“O, wind, if winter comes, can spring be far behind?” - Percy Bysshe Shelley
Of all the phenological firsts that will excite and inspire season observers this year, tree buds opening are an anticipated signal of Spring. How do Minnesota trees time budbreak to maximize their growing season but minimize risk of frost? While not a hard and fast rule, the incremental increase in hours of sunlight seems to play a smaller role than the interplay of cold and warm temperatures that a plant experiences in the days and weeks leading up to budbreak.
Experiments have shown that leaf budbreak is accelerated with increasing spring temperatures, but can also be delayed when warmer winters reduce exposure to cold temperatures. In a recent publication, Claudia Nanninga and her colleagues observed five trees native to Minnesota (red maple, tamarack, quaking aspen, northern pin oak, paper birch) and one invasive species (common buckthorn). The used cut branches to manipulate the amount of time the twigs were exposed to temperatures below 40 degrees fahrenheit (chilling temperatures). Across all of the species studied, they observed earlier budbreak as the time exposed to chilling increased. These results confirm the importance of winter chilling on tree phenology, but also raises some questions about how trees may respond to our changing climate.
For the most part, how plants sense both warm and cold temperatures is a question that remains largely unanswered.
Increasing global temperatures have the potential to profoundly change the timing of plant and animal life cycles.. One of the strongest signals of changing climate has been the change in plant phenology, especially the observation of earlier spring budbreak and flowering. Nanninga et al. (2017)’s research suggests that global climate change may result in earlier budbreak at first, but if winter temperatures continue to increase as predicted, tree buds will be exposed to lower amounts of time at chilling temperatures. This could lead to the counterintuitive result that trees will have delayed budbreak in a warmer future.
Nanninga at al. (2017)’s research may raises more questions than answers, but highlights a clear opportunity for engaged citizens to observe and record spring budbreak and flowering. These observations, when taken together, can offer insight into how our climate and the seasons impact the places where we live.
Citation: Claudia Nanninga, Chris R Buyarski, Andrew M Pretorius, Rebecca A Montgomery. 2017. Increased exposure to chilling advances the time to budburst in North American tree species. Tree Physiology 37: 1727–1738.