Variation in timing of bud-burst of Ulmus minor clones from different geographical origins

The vegetative bud-burst phenology of field elm (Ulmus minor Mill.) clones originating from a large geographical range within the species natural area was studied at three European sites over a 5-year period in ex situ collections constituted for the conservation of elm genetic resources. At all the sites and in all the years of study, the date of bud burst and the required thermal time to bud burst were directly related to latitude and altitude of origin of the clones. Nevertheless, the differences among origins were reduced with increasing the chilling duration, suggesting that the clones from more northern latitudes and higher altitudes required longer chilling for dormancy release. Analyses of variance performed on selected groups of clones from different latitudinal origins showed that the order of bud burst was stable among years and that the thermal time requirements decreased in all the groups with increasing chilling, with significant differences among groups; southern clones satisfied their chilling requirements after a shorter chilling duration.