The influence of hybridization on epidermal properties of birch species and the consequences for palaeoclimatic interpretations

The Fennoscandian birch population primarily consists of Betula nana, B. pe ndula and B. pubescens ssp. czerepanovii, the Mountain birch. Frequent hybr idization between the Mountain birch and B. nana generates a wide range of genotypic and phenotypic plasticity in the subarctic birch zone of Fennosca ndia. Phases of subarctic conditions prevailed during the Late Glacial in l arge parts of NW Europe, and palynological as well as macrofossil analysis provide some evidence for the occurrence of birch hybrids during these inte rvals. Leaves from genetically controlled specimens of Betula pendula, B. p ubescens ssp. czerepanovii, B. nana and the hybrids B. pubescens ssp. czere panovii x nana and B. nana x pubescens ssp. czerepanovii are investigated f or their specific characteristics of the epidermis morphology. Frequency an d size of epidermal cells and stomata reveal a close affinity of both hybri ds to B. nana and allow a differentiation of the intermediate forms between B. nana and the Mountain birch. With respect to palaeoatmospheric CO2 reco nstructions based on stomatal index, epidermal analysis shows that a possib le occurrence of hybrids in fossil leaf assemblages has no profound consequ ences for combined species records. However, the significant differences ob served in B. nana demand the separation of this species. A comparison of th e cuticle properties of B. pendula and B. pubescens from Finnish Lapland an d leaf material from The Netherlands reveals a divergence of the stomatal i ndex that may be due to differences in day light length.