Modelling bud dormancy release in trees from cool and temperate regions

A model of the effects of air temperature on bud dormancy release was developed on the basis of experimental results with two-year-old seedlings of Pinus sylvestris and Picea abies and a study of previous models. Bud dormancy is generally released when buds are exposed first to chilling temperatures (e.g. -5 degrees C to 10 degrees C) and then to forcing temperatures (e.g. >0 degrees C). Seedlings from a central and a northern Finnish provenance of Pinus sylvestris, and from a central Finnish provenance of Picea abies, were raised in paper pots in a mixture of peat and sand, and grown outside. At the end of the second growing season, the seedlings were transferred to dark growth chambers and subjected to chilling conditions. In one experiment the chilling took place outside. The seedlings were then subjected to forcing temperatures (12-22 degrees C) in growth chambers. Bud burst was said to have occurred if the total growth of the terminal bud exceeded 5 mm. Behaviour of the Finnish material differed from that predicted by models based on studies of more southerly provenances. The new model is described, and further model development outlined.