The Coming of Spring and Trees

This was a different winter. It had a good start - cold with some snow. Then, it was February. Instead of the coldest month, it was light jacket season - it seemed as if spring was here to stay. Now, we are just past the spring equinox and most of the state recently saw a lot of white blanketing the ground. If we are confused, imagine how the plants "feel."

Some trees and other woody plants have started to respond to the unnatural weather conditions with swollen buds and subtle changes in twig color (yellow on willow, and reddish purple in black birch). Fruit growers across the state are anxiously watching the weather, wondering if we will get another cold snap that may hold off flowering until later in April and our more traditional frost-free dates. Similarly, strawberry producers are expressing concerns about cold temperatures affecting those delicate flowers. People care about these domestic plants and ask questions: Will the crops be safe? Will our weather affect supply and prices?

We can all appreciate how weather affects our crops and landscape plantings; however, what is happening in forests and woodlands? The astute observer will note changes that suggest forests are ahead of schedule as well. By the end of February in central Pennsylvania, some red maples were already opening flower buds. At the same time, buds on some shrubs (e.g., elderberry) were expanding and showing the yellow-green edges of juvenile leaves under the brown bud scales. By the second week in March, it was common to see aspen flowers displayed high in tree crowns.

Our native trees have over millennia adapted to our climate and go through a process of acclimation in the fall to prepare for cold and deacclimation in the spring to initiate growth. Both of these processes are set in motion by the interaction of available light and temperature. As trees begin the process of shutting down or going dormant for the winter, many internal changes at the cellular level prepare them for cold temperatures, and once they are dormant, they will remain in that state until they meet some external and internal thresholds. Again, these include light and temperature. In addition, for some tree species, they need to attain a chilling threshold where temperatures are cold enough for an extended period. Once dormant, most of our trees can tolerate extremely low temperatures.

Trees begin to "awaken" or come out of dormancy (i.e., deacclimate) in response to photoperiod or the amount of light and the amount of time where temperatures are above freezing. Just like many of us, coming awake is a much slower process than falling asleep or going dormant. Apparently, once the deacclimation process reaches a certain point, it is increasingly difficult to reverse; however, it is possible to "stall" growth initiation with cold temperatures if it is early enough. As dormancy begins to break, growth initiation will occur and most sensitive are flower and leaf buds. Most often, though, for many of our forest trees, flowering buds open after the leaf buds, and leaves are generally more cold tolerant.

Studies have determined that our native trees deacclimate differently in response to the amount of light or photoperiod and the accumulation of warmth when "deciding" to awaken in the spring. Among those that response more to light are white ash, sweet gum, white pine, white oak and swamp white oak. Those that awaken to increasing amounts warming include sugar maple, box elder, green ash, butternut, quaking aspen, black cherry, and red oak.

In a normal year, the combination of light and temperature provides the means for predicting plant processes such as spring greening and fall coloration. However, one of the unknowns is how our trees will have responded to the warm February. We may well see some trees leaf out earlier. Hopefully, these young leaves will do well. More problematic is if forest tree flowers appear too early and their subsequent response to early frosts, which are extremely damaging to flowers. Many wildlife species depend on successful seed years for mast production (i.e., acorns, nuts, and fruits). Increasingly changes in weather and climate put seed production at risk and the resulting impacts to forests and dependent species and processes are important to consider.

Documenting and understanding the timing of these and other events in a plant's life involves the study of phenology. Phenology is very useful as it relates to timing of anticipated natural events and may help us understand how weather and climate change, as well as how those factors affect wildlife behavior and lifecycles. People, both scientists and citizens, have followed and document phenological events for years. To enlarge the number of people watching events across large regions, scientists seek input from the public in a partnership to do citizen science. As an example, one citizen-science project - Project BudBurst - has depended on people across the country to report when common plants first flower. This effort and others like it provide data on how changing climate is affecting plants.
If you would like to learn more about phenology, look to Center for Private Forests at Penn State's website to purchase a copy of its Woodland Nature Journal to record your local observations and to find information on other citizen-science projects relating to phenology.