Non-structural carbohydrates’ (NSCs) allocation pattern is closely related to environmental factors, plant metabolism, and xylem function. At the same time, we know little about whole-tree NSC allocation patterns in different seasons, especially the high-elevation species which suffer from environmental stress. We examined the concentration of NSCs in various parts of Faxon fir trees (needles, branches, trunks, and roots) at five elevations (2800 m, 3000 m, 3200 m, 3400 m, and 3600 m) over four months (July 2019, October 2019, January 2020, and April 2020). The goal was to understand how NSC allocation patterns vary by location and time in high-elevation species and what factors contribute to these variations. The results showed that the needles had the highest concentration, followed by roots, branches, and trunks. The NSC concentration was highest in January 2020 and lowest in July 2019. The total non-structural carbohydrates (TNSCs) and soluble sugar concentrations of roots and needles were substantially higher in the cold (non-growing season) than in the warm (growing season) season. At different elevations, the soluble sugar concentrations in the needles and trunks remained the highest and lowest, respectively. Branches and roots’ soluble sugar concentrations alternated and varied with the seasons at all elevations. Many factors, such as climate, morphological traits, and carbon content, affected the spatial and temporal patterns of non-structural carbohydrates, with temperature, plant moisture conditions, and carbon content being the main driving factors. Various factors’ interaction mainly influenced NSCs’ spatial and temporal patterns. Non-structural carbohydrates significantly improve the resistance of Faxon fir trees’ terminal organs in adverse environments.

Soil fungi play an indispensable role in maintaining soil ecosystem functions. However, how forest succession and soil depth interactively shape fungal community composition and diversity remains poorly understood. To address this, we investigated fungal communities across four successional stages and two soil depths (0–10 cm and 40–60 cm) in a subalpine forest on the eastern Qinghai–Tibetan Plateau using Illumina high-throughput sequencing. Results showed that the soil fungal community composition of different trophic modes varied significantly with both succession and soil depth. The α-diversity of symbiotic and saprotrophic fungi responded to succession in a depth-dependent manner, while β-diversity across all trophic modes was primarily driven by species turnover. Soil properties and vegetation factors collectively explained 69.85–82.91% of the variation in soil fungal community composition, with their effects being dependent on both soil depth and trophic mode. Specifically, in topsoil, the β-diversity of symbiotic fungi was influenced only by soil property heterogeneity, whereas that of saprotrophic and pathogenic fungi was shaped by both vegetation and soil property heterogeneity. In subsoil, symbiotic fungal β-diversity was co-regulated by vegetation and soil properties heterogeneity, while saprotrophic fungal β-diversity was driven solely by soil properties heterogeneity. This study demonstrates that soil depth modulates the successional dynamics of soil fungal communities and highlights the trophic-dependent drivers of fungal assembly in forest soils.

Morphological plasticity (MP) is an essential strategy for plants in nutrient acquisition, disturbance alleviation, and community coexistence during environmental and climatic changes. However, to date, there has been little research concerning the MP for alpine–subalpine forests on the Qinghai–Tibet plateau. These forests are representative of the ectomycorrhizal (ECM) type, and morphological traits of these ECM roots, such as root tip lengths, diameters, and their adherent hyphal lengths and exploration types, have rarely been studied in the context of nutrient and environmental gradients. In this study, we examined the morphological traits of ECM roots for faxon fir (Abies fargesii var. faxoniana), which dominated in subalpine forests across nine elevations on the Eastern Qinghai–Tibet plateau. By quantifying ca. 90,000 root tips, the hyphal lengths of ectomycorrhizal extraradical mycelium (EEM, i.e., short- and long-distance exploration types) reached up to 1.1 × 10⁶ cm/m³ in soil, which decreased significantly due to gradually increasing altitude. In contrast, the variability of ECM root traits (diameter, length, and superficial area) was highly conserved along the altitudinal gradients, yet the root tip lengths were positively associated with soil protease enzyme activity. The increase in diameter and length of ECM root tips was climate-independent yet significantly associated with increasing root N concentration. In the studied forests, a long-distance exploration type of ECM hyphae was controlled by precipitation (p < 0.05), whereas the short-distance one was controlled by precipitation and temperature simultaneously. The EEM lengths of short- and long-distance exploration types were associated with high C concentration and low N concentration in host tree root tissues. Our findings demonstrated that MP expression in nutrient-foraging strategies for the dominant coniferous trees facilitates the adaptation to changing environments by specialized hyphal structures. In conclusion, ECM root tips and hyphal structures are two dimensions of functional traits linked to root N concentration in opposite ways, and their MP collectively ensures the temporal stability and resistance of subalpine forests on the Qinghai–Tibet plateau. These results provide new insights into ECM morphological traits and their adaptation in changing environments, which is valuable for understanding responses of subalpine forests to climate change.