A changing climate: Selecting trees that flourish in urban settings

Bert Cregg and a team of MSU researchers are studying trees in urban settings with an eye on identifying species that are adaptable to increasing stress.

The thought of climate change evokes many troubling images — a lean polar bear scavenging for food on a shrinking arctic ice sheet or thick smoke billowing from factory stacks. But climate change impacts a location much closer to home for most.

Urbanization has led to more than half of the world’s population living in cities, and that figure is only expected to rise. As the environment gets warmer, those city dwellers feel the effects more drastically. And for other living creatures of these sprawling cityscapes, such as trees, existence can be challenging.

“The temperature in urban areas is roughly 5 to 10 degrees warmer than in the outlying rural areas, and in the summer it can be even more than that,” said Bert Cregg, an associate professor in the Department of Horticulture at Michigan State University (MSU). “This effect is known as an urban heat island because cities are so much warmer than the spaces around them. That increased temperature puts an added strain on any vegetation in the city.”

Cregg and a team of MSU researchers are studying trees in urban settings with an eye on identifying species that are adaptable to increasing stress. Cregg, who also serves as a faculty member in the MSU Department of Forestry, explained that it’s critical for landscapers and urban foresters to consider climate projections when planting trees.

“If you’re a wildebeest in Africa and your water source dries up, you move to a different spot,” Cregg said. “Trees don’t have the option of moving if their environment changes. However, some species may be able to acclimate or adjust their physiology in response to climate change. We’d like to find trees that can take the warmer temperatures that are predicted for Michigan and the Midwest but can also withstand the occasional harsh winter that will still occur.”

With funding from MSU's Project GREEEN (Generating Research and Extension to meet Economic and Environmental Needs) and the Michigan Nursery and Landscape Association, among others, Cregg’s group embarked on a two-phase project in 2012.

In the first stage, the team received eight tree cultivars from a major nursery in Oregon. Each tree was planted in a 10-gallon container with a mixture of pine bark and peat moss and placed in the greenhouse at the MSU Horticultural Teaching and Research Center on campus. The trees were divided into three climate scenarios: average temperature for the region, 5 degrees Celsius warmer than average and 10 degrees Celsius warmer.

After eight weeks, scientists recorded photosynthesis and leaf respiration data. The tree species responded differently, as predicted, but Cregg was intrigued by some of the findings. Under hotter conditions, some cultivars responded by developing more stomata. These small pores under the leaves are vital to plants’ ability to regulate their temperature through transpiration. Stomata also allow for the exchange of carbon dioxide and oxygen, a necessary part of photosynthesis.

For phase two of the project, new trees were planted in 20-gallon containers to promote growth and observed for a year. In 2013, with the assistance of volunteers from community partner Greening of Detroit, the trees were planted in two contrasting locations in the city. Eighty trees, 10 of each type, were planted at Lafayette Park on Detroit’s east side, while another identical 80 were planted in the nearby St. Aubin Avenue median.

Cregg’s team installed data loggers and sensors at each location to measure temperature and relative humidity, and the group is continuing to monitor long-term growth.

“We’ve noted that the elm trees have done really well,” Cregg said. “Some of the oaks have been a pleasant surprise as well. It’s important to remember, however, that diversity is still our main risk-management tool. We’re looking for the best trees, but we have to be mindful of risk and make sure we’re protecting ourselves in that regard.”

Some forestry researchers have suggested planting trees in Michigan that thrive in the southern United States. Cregg believes that is a shortsighted solution.

“Maybe something like that could work, but there are factors at play that may derail those ideas,” Cregg said. “Tree cultivars from southern states can’t necessarily deal with the cold in Michigan. The general trend of climate change is toward warming, but that doesn’t mean Michigan won’t experience harsh winters, even if it’s only one in 20 winters. If we have really cold temperatures, those trees adapted to mild winters could be in serious trouble.”

Besides beautifying urban settings, trees produce oxygen for human consumption and help with flood protection by absorbing rainwater. Cregg said that trees can also help improve psychological and emotional health.

“There are many studies that show trees have a positive impact on the well-being of city residents,” Cregg said. “But there are a host of challenges for trees in addition to temperature. Urban locations have compacted soils, as well as small spaces that create restricted root volumes. “Everyone gets excited about tree plantings because of the benefits, but who is there to make sure the trees stay healthy? Maybe no one, so we need trees that can self-regulate and acclimate. For that reason, species selection is the key.”

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