Epicuticular wax lipid composition of endemic European Betula species and its application to chemotaxonomy and paleobotany

Abstract

Plants, in particular trees with specific habitat demands are excellent indicators of climate state. Vegetation successions in subrecent and deep geologic time is recorded in fossil macro-remains or pollen accumulating in geological archives like limnic and marine sediments, peat bogs and mires. Birch trees in Europe form a major part in plant successions and constitute the dwarf species Betula nana and Betula humilis representing cold-adapted habitats or climates and two tree birches, Betula pubescens and Betula pendula characteristic for temperate habitats or climates. These birch species exhibit highly similar pollen shape and size, preventing their unambiguous application as paleoclimate/paleovegetation proxies. We here present a chemotaxonomic differentiation of the four European birch species based on their epicuticular wax lipids. The dominating lipid classes in epicuticular birch waxes were found to be n-alkanes (in the range of n-C23 to n-C33), straight-chain primary alcohols and fatty acids (in the range of n-C20 to n-C32), and long-chain wax ester (in the range of n-C38 to n-C46) in variable amounts and distributions. When preserved in geological archives these lipids may serve in paleovegetation/paleoclimate reconstruction. Long-chain wax esters are susceptible to hydrolysis and upon diagenesis the release of ester-bound alcohols and fatty acids may modify the distribution pattern of the corresponding primary free lipids. Quantitative analysis of the hydrolyzable wax ester proportion revealed primary distribution patterns of birch lipids not to change substantially upon release of bound analogues. The specific composition and abundance of epicuticular wax lipids facilitates unambiguous chemotaxonomic separation of the four European birch species. Wax lipid-based discrimination in field application, however, is complicated by mixing of alkyl lipids derived from different birch species and contribution of wax lipids from other plants. In cases, where palynology indicates a high contribution of Betula species to European vegetation associations, wax lipids may serve for differentiation of the species contributing.