Vegetative bud-burst variability of european elms.

The bud-burst dates of clones of Ulmus minor, U. glabra, and U. laevis were recorded in the field during the spring of 2000, 2001 and 2002 at six locations in five European countries (Belgium, France, Spain, Italy and Germany). Meteorological data were obtained from stations close to the plots. Thermal time to bud-burst (T) and chilling (C) were calculated. In the three species, winter rest release seems to be controlled by two opposite quantitative effects of the temperature, which interrelate according to an inverse exponential relation. Besides, the bud-burst date was found to be stable between years characterized by different winter thermal trends. These 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.