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Root exudation fulfills fundamental roles in regulating carbon (C)-nutrient cycling in forest ecosystems, yet the main ecological drivers of root exudation and underlying mechanisms in forests under natural gradients remain poorly understood. Here, we investigated the intraspecific variation of root exudation rates in two alpine coniferous forests (Abies faxoniana Rehder et Wilson and Abies georgei Orr) along two elevation gradients on the eastern Tibetan Plateau. Meanwhile, the fine root traits and associated climate and soil parameters were assessed to examine the effects of elevation-dependent changes in climatic and soil nutrient conditions on root exudation. Results showed that root exudation rates decreased with increasing elevation and were positively correlated with mean air temperature. However, the relationships of root exudation with soil moisture and soil nitrogen availability were not significant. The structural equation model (SEM) further revealed that air temperature affected root exudation both directly and indirectly through the effects on fine root morphology and biomass, implying that the adaption of root C allocation and fine root morphological traits to low temperatures primarily resulted in declined root exudation at higher elevations. These results highlight the perceived importance of temperature in determining the elevational variation of root exudation in alpine coniferous forests, which has foreseeably great implications for the exudate-mediated ecosystem C and nutrient processes in the face of drastic warming on the eastern Tibetan Plateau., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Elucidating the mechanisms that control the leaf stable carbon isotope values (δ 13 C leaf ) is the prerequisite for the widespread application of δ 13 C leaf . However, the competing effects of physiological and environmental factors on δ 13 C leaf variations of the different plant functional types (PFTs) have not been disentangled, and the corresponding mechanisms remain unclear. Based on large-scale δ 13 C leaf measurements on the eastern Qinghai-Tibetan Plateau, the relative contributions and regulatory pathways of leaf functional traits (LFTs) and climatic factors to δ 13 C leaf variations of the different PFTs were investigated. We found that δ 13 C leaf of the different PFTs was correlated with annual mean precipitation negatively, but not a simple linear relationship with annual mean temperature and varied by PFTs. Leaf nitrogen content per unit area and leaf mass per area (correlated with δ 13 C leaf positively) had more substantial effects on the δ 13 C leaf variations of the different PFTs than other LFTs. The relative contributions of LFTs to the δ 13 C leaf variations were greater than that of climatic factors, and the direct and indirect effects of climatic factors on δ 13 C leaf variations varied by PFTs. Our findings provide new insights into understanding key drivers of δ 13 C leaf variations at the PFT level on a regional scale., Competing Interests: Declaration of competing interest The authors assert that we have no financial or other non-financial interests that could influence the reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)