Increased Risk of Freeze Damage in Woody Perennials VIS-À-VIS Climate Change: Importance of Deacclimation and Dormancy Response

Winter survial during an annual cycle of temperate-and boreal-zone woody perennials is a multicomponent response.Two of these components are: (1) the timing, rate, and extent of cold acclimation (CA; Box 1), and (2) plants' ability to sufficiently maintain its freezing tolerance (FT) until the danger of killing frosts is passed which is determined by, among other factors, plants' ability to resist any premature/unseasonal deacclimation (DA; Box 1), which, under a predictable seasonal pattern, typically occurs during spring-warming. DA RESPONSE IS MORE CRITICAL THAN CA UNDER CLIMATE CHANGE SCENARIOCA is primarily initiated by the shortening day-length in autumn but further enhanced by cold temperatures by mid-winter.Research shows that short-days alone are sufficient to induce significant FT in woody perennials.Following are just of many examples that support this observation: (1) Pinus sylvestris acclimated to -22 • C under short-days alone compared to -40 • C under both the short-days and cold (Smit-Spinks et al., 1985), (2) Taulavuori et al. (2000) reported significant induction of FT in pine seedlings by short-days at 20 • C, and (3) Fagus sylvatica are already hardy to below -15 • C in October for an average year (Tranquillini and Plank, 1989).Moreover, FT of overwintering tissues in mid-winter is generally much higher than the absolute minimum temperatures typically encountered in nature (Vitasse et al., 2014).Therefore, the timing or capacity of CA does not constitute a critical factor regarding the risk of freeze-damage unless, however, an unusually early freeze in autumn causes premature leaf-loss and interferes with the preparation for maximal CA.The dominant driver of DA, on the other hand, is warm temperature, and DA proceeds much faster (hours to days) than autumnal CA (weeks to months) (Fuchigami et al., 1982;Taulavuori et al., 2004;Kalberer et al., 2006).Consequently, erratic temperature fluctuations, i.e., "unseasonal" spring-like conditions followed by a freeze, could render partially or fully deacclimated tissues vulnerable to freeze-damage.Indeed, magnitude and frequency of such fluctuations have been on the rise in recent years (Jentsch et al., 2007; IPCC, 2014), and some of the most devastating killerfrosts across North America are attributed to such vagaries of climate, e.g., Easter freeze of 2007