Divergent patterns of foliar 13C and 15N in Quercus aquifolioides with an altitudinal transect on the Tibetan Plateau: an integrated study based on multiple key leaf functional traits

Aims With a close association with plant water availability, foliar delta C-13 had been investigated extensively in alpine regions; however, foliar delta N-15 has rarely been concurrently used as an indicator of plant nitrogen availability. Due to the positive correlations between leaf nitrogen content and foliar delta C-13 and delta N-15 found in previous studies, we expected that they should show consistent patterns along an altitudinal gradient. Methods To test our hypothesis, we measured foliar delta C-13 and delta N-15 in conjunction with multiple key leaf functional traits of Quercus aquifolioides, a dominant species of alpine forest on the eastern slopes of the Sygera Mountains, southeastern Tibetan Plateau from 2500 to 3800 m. Important findings (i) Contrary to our hypothesis, foliar delta C-13 exhibited a significant positive linear relationship with altitude; in contrast, foliar delta N-15 initially increased and subsequently decreased with altitude, the change in trend occurring around 3300 m. (ii) Our analyses indicated that leaf internal resistance and stomatal conductance, rather than photosynthetic capacity indicated by leaf N concentration, apparently explained the altitudinal variation in foliar delta C-13, while differences in foliar delta N-15 were likely the result of soil N availability. (iii) Principal component analysis revealed a clear association between delta C-13 and a tradeoff between water loss and carbon gain, indicated by traits related to gas exchange such as leaf thickness, density, stomatal properties. In contrast, the second axis was associated with delta N-15 and nitrogen acquisition strategy in Q. aquifolioides across its altitudinal distribution, represented by traits related to nitrogen concentration and stomata per gram of leaf nitrogen.