Some early signs of spring such as the sight of robins returning to the yard or the sound of frogs chirping in the woods are obvious and welcome. Other signs like the swelling tree buds are more subtle. We usually notice them first with itchy eyes or a dripping nose.
Wildlife survives winter by migrating, hibernating, or toughing it out. Trees, however, go dormant, a process triggered in the fall by decreasing daylight. Leaves drop, branches harden off, and the flow of water and sugars subsides.
A midwinter warm spell does not wake the tree from dormancy. The tree waits until it has experienced a sufficient number of chilling hours and then forcing (or warming) hours before beginning the process of waking from dormancy. The timing is different for each tree species.
These environmental cues, chilling and forcing, alter the balance of hormones that keep a tree dormant or signals growth. This evolutionary process helps a tree to take advantage of favorable spring growing conditions while avoiding tissue damage from late frosts.
By the time a tree drops its leaves in the fall, the buds are already formed and are packaged up in a protective armor of scales. When the sap rises in the spring from the tree’s roots to its branches, buds swell, and scales fall off. The leaves, stem, and flowers unfurl from the bud releasing pollen.
Take a look around your house or woods and look closely at the buds on the branches. Do they look different? Maple, cherry, and buckeye trees tend to leaf out earlier than oak, hickory, walnut, and ash trees. Take pictures or notes about how they differ, both among species or by location. Are the trees facing the south further along? Keep track of how they change throughout the spring.
Scientists and citizens are doing this across the world. Henry David Thoreau started tracking the leafing out of plants in the 1850’s around Walden Pond, and Aldo Leopold kept notes on plants and wildlife across Wisconsin from 1945-1999. This study of how environmental factors affects plant and animal life cycles is called phenology.
Project Budburst is tracking data on over 250 plants across the United States, examining how our warming planet is affecting the phenology of plants. By engaging everyday citizens, schools, and scientists, a large database is developing.
Trees are waking earlier from dormancy today than they did in Thoreau and Leopold’s time. We know certain parts of our earth are warming more quickly than others. The migration of birds and the life cycle of insects are not changing to coincide with this earlier budburst. Invasive plants require a much shorter chilling period during winter than native trees. What will the effects be?
Scientist currently can only hypothesize about the repercussions of climate change on the phenology of trees. With plants at the base of the food system in our environment, the effect will be felt by everyone in the ecosystem.