by warming and drying. Under conditions of low N availability, both the frequency of contact exploration type and ECM root tip density prevailed, whilst the frequency of both short-distance and medium-distance exploration types tended to decline, suggesting conservative carbon cost of ECM root systems in low fertility soil. Overall, our findings suggest that the ECM symbionts in Faxon fir adopt the strategy of C investment in root systems in response to warming and drying while relying on the functional efficiency of the ECM root tips toward the colder and more humid environments, and that Faxon fir may sustain moderate changes in the regional climate and maintain population stability, partially owning to adjustment of the ECM foraging strategies.