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The utility of DNA barcoding for identifying representative specimens of the circumpolar tree genus  Fraxinus  (56 species) was investigated. We examined the genetic variability of several loci suggested in chloroplast DNA barcode protocols such as  matK ,  rpoB ,  rpoC1  and  trnH-psbA  in a large worldwide sample of  Fraxinus  species. The chloroplast intergenic spacer  rpl32-trnL  was further assessed in search for a potentially variable and useful locus. The results of the study suggest that the proposed cpDNA loci, alone or in combination, cannot fully discriminate among species because of the generally low rates of substitution in the chloroplast genome of  Fraxinus . The intergenic spacer  trnH-psbA  was the best performing locus, but genetic distance-based discrimination was moderately successful and only resulted in the separation of the samples at the subgenus level. Use of the BLAST approach was better than the neighbor-joining tree reconstruction method with pairwise Kimura's two-parameter rates of substitution, but allowed for the correct identification of only less than half of the species sampled. Such rates are substantially lower than the success rate required for a standardised barcoding approach. Consequently, the current cpDNA barcodes are inadequate to fully discriminate  Fraxinus  species. Given that a low rate of substitution is common among the plastid genomes of trees, the use of the plant cpDNA “universal” barcode may not be suitable for the safe identification of tree species below a generic or sectional level. Supplementary barcoding loci of the nuclear genome and alternative solutions are proposed and discussed.

Deciduous Trees and the Application of Universal DNA Barcodes: A Case Study on the Circumpolar Fraxinus