Using thermal time models to predict the impact of assisted migration on the synchronization of germination and shoot emergence of oak (Quercus robur L.)

Climate change will affect regeneration. Assisted migration is a climate change adaptation strategy that is associated with risks regarding transfer distance. Thermal time models can provide information about the synchronization of regeneration with seasons and the implications for assisted migration. Climate change may prevent trees from adapting or migrating fast enough to track their climatic envelopes. Assisted migration facilitates gene flow by sourcing pre-adapted provenances usually from southerly regions representative of future climates. The aims were to develop thermal time models for the germination and shoot emergence of two provenances of Quercus robur and to predict the impact of assisted migration on the synchronization of regeneration with seasons. Using cumulative germination (radicle emergence) and shoot emergence data from a laboratory experiment, thermal time models were developed for the seedling emergence of Q. robur. Thermal time parameters were then used with climatic data to predict the timing of germination and shoot emergence for English and Italian provenances in southern England. The thermal time parameters were lower for germination than shoot emergence, resulting in their temporal separation. For Italian acorns, base temperature was lower, but thermal time required was higher for germination and shoot emergence compared to English acorns indicating local adaptation. Predictions suggest little difference in the timing of germination and shoot emergence for the two provenances in the future climate of southern England (2080s). Q. robur has a robust regeneration mechanism where the thermal time parameters constrain germination and shoot emergence to safe windows and thereby ensure synchronization with seasons.