The tree that fell has long since rotted away, but we know it was there. How? Because of the tales that tree rings tell.
Those rings record how much a tree grew every year. Reading that record can tell experts at The Morton Arboretum not only about the life of that tree, but about the trees around it, the entire forest, the weather, the climate, and the effects of what people have done. “Tree rings are a record of so much that has happened,” says Forest Ecologist Robert Fahey.
That record may guide how the Arboretum manages its own woods, help predict what climate change will do to trees all over the Midwest, guide how the Arboretum manages its own woods, and inform the choices that cities make about what kinds of trees to plant.
Consider the tale of that tree that died and rotted away. How does Fahey know it was there? Because its death affected other trees and he can read those effects in their rings.
A new ring is formed each year when a tree adds wood just under the bark. If the tree gets more sunshine and more water, it can grow more and the ring is fatter. The ring is thinner when a tree’s growth is curbed, perhaps because the year is dry or the tree is shaded by larger trees overhead or it has to compete with other trees for water.
When the rings of a small group of trees in a forest reveal that they had a sudden growth spurt when they were young, it tells Fahey that they suddenly got more sunlight and more water compared to other trees nearby. The most likely explanation is that a larger tree fell, opening a gap in the forest canopy where light could pour down to reach saplings that had been in the shade. By comparing the rings of hundreds of trees, Fahey can puzzle out the sequence of events. “You can use the rings to spot the gaps,” he says. “It tells you the history of a forest.”
Tree rings also tell tales about people. They told Kurt Dreisilker, the Arboretum’s manager of natural resources, that a disproportionate number of oaks in the East Woods are roughly the same age, dating back to the 1840s. Why? Most likely because the first settlers in the 1830s quickly cut down most of the existing oaks for lumber and firewood. Over the next few years, acorns sprouted to replace them, and those replacement trees are the oaks we know today.
Dreisilker also studied rings to learn that that many of the East Woods’ honey locusts—although they are a native species—were planted in the 1920s, shortly after the Arboretum was founded, and had not been part of the forest before that. He couldn’t have known that the locusts were all the same age by their size, since some, planted in more favorable conditions, were larger than others. But the rings don’t lie.
Plant Conservation Biologist Marlin Bowles studies tree rings to understand how important fire is to the life and balance of tree communities.
Shortleaf pines in the Shawnee National Forest, which need fire to create conditions in which they can reproduce, were in trouble once the US Forest Service took over and suppressed forest fires to protect property. Open savanna at Illinois Beach State Park, with trees scattered over prairie, started closing in after 1950, when a nearby railroad line switched from steam engines, whose cinders sparked fires, to diesel locomotives that threw no sparks.
How does Bowles know all this? Because fire leaves its mark in the rings of the trees that survive and keep growing. He can count fires in the rings and compare their timing to human records.
It’s no simple thing to decipher the tales of the rings. For one thing, scientists usually can’t see them, at least not as handily as you can see them in “tree cookies” in the Children’s Garden. It’s fascinating to examine the concentric rings of a slice from the trunk of an old tree and imagine the history that took place in that tree’s lifetime. But “you can’t cut down trees any more just to see how old they are,” Bowles says.
Sometimes cross-sections are available from trees that fall in storms, such as a huge white oak—dated back to 1682—that toppled on the Arboretum’s West Side in 1984. When a windstorm topples a whole group of trees, as it did in Illinois Beach State Park in 2010, scientists pounce, eager for a large sample of data. “We were in there with chainsaws right away,” Bowles says.
But most ring studies are of living trees. To see inside a tree’s trunk, researchers use a hollow drill to remove a core, a long, pencil-thin sliver of wood with brown-and-tan stripes. Each stripe, from the bark to the middle, is a bit of a ring that can be counted and measured. The tree can easily grow over the small wound left behind.
Another problem is that the record may be incomplete. A tree may be damaged, or it may be completely hollow in the middle where the oldest wood has rotted out. Bowles, who took cores of more than 600 old-growth trees in the 1980s to reconstruct the presettlement forest of the Chicago area, recalls a red oak in Joliet’s Pilcher Park. It could be dated back to 1707 for sure, but since the middle had rotted out there is no way to know for sure how much older it was. The mere diameter of a tree can’t always tell you its age, since the amount of growth a tree puts on each year varies so much with the conditions.
It is that very variation, however, that makes tree rings so useful to researchers as a record of weather and climate.
Reading the climate
Trees put on the most wood when there is plenty of water, Fahey says; the tree needs large vessels to carry water from the roots out to the leaves. A tree usually will grow the most and largest cells in spring, when rain is plentiful. Growth will slow and cells will get smaller in summer, as the water supply dries up. That pattern is what makes the rings show up: Broad, pale stripes show fast spring growth, and darker stripes are the smaller cells that grow in late summer and early fall.
In an especially wet year, the ring will be wider, says Ed Hedborn, manager of Plant Records. But in a dry year, it will be especially narrow. Skinny, fat, skinny, fat: The variations in the rings record the weather over a sequence of years. Severe drought years are particularly useful markers because they show up so well.
Of course, “telling the climate does not depend on one tree,” Hedborn says, since each tree grows in slightly different conditions. But scientists have built up huge databases of tree ring records from many different trees over wide areas. Put together, their rings form a continuous record that can go back thousands of years and provides evidence about the ebbs and flows of weather and changes in overall climate and water flow.
Consider that 302-year-old white oak that fell on the West Side. It hadn’t always had an easy life, Hedborn says: A series of narrow, dark rings in the 1720s showed it was growing slowly under some kind of stress. Sure enough, the North American Drought Atlas, a database based on many tree rings that maps drought going back 2,500 years, shows a series of severe dry years in Northeastern Illinois starting in 1720.
With this long accumulated record, sequences of rings can be read like a bar code and matched to other sequences. If you have a fragment of wood, you may be able to match its rings to a known sequence and figure out what years they represent. This technique is used by antiques dealers and archaeologists to date old furniture or wooden posts from ancient digs, Fahey says.
But the rings also tell larger sagas. The effort to understand how our climate is different from that of the past has relied extensively on tree ring data. And scientists at the Arboretum now use the rings to explore how trees are likely to respond to climate change to come.
Using tree rings, Soil Scientist Bryant Scharenbroch and Fahey are leading a team to study how resilient different tree species are in the face of disturbances, including climate change. Which trees are likely to handle it best? What does that mean for the city streets and yards of the future?
Since one of the shifts expected from climate change is more severe droughts, Fahey is looking at past drought years, such as 2005, which shows up as a dark mark on nearly every Chicago-area tree core. How have different species recovered in the years since?
It’s a novel effort to look at urban rather than forest trees, using some of the same research plots and landscape types counted in the tree census of the Chicago region recently completed by the Arboretum and the US Forest Service. And it’s tricky, because there are a lot more variables in urban areas than in the forest: Tight space for roots, pollution, paving, altered water flow as creeks become culverts, and how much care trees get from people.
Scharenbroch also uses tree rings to study how different soil conditions affect tree growth. He’s checking cores as part of his effort to create better guidelines for how to manage soils so urban trees can get a better start and live longer. And Forest Ecology Research Assistant Emma Bialecki uses tree rings to study the response of different tree species to flooding—an important question, since climate change is also expected to bring more major storms and heavy rains.
In its rings, every tree can tell a tale of struggle and survival in the face of change. The trees of the Arboretum are a priceless library of knowledge about the world of the past and the world to come.
An abbreviated version of this story appeared in the Spring 2014 issue of Seasons, the member magazine of The Morton Arboretum.