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We investigated the relationship between stomatal frequency and a range of atmospheric CO2 concentrations ([CO2]atm) in Betula pubescens and Pinus sylvestris, two important boreal trees in Scandinavia. If strong relationships exist, they can be used to reconstruct past [CO2]atm from stomatal frequency of fossil Betula and Pinus leaves. Responses of epidermal characters (stomatal density (SD), epidermal cell density (ED), stomatal index (SI)) to different CO2 concentrations were investigated utilising (1) the lower partial pressure of CO2 at increasing altitudes for B. pubescens, and in herbarium specimens of B. pubescens and P. sylvestris collected during the post-industrial rise of [CO2]atm from c. 280 ppmv to c. 360 ppmv in 1997 and (2) concentrations (560 ppmv) and temperatures (3° summer) above present day in the CLIMEX greenhouse experiment. All the results show no clear relationship between SD or SI and [CO2] atm for either B. pubescens or P. sylvestris. Most likely there are stronger genetically and environmentally induced factors that affect the development of the leaves. Problems with collecting representative samples from herbarium specimens are discussed. Since the effects of changes in [CO2]atm cannot be statistically modelled, B. pubescens and P. sylvestris are not suitable for reconstructing past atmospheric CO2 concentrations from fossil leaves using stomatal density or stomatal index