Anatomical variations of living and fossil Liquidambar leaves: A proxy for paleoenvironmental reconstruction

Abstract

Based on living L. formosana, we have established that such epidermal cell properties as epidermal cell circumference (Circum.), undulation index (UI), epidermal cell area (EA) on both upper and lower epidermis and the corresponding epidermal cell density (ED), are suitable to differentiate between sun and shade leaves. Also, stomatal properties on the lower epidermis, including stomatal length (SL), stomatal pore length (PL) and the corresponding stomatal density (SD), are reliable parameters to identify sun/shade morphotypes. Then this anatomical approach was applied to 18 leaf remains of L. miosinica from the Miocene Xiannanshan Formation, Zhejiang Province (eastern China), and the fossil leaves were sorted into two groups, namely, sun and shade morphotypes. It indicates that anatomical differences between sun and shade leaves found in living species could be used for fossil leaves. Notably, the variation of UI between sun and shade morphotypes is larger on the upper epidermis as compared with the lower epidermis, showing that the UI on the upper epidermis is more sensitive to environmental changes. In addition, anatomical features are more variable in sun leaves than in shade leaves for both modern and fossil Liquidambar, suggesting sun leaves may be more sensitive to environmental changes. Sun leaves can then be a better indicator of paleoenvironmental change. Thus, stomatal parameters of sun morphotype should be adopted on reconstructing paleoenvironments using Liquidambar leaf remains. On the other hand, the three-lobed fossil L. miosinica leaf assemblage in the Xiananshan Formation is represented by an almost similar percentage of sun and shade morphotypes (56% and 44%, respectively), which is attributed to the short transport before deposition. As a result, identifying sun/shade morphotypes are very useful in reconstructing paleoenvironments based on fossil leaves of Liquidambar.