Understanding the environmental regulation of tree phenology

Sammanfattning: In temperate and boreal climates, trees synchronise their annual growth cycles with seasonal changes in daylength and temperature. Understanding how environmental cues regulate tree phenology is important to our ability to capture the potential responses of trees to climate change, with implications for forest productivity.In this thesis, we demonstrated that different research methods can be applied to study the regulation of tree phenology and that this knowledge can be used to generate climate change impact assessments. The knowledge gaps identifiedby summarising recent advances on the molecular regulation of growth cessation and bud set were addressed by modelling. Modelling autumnal bud development showed that both photoperiod and temperature help to predict thetiming of bud set in non-stressful conditions, while additional regulatory mechanisms may be involved under stressful conditions.The differences in phenological response to environmental signals between populations and provenances were accounted for via the values of model parameters. The provenance specific temperature sum requirements for bud burst for Norway spruce were used to calculate the risk of spring frost damage under current climate conditions and future climate scenario. The timing of bud burst will occur earlier in future and will be associated with an increased risk ofspring frost damage due to the increased frequency and severity of spring frost events. The information on the provenance specific frost risk will facilitate forest management decisions on choosing suitable plant material for regeneration.The ecosystem model was used to assess the effect of phenology parameterisation on simulating carbon uptake. Model simulations with calibrated phenology parameterization predicted enhanced forest productivity by the end of the century due to earlier timing of bud burst. However, uncertainty remains whether reduced winter chilling may slow down future bud burst advancement and forest productivity increase, highlighting the need for a mechanistic understanding of environmental regulation of dormancy release and bud burst.Further progress towards a better understanding of regulation of tree phenology can be achieved through integrating molecular and modelling approaches, by incorporating knowledge on molecular pathways of environmental control ofstages of the annual growth cycle into phenological models.