Abstract: Fine-root morphological and stoichiometric traits are key regulators of plant nutrient use strategies. However, altitudinal variations in these traits and their trade-offs in Picea schrenkiana remain poorly documented. In this study, fine-root samples of P. schrenkiana were collected by excavation at four altitudes (1800, 2100, 2400, and 2700 m) in the Tianshan Mountains. Morphological and stoichiometric traits of different root orders were measured. We investigated the variations in root diameter (RD), specific root length (SRL), specific root area (SRA), root tissue density (RTD), and carbon (C), nitrogen (N), and phosphorus (P) concentrations and ratios (C∶N, C∶P, N∶P) of different root orders across altitude levels to elucidate the resource acquisition strategies. The results showed that: (1) Altitude and root order significantly influenced the morphological traits of fine roots (P<0.01). RD and SRL of the same root order generally increased with altitude, whereas SRA and RTD exhibited a unimodal pattern, peaking at mid-altitudes. At a given altitude, RD and RTD increased with increasing root order, while SRL and SRA decreased. (2) Altitude and root order significantly affected fine-root C, N, and P concentrations and ratios (P<0.01). With increasing altitude, C content in absorptive roots (orders 1-3) increased, while that in transport roots (orders 4-5) remained stable. The N content and N∶P ratio in both root functional categories followed a unimodal pattern, contrasting with the trend of C∶N ratio. The P content increased with altitude, whereas C∶P ratio decreased. Absorptive roots generally had higher N and P contents but lower C content, C∶N, and C∶P than transport roots. (3) Functional traits of P. schrenkiana fine roots revealed two primary dimensions of ecological strategy variation across altitudes: “do-it-yourself versus outsourcing” and “resource acquisition versus conservation”, with distinct strategic preferences at different altitudes.

Key words: fine root, root order, trait, altitude, spruce forest