Seasonal variation of leaf wax n-alkane δ2H values: Differences between Quercus aquifolioides (an evergreen tree) and Stipa bungeana (a perennial grass) from the southeastern Tibetan Plateau

Abstract

To understand the cause of known isotopic differences between grasses and trees, their seasonal variability and environmental drivers, we present a parallel study of a perennial grass, Stipa bungeana, and an evergreen tree, Quercus aquifolioides, in the monsoon-dominated southeastern Tibetan Plateau (TP), by conducting fortnightly paired δ2H analyses of precipitation (δ2Hp), leaf water, and n-alkanes (δ2Hwax) over a period of 2 y. Similar to previous investigations, we show that δ2Hwax in both species are correlated to δ2Hp throughout two growing seasons. However, for the first time, we identified a time lag of 2 months in variation of δ2Hwax relative to δ2Hp in new Q. aquifolioides leaves, whereas nearly no lagged phenomena were observed in S. bungeana and old mature Q. aquifolioides leaves. The δ2Hwax in new Q. aquifolioides leaves correlated significantly with the total amount of n-alkanes. These findings suggest that the turnover rate of n-alkanes in new Q. aquifolioides leaves is possibly changing with their synthesis, and that is a possible mechanism of the observed delayed phenomenon. A paired t-test showed that the difference of δ2Hwax between trees and grasses becomes insignificant after time-lag correction. Moreover, the apparent fractionation in Q. aquifolioides was significantly correlated with weekly or fortnightly precipitation amount, temperature, relative humidity, and wind velocity, while in S. bungeana only weekly or fortnightly precipitation amount. Our high temporal resolution observations hence confirm that climate variables are important drivers of plant δ2Hwax and, give significant implications for δ2Hwax-base paleoclimate reconstructions in global monsoonal regions.