Species-specific fine-root traits dominate the response of root water uptake to elevational gradients in subalpine...

Subalpine forests are one of the regions where the adjustment of fine-root water uptake becomes important for tree adaptation; however, this process has not been adequately investigated. Here, we aimed to detect species-specific elevational variation in fine-root water uptake and its relationship with the variation in fine-root functional traits in subalpine forests. Fine-root water flux (WFsoil-root) was evaluated from direct measurement of the water potential difference between the soil and fine roots, and the hydraulic conductivity of fine roots of Abies mariesii Mast. and Betula ermanii Cham. Additionally, we measured the average diameter, specific root length and root tissue density (RTD) as morphological traits, and nitrogen content (N) as a chemical trait. These traits were compared at different elevations (2000, 2300 and 2500 m), and the relationships between WFsoil-root and root morphological and chemical traits were evaluated. The WFsoil-root of A. mariesii was highest at 2500 m compared with the WFsoil-root value of B. ermanii at 2300 m. These results suggest that the limiting factors of fine-root water uptake differ between A. mariesii and B. ermanii in subalpine forests. Additionally, WFsoil-root covaried with the RTD-N axis along the elevational gradient, and trees increased WFsoil-root with increasing RTD. This result brings the new insight that higher RTD of fine root could function as the acquisitive traits for water uptake in subalpine forests. However, covariation of WFsoil-root with RTD-N axis was less obvious in A. mariesii than B. ermanii indicating different driving mechanisms of WFsoil-root between the species. Trees must cope with several factors limiting their growth in subalpine forests. Adjustment of WFsoil-root may contribute to the species-specific strategy, which compensates for their physiological processes and growth, and coordination with the RTD-N axis would be important for effective water uptake in cold and carbon-limited environments.