Vegetative bud-burst variability of European elms

The bud-burst dates of clones belonging to the Ulmus minor Mill., Ulmus glabra Huds. and Ulmus laevis Pall. species were recorded in the field during the spring of 2000, 2001 and 2002 at six locations in five European countries. Meteorological data were obtained from stations close to the plots. Thermal time to bud-burst (T) and chilling (C) were calculated. A relationship of the form T = a + b erC was fitted for each species. In the three species, the interruption from dormancy seems to be controlled by two opposite quantitative effects of the temperature, which are linked by an inverse exponential relation. The bud-burst date was found to be stable also in years characterised by considerably different winter thermal trends. The three Elm species have small chilling requirements. U. minor has chilling requirements lower than those of the other two species, in agreement with the more southern distribution.