Modelling Populus autumn phenology: The importance of temperature and photoperiod

In late summer-early autumn, trees undergo growth cessation culminating in bud set. For a wide range of plants, including Populus, photoperiod is considered as the primary environmental cue determining timing of growth cessation and shoot to bud transition. However, studies on Populus have revealed temperature influence on seasonal growth cessation and bud set. In this study we examine the role of temperature in regulating the transition between phenological stages of bud development, from a growing apex to a closed bud. We test different model structures incorporating cues from both temperature and photoperiod (M1 and M2) as compared to the previously established model based on photoperiod only (M0, null hypothesis). All models simulate the date of bud set for Populus trees from 12 latitudinal populations grown in the common garden of Sävar. For two of the three years (2005 and 2007) M1 and M2 models outperformed the null model in predicting bud set date. Poor predictions for 2006 were related to stressful weather conditions. This indicates that under non-stressful conditions temperature can be a factor modifying the photoperiod response, while other regulatory mechanisms can be at play during stressful conditions. A phenological model of bud development has to account for both types of responses. Thus, our study highlights the importance of temperature as a factor that should be considered in addition to the well-established photoperiodic signal in studies on autumnal bud set under natural conditions.