Abstract
Plants are excellent climate indicators and their macro-remains or pollen accumulating in geological archives serve as recorders of environmental change. In Europe birch trees contribute importantly to Holocene plant successions. They constitute the dwarf species Betula nana and B. humilis, representing colder and two tree birches, B. pubescens and B. pendula indicative of more temperate climate. Birch pollen is highly similar preventing species differentiation. We obtained unambiguous chemotaxonomic differentiation of four European birch species via cuticular wax lipids. Dominating lipid classes in recent epicuticular birch waxes were n-alkanes (nC23 to nC33), n-alcohols and n-alkanoic acids (nC20 to nC32), and long-chain wax ester (nC36 to nC48) differing in amount and distribution. After plant senescence and in geological archives lipids undergo diagenetic alteration modifying the distributions found in recent plants. Long-chain wax esters via hydrolysis release bound n-alcohols and n-fatty acids, adding to their free analogues. Simulated release of bound lipids increased the pool of n-alcohol and n-fatty acids up to 400%. Such modification of primary lipid patterns is unaccounted for in most paleovegetation studies. Proceeding diagenesis, e.g. by decarboxylation will convert these functionalized primary and secondary lipids into their corresponding n-alkanes, the compound class mostly applied in paleoenvironment reconstruction. The simulated n-alkane pattern changed significantly, evidenced by an increase of mid-chain (nC23,nC25) homologues. Release of bound lipids may not only alter molecular but also isotopic composition, which may cause errors in paleoclimate reconstruction. We assessed the potential contribution of secondary (free lipid decarboxylation) and tertiary (bound lipid decarboxylation) wax metabolites and compared the cumulative n-alkane patterns with birch n-alkane distributions reported in the literature. Two statistically different patterns were separated, one dominated by primary, the other by secondary and tertiary formed n-alkanes. This may explain the inconsistency in previous birch wax analysis reported and needs consideration in paleoenvironment reconstruction.
Highlights
Environmental demands, in particular climate, govern the present-day habitat and distribution of trees, which in turn facilitates determination of climate regimes in the contemporaneous as well as in the fossil domain
Taxonomy of present-day trees is based on anatomical, morphological, genetic and biochemical studies, whereby such features in the fossil record are best preserved in pollen distributions, due to a higher recalcitrance of pollen versus other plant organs, e.g. leaves and rare findings of other macro-remains, e.g. fruits
All samples were analysed by gas chromatography-mass spectrometry (GC/MS)
Summary
Environmental demands, in particular climate, govern the present-day habitat and distribution of trees, which in turn facilitates determination of climate regimes in the contemporaneous as well as in the fossil domain. Taxonomy of present-day trees is based on anatomical, morphological, genetic and biochemical studies, whereby such features in the fossil record are best preserved in pollen distributions, due to a higher recalcitrance of pollen versus other plant organs, e.g. leaves and rare findings of other macro-remains, e.g. fruits. Birches (Betula L., Betulaceae) are common broadleaf trees and shrubs occurring in diverse habitats of the boreal and the cold-temperate zones of the Northern Hemisphere [9,10]. Ranging from temperate zones in Northern America over Eurasia to East Asia and the circumpolar regions, birches populate different habitats including forests, swamps, tundra and mountainous terrains [11]. The two tree birches, Betula pubescens (downy birch) and Betula pendula (silver birch) occur throughout most of Europe, whereby
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