Despite experimental evidence of the individual and interactive effects of photoperiod and temperature on bud growth, photoperiod has not yet been effectively accounted for in models of budburst. However, in some tree species, such as Betula pubescens (birch), photoperiod has an important role in phenological control, and its inclusion in process-based models of budburst might affect phenological projections under climate change scenarios. The aim of the present study was to integrate photoperiod into a process-based phenological model (Chuine 2000; J Theor Biol 207: 337-347; Unified model), using experimental findings in which photoperiod was found to significantly affect budburst in B. pubescens (Caffarra et al. 2011; Clim Res 46:147-157, this issue). The effect of photoperiod was integrated into the model at 2 levels. Firstly, photoperiod, in interaction with temperature, affects the course of dormancy induction. Secondly, photoperiod modifies the response to temperature during the phase of forcing. The resulting model (DORMPHOT) for the simulation of birch budburst was fitted to a large phenological dataset, including data from different latitudes, and validated with 7 datasets from 4 different European countries. Besides giving more biological realism to the model, the newly introduced mechanisms improved its predictive performance. The DORMPHOT model outperformed the Unified model, the linear regression model (budburst date vs. spring average temperature), and the UniForc model. It also proved to be more accurate at predicting budburst in extremely warm years, which suggests it might be more reliable than previous models when using future climate change scenarios.
KEY WORDS: Betula pubescens · Budburst · Calibration · Phenological models · Photoperiod · ValidationResale or republication not permitted without written consent of the publisher Clim Res 46: 159-170, 2011 temperature changes as dormancy progresses, and its responses are, in some cases, proportional to the amount of time of exposure to a particular environmental condition. These findings have resulted in the elaboration of models that describe budburst timing as the end point of 2 consecutive phases: endodormancy, which is released upon the cumulative effect of chilling temperatures (cool, autumn-winter temperatures), and ecodormancy, during which the cumulative effect of forcing temperatures (warm, spring temperatures) promotes cell growth (Hänninen 1990, Battey 2000. These models account for the effects of temperature in terms of developmental units, and different functions relate temperature to the rate of growth or the rate of dormancy release of the buds.Among these models, the simplest consider only the effects of forcing temperatures for the prediction of budburst timing. In the Spring Warming model (Cannell & Smith 1983, Chuine et al. 1998, Pop et al. 2000, also called the Thermal Time model (Hunter & Lechowicz 1992), and in the UniForc model (Chuine 2000), temperature is linearly or sigmoidally related to the rate of growth...