Shifting Seasons, Rising Risks: Spring Frost Predictions for Pedunculate Oak in Hesse, Germany

Advanced bud burst in deciduous trees extends the growing season to an earlier date, increasing their vulnerability to spring frost damage. Such frost events, though understudied in current research, can cause long-term damage by compromising the recovery capacity of trees and adding stress to their carbon and water balance. The resilience of pedunculate oak (Quercus robur L.) to the increasingly dry and warm climate positions them as crucial tree species in the planning of climate-adapted forests. Understanding the balance between drought resilience and frost vulnerability is essential for informed decisions about future forest management strategies. In this study, we used a dense monitoring network of phenological observations to i) highlight a significant increase in spring frost risk in recent years and ii) calibrate two phenological models and apply them to the output of seven coupled RCP8.5 climate projections. The models predict a mean advance in oak bud burst by 2-2.7 days per decade, consistent with a 2 days per decade advance observed historically. Despite the general warming trend, last spring frost events have not preponed to earlier dates in the observations. This is probably linked to stable high-pressure systems in spring with enhanced radiative cooling at the surface during the night. However, the projections fail to accurately capture the last frost, likely linked to a known weakness in blocking events. Consequently, we suggest that future frost risk may be underestimated in current projections.