Does leaf wax isotopic characterisation of gymnosperms and angiosperms capture environmental gradients in Himalayas?

Abstract

The responses of angiosperms and gymnosperms to environmental variables (e.g., rainfall [MAP], temperature [MAT] and pCO2) remain ambiguous and require systematic investigation. This study monitored the plants' responses to changing environments and measured variations in molecular n-alkane indices, δ13Cbulk, δ13Cn-alkanes, and δ2Hn-alkanes values in woody gymnosperms (n = 36) and angiosperms (n = 17) from an altitudinal gradient (0.9 to 3.8 km) in the Himalayas The results indicate that δ13Cbulk values in angiosperms show a moderate correlation with altitude, MAP, and pCO2, while gymnosperms show no response in their δ13Cbulk. The δ13Cn-alkanes values in angiosperms remain unaffected by MAP, MAT, and pCO2, whereas gymnosperms exhibit a moderate negative correlation. The fractionations between δ13Cn-alkanes and δ13Cbulk values (ɛalk/leaf) are approximately –4.0 ± 1.6 ‰ and –5.6 ± 1.5 ‰ for gymnosperms and angiosperms, respectively. In gymnosperms, the fractionation between δ2Hn-alkane and δ2Hrain (ɛalk/rain) is –133.2 ± 39.8 ‰ and –122.8 ± 38.0 ‰ (C31), while in angiosperms, it is –88.5 ± 44.6 ‰ (C29) and –62.4 ± 22.9 ‰ (C31). The δ2Hn-alkane values in gymnosperms and angiosperms are weakly and positively influenced by MAT and MAP, respectively. We found that gymnosperms are enriched in 13C (∼1.5–3.0 ‰) and depleted in 2H (∼56–60 ‰) compared to angiosperms; this pattern is consistent throughout the latitudes sampled in this study. The species-specific isotopic response to environmental factors is primarily driven by lower stomatal conductance, smaller leaf-size, and presence of complex tracheids in gymnosperms compared to angiosperms. The statistical K-means algorithm of dual isotope analysis (δ13Cn-alkanes and δ2Hn-alkane) provided two distinct clusters with an accuracy of 70 % for angiosperms and gymnosperms, which has implications for studying past vegetation transitions.