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The Science of What Blooms When

A woman wraps a band around an oak tree
Christy Rollinson studies an oak on the Arboretum's grounds.
April 27, 2017

One of the signs of spring is when we see the first flowers. Those blooms don’t come at the same time every year: There are early springs when warm weather spurs plants to bloom early, chilly late springs when they wait to bloom, and hot springs when the blooms seem to last for just a day.

Researchers track these variations in bloom as part of the science of phenology—the study of timing in nature and how the life cycles of different organisms, such as flowering trees and pollinating insects, are coordinated (or not).

Christy Rollinson, forest ecologist at The Morton Arboretum, studies phenology as she tries to understand how climate change is altering the lives of living things and possibly scrambling the ways they interact. For example, she says, the timing of spring bloom is “a bioindicator of climate change.” This year, phenology data indicate that spring came to most of North America about a month earlier than the long-term average.

That’s not necessarily a good thing, Rollinson says: If plants bloom before their pollinating insects have hatched, they may not be able to reproduce. An early spring may cause plants to use up all the moisture in the soil and face stress from drought. If trees in an oak woodland open their leaves early, the wildflowers on the forest floor may be starved for light. Climate change “can alter how species relate to each other in a community," she says. 

The Arboretum is an especially good place to study plant phenology because it has trees from all over the globe growing in the same climate.

“You can see how species from around the world do or don’t respond to the same weather fluctuations,” she says. “You can assess their vulnerability to climate change.”

That can begin to tell scientists about the future of entire forest ecosystems.

Phenology measures more than spring bloom. In the Arboretum’s Oak Collection near Parking Lot 8, some trees have metal bands strapped around their trunks with instruments called dendrometers. Rollinson and her associates are measuring something very subtle: Exactly when and how fast the trunks get wider as the trees add cells under their bark.

“We can count the tree rings to tell how old a tree is,” she says, “but a ring only tells you about a whole year. We want to know when during the year the growth is taking place.”

Rollinson, who is relatively new to the science staff, is recruiting volunteers to help gather phenology data for her research. She hopes to have more volunteers in place to track when leaves at the Arboretum drop this fall, especially among the oaks. If you’re interested, contact the Volunteer Department

In the meantime, she says, the Arboretum has joined the U.S. National Phenology Network, which encourages anyone to contribute to science by tracking changes in trees, other plants, and animals. The pool of data from citizen scientists all across the nation has proved its value to science, she says.

“You can monitor a tree at the Arboretum or in your own backyard,” she says. “You can see in maps how your data compare to other measurements from the whole country. And it will help me and other scientists understand what is happening to our trees and forests.”