Frost hardening and dehardening potential in temperate trees from winter to budburst

We investigated how deciduous trees can adjust their freezing resistance in response totemperature during the progress of the ecodormancy phase, from midwinter to bud burst. We regularly sampled twigs of four different temperate deciduous tree species from Januaryto the leaf-out date. Using computer-controlled freezers and climate chambers, the freezingresistance of buds was measured directly after sampling and also after the application of artifi-cial hardening and dehardening treatments, simulating cold and warm spells. The thermaltime to budburst in forcing conditions (c.20°C) was also quantified at each sampling as aproxy for dormancy depth. Earlier flushing species showed higher freezing resistance than late flushing species at eithersimilar bud development stage or similar dormancy depth. Overall, freezing resistance and itshardening and dehardening potential dramatically decreased during the progress of ecodor-mancy and became almost nil during bud burst. Our results suggest that extreme cold events in winter are not critical for trees, as freezingresistance can be largely enhanced during this period. By contrast, the timing of budburst is acritical component of tree fitness. Our results provide quantitative values of the freezing resis-tance dynamics during ecodormancy, particularly valuable in process-based species distribution models.