A street tree planted in Chicago in 2022. (Patty Wetli / WTTW News)

A lot is riding on trees.

When it comes to alleviating the impacts of climate change, trees have been promoted as the ultimate “green infrastructure”: they store carbon, absorb pollutants and clean the air, provide shade and cool heat islands. 

Yet unlike gray infrastructure — roads, bridges, tunnels — trees are living organisms, and they can be sensitive to some of the same climate stressors as people. But just how sensitive?

Morton Arboretum, along with partners including Argonne National Laboratory and State Climatologist Trent Ford, is undertaking a study on trees' response to drought. For urban treekeepers, this is vital information, particularly as more heat and drought are expected.

Among the team’s very first findings: There’s not a lot of information out there on the drought tolerance of trees, and in many cases what does exist is a “mess,” said Christy Rollinson, the arboretum’s forest ecologist.

There are species like the river birch, which is definitively intolerant of drought. But then there are mixed reviews on species like basswood, and question marks for plenty of others, including silver maple and hackberry. The team is aiming to back up any assertions with data, which then can be disseminated among tree specialists, Rollinson said.

In a case of pure coincidence, the Chicago region happens to be experiencing a drought just as funding came through for the study. 

Rollinson said she hadn’t seen many early effects of the lack of water, in part because March and April were wet enough for trees that leafed out before things dried up. (Oaks especially have benefited, having suffered root rot during prior rainy Mays.) And apart from a brief run of hot days, temperatures have been comfortable. 

“If we get heat without water, that can be ‘Bad News Bears,’” she said.

Using satellite images to track the “green-ness” of the region’s tree canopy, Rollinson said she’s seeing patterns that look similar to 2012, which was a bad year for drought. 

According to the imagery, forests are doing better than their urban counterparts because their very nature creates more shade and more humid conditions, the latter of which means less evaporation.

But there are more dips in the green in the urban canopy, and urban trees are already operating at a deficit, she said.

Street trees, in particular, are often planted in isolation, next to concrete and blacktop, in highly compacted soil that even in non-drought years has “such low capacity for absorbing and holding water,” said Rollinson.

Short-term mitigations, most needed by young trees that haven't had a chance to establish substantial root systems, boil down to water, water and more water. 

“You want to make sure you’re saturating a couple inches down and providing mulching to reduce water evaporation,” Rollinson said. But not “volcano mulching,” she emphasized: Spread it shallow and wide, not mounded against the tree’s trunk.

Amending the soil prior to planting, making sure the hole for the tree is deep enough but not too deep — all this prep work will give a new tree a solid start, she said: “Otherwise we’re fighting uphill.”

Though trees have evolved to live for a long time and can handle a year or two of stress, continual cycles of drought-early freeze-drought will take their toll, Rollinson said.

The study that Morton Arboretum is conducting now aims to identify which species are most vulnerable to drought, what warning signs to watch for and how to keep a tree healthy in tough conditions.

Because ultimately, trees can’t help us if we don’t help them, Rollinson said.

The cooling effect of trees, for example, occurs through transpiration: Trees draw up water through their roots and “sweat” it out their leaves. As this sweat evaporates, it removes heat from the air.

Think of an air conditioning unit that runs on electricity. No electricity, no a/c. It’s kind of the same with trees and water, said Rollinson. 

If we’re going to treat trees like infrastructure, we have to monitor them like infrastructure. Do they have the “surge capacity” to provide benefits during a heatwave?

“If we want trees to cool cities, they need water,” Rollinson said.