Pressure-volume curves of fine roots reveal intraspecific variation across different elevations in a subalpine forest

Water conservation in fine roots can be important for the adaptation of trees to cold, nutrient-poor ecosystems. Although pressure-volume (p-v) curve traits are commonly used to assess leaf water conservation, little is known about their intraspecific variation in fine roots and their association with root functional traits, such as morphology and chemistry. Here, we aimed to determine the p-v curve traits of Betula ermanii and Abies mariesii fine roots at 2,000 and 2,500 m elevations and explore their intraspecific variation with root morphological and chemical traits in a subalpine forest. Turgor loss point (π_tlp), relative water content at π_tlp, osmotic potential at full hydration, and capacitance at full turgor (C_ft) were evaluated as p-v curve traits. Additionally, root diameter, specific root length, and root tissue density (RTD) were assessed as morphological traits, and nitrogen (N) content was measured as a chemical trait. For A mariesii roots, the C_ft was lower, and π_tlp was more negative at 2,500 m than at 2,000 m. The p-v curve traits of B ermanii roots remained unchanged with elevation. There were strong correlations between RTD and π_tlp and between N content and π_tlp and C_ft, especially for A. mariesii. These results indicated A. mariesii adjusted p-v curve traits with RTD and N content and achieved water conservation in fine roots at higher elevations. The p-v curve traits, particularly π_tlp and C_ft, reflected diverse tree strategies for environmental acclimation with fine-root carbon economy. Our findings highlighted the importance of adjusting water relation traits for acclimation to cold and nutrient-poor subalpine regions, particularly for evergreen coniferous species. The p-v curve traits revealed diverse fine-root water relation traits as a basis for water conservation capacity by preserving root function under stress conditions and enabling prolonged resource acquisition in a subalpine forest.