Models of the spring phenology of boreal and temperate trees: Is there something missing?

According to prevailing theory, air temperature is the main environmental factor regulating the timing of bud burst of boreal and temperate trees. Air temperature has a dual role in this regulation. First, after the cessation of growth in autumn, prolonged exposure to chilling causes rest completion, i.e., removes the physiological growth-arresting conditions inside the bud. After rest completion, prolonged exposure to warm conditions causes ontogenetic development leading to bud burst or flowering. During the past three decades, several simulation models based on chilling and forcing have been developed and tested. In recent modeling studies of the timing of bud burst in mature trees, the simpler thermal-time models that assume forcing starts on a fixed date in the spring have outperformed the chilling-forcing models. We hypothesize that this discrepancy may be due to some element missing from the chilling-forcing models. We tested two new model formulations by introducing reversing, temperature-driven elements that precede forcing and by fitting the models to seven historical time series of data of flowering and leaf bud burst of common boreal tree species. In these tests, both of the new models were generally more accurate in predicting the timing of bud burst than a classical chilling-forcing model, but less accurate than the simple thermal-time model. We therefore conclude that besides chilling, other environmental factors are involved in the regulation of the timing of bud burst. Further work is needed to determine if the regulatory factors derive from air temperature or from some other environmental condition such as changes in light conditions, like day length or night length.