PREDICTING THE TIMING OF BUDBURST IN TEMPERATE TREES

1. Four models for predicting budburst in northern hardwood trees, based on response to spring warming alone, or with the response to spring warming modified by winter chilling and photoperiod, were compared. An historical, 18-year budburst record, and artificial datasets with budburst dates generated according to each of four conceptual models, were used to analyse the abilities of the models to predict budburst dates. 2. The four models all gave better predictions than could be obtained by taking the average date of budburst of a species. The historical budburst dates were most accurately predicted by models based only on spring warming from a fixed start date, or from a start date determined by the satisfaction of a chilling requirement. A photothermal model was only useful for species with late budburst dates, and gave relatively little improvement over the average date of budburst as a predictor. 3. Analysis of artificial datasets, in which budburst dates were generated according to the biological assumptions of each conceptual model, reveals little connection between the ability to predict budburst with accuracy and the underlying biological response to temperature. This should be a general caveat to modellers; even biologically incorrect models can give reasonably good predictions of budburst phenology.