The potential for abnormal leafing phenology under climate change: differences in sensitivity of spring budburst to winter chilling in six North America deciduous tree species

Background/Question/Methods

Phenology, the timing of seasonal biological events such as budburst and blossom dates has provided a globally consistent signal of plant response to climate change. For trees in seasonally cold climates, change is most commonly manifest as earlier budburst, likely due to earlier onset of warm temperatures. It is often assumed that climate change will result in earlier spring and concomitantly, a longer growing season, in temperate and boreal forests. However, winter chilling and photoperiod can also be important determinants of spring leafing phenology. Here we examine the role of mid- to late winter chilling for timing of budburst in six deciduous tree species (Larix laricina, Acer rubrum, Quercus ellipsoidalis, Betula papyrifera, Populus tremuloides, and Rhamnus cathartica[invasive]) that co-occur in central Minnesota, USA.  Shorter and milder winters with long periods of inter-seasonal winter warm-ups now occur more frequently. We hypothesized that reduced duration of chilling, associated with warmer winters, could lead to delayed budburst. Further, we hypothesized that species may differ in the sensitivity to mid- to late winter chilling. We collected branches every two weeks from January-March 2011 and December-March 2012, forced them under long day conditions in a greenhouse (21ºC/16ºC; 16h photoperiod), and monitored phenology every 2 days.

Results/Conclusions

Results support our working hypothesis that timing of budburst will be influenced by duration of chilling and that species will differ in senstitivity to chilling. We found no relationship between chill days and date of budburst date for two boreal species L. laricina and B. papyrifera, nor for the invasive species R. cathartica.  Conversely, A. rubrum was highly sensitive to mid- to late winter chilling: time to budburst was shorter as chilling increased for Acer rubrum, a temperate species.  P. tremuloides, Q. ellipsoidalis had an intermediate response to chilling. Insensitivity of some species to warming suggests the potential for inappropriate during winter warm spells. Indeed, for L. laricina 8 days at 21ºC/16ºC was enough to induce budburst even in mid December. Conversely, high sensitivity to chilling suggests the possibility for delayed rather than advances in budburst in a warmer future.