Divergent patterns of δ13C versus δ15N in Quercus aquifolioides along an altitudinal gradient: An integrated study based on multiple foliar traits

Background/Question/Methods

1. Due to the different mechanisms underlying δ¹³C and δ¹⁵N, we hypothesized that they could exhibit differing patterns across an environmental gradient, particularly an elevation gradient.
2. In this study, we investigated the shifting patterns of both δ¹³C and δ¹⁵N of Quercus aquifolioides in the Tibetan Plateau with altitudes from 2500 to 3800 m based on measures of multiple foliar traits.

Results/Conclusions

1. Foliar δ¹³C significantly increased linearly toward increasing altitude. By contrast, foliar δ¹⁵N initially increased with the altitudinal gradient, but subsequently decreased around 3300 m. Principal components analysis (PCA) showed that the first principal component (PC1) associated with the δ¹³C, LMA, LM, LT, LA, C_mass, C_area, SD, SL and St/LM accounted for 45.78% of total variance of leaf traits and represented an axis of water availability, and the second principal component (PC2) related to the δ¹⁵N, N_mass, N_area, C: N, St/N accounted for 26.57% of total variance and represented an axis of N availability across the entire elevation gradient.
2. Further analyses showed that (1) δ¹³C showed significantly positive correlations with LMA, LM and LT, and negative correlations with stomatal parameters (SD, SL and St/LM), but no correlation with N_mass and N_area; and (2) δ¹⁵N is positively correlated with N_mass and N_area whereas negatively correlated with C: N and St/N. These suggest that leaf conductance, rather than photosynthetic capacity, could result in altitudinal variations of foliar δ¹³C while altitudinal variations of the δ¹⁵N may be closely correlated with leaf N concentrations and N availability.
3. We therefore concluded that the foliar δ¹³C with LMA, LM, LT, SD, SL and St/LM could reflect water use strategy, while foliar δ¹⁵N with leaf N_mass, N_area and C: N could be related to nutrient use strategy, particularly N availability in the studied species along the altitudinal gradient in the Tibetan Plateau.

Abbreviations: δ¹³C, ratio of ¹³C to ¹²C; δ¹⁵N, ratio of ¹⁵N to ¹⁴N; LMA, leaf mass per unit area; N_mass and N_area, leaf N concentration per unit mass and area; C_mass and C_area, leaf C concentration per unit mass and area; C: N, ratio of leaf N concentration to leaf C concentration; SD, stomatal density; SL, the length of stomata; St/N and St/LM, number of stomata per gram of leaf nitrogen and mass; LA, leaf area; LM, leaf mass; LT, leaf thickness.