Stable isotope compositions of recent and fossil sun/shade leaves and implications for palaeoenvironmental reconstruction

Abstract

Stable carbon isotope values (δ13C) obtained from terrestrial plant leaves are increasingly being used to infer palaeoenvironmental trends. However, there are considerable variations in δ13C values even among leaves of a single plant due to different microhabitats, which can bias palaeoenvironmental interpretations. One important factor causing microhabitat variations is leaf position on a tree (i.e. sun and shade leaves). It is extensively known that δ13C varies markedly between sun and shade leaves in modern plants, with sun leaves containing more enriched 13C. Yet, the δ13C variations of fossil leaves in this respect are not investigated systematically. Here, we examine bulk carbon and nitrogen isotopic variabilities of fossil Liquidambar leaves between sun and shade leaves. For comparison, bulk isotopic data are analyzed in modern Liquidambar. Our results show that carbon and nitrogen content, C/N ratio, δ13C and nitrogen isotope composition (δ15N) are notably different between sun and shade morphotypes in modern Liquidambar. When these criteria are applied to fossil Liquidambar, we found that the difference in stable carbon isotope compositions between fossil sun and shade morphotypes is narrow (only 0.33‰) and statistically not significant (P>0.2). Also, the aforementioned other parameters do not present apparent differences between sun and shade morphotypes (P>0.2). Comparing isotope data with anatomic characteristics previously studied in fossil Liquidambar miosinica, we conclude that anatomical characters are better indicators to distinguish sun/shade leaves for fossil L. miosinica. Here, carbon and nitrogen contents of the sun morphotype are higher than that of the shade morphotype in fossil Liquidambar leaves, suggesting that sun leaves perhaps are more resistant against decomposition. Moreover, the variation of δ13C values is more profound in sun leaves than that in shade leaves for both modern and fossil Liquidambar, suggesting that sun leaves may be more sensitive to environmental changes. Together, our data indicate that δ13C of sun morphotypes is a better proxy in reconstructing palaeoenvironments.