♫ Hot town, summer in the city. ♫ When it’s hot out, city centers are even hotter, thanks to all the concrete, asphalt, and machinery. This well-established heat island effect has led many urban planners to assume that urban parks aren’t good places for local native trees to live, instead choosing to plant species from Europe and Asia. A new study from researchers at the Holden Arboretum reminds us what happens when we assume. 

Urban parks can be perfectly suitable for native trees. 

Plants have the remarkable ability to adjust their physiology so they can stay hydrated across a wide range of growing conditions. But many people have thought that conditions at urban parks were outside of the range of adjustments a native tree could make, and therefore non-native trees — from hotter climates — would be better suited for the hotter cities. In the new study, researchers showed not only that native trees in urban parks adjusted to summer conditions fine, but also that they performed on par with a commonly planted Asian ornamental tree.  

The study was published this month in the scientific journal, Arboriculture & Urban Forestry.

“Everyone assumes cities are stressful for trees, but our data suggests parks are fine to include native trees,” says Juliana Medeiros, plant physiologist at the Holden Arboretum who supervised the study. “The parks we studied were not particularly stressful locations.”

Testing Trees

The study was led by Sharon Danielson, as part of her Ph.D. research with Holden and the Case Western Reserve University Bioscience Alliance. Medeiros and Danielson collaborated with Kevin Mueller at Cleveland State University and Milos Simovic, previously a Master’s student at Cleveland State and now a Ph.D. candidate at the University of British Columbia. 

The team tested five tree species growing at two different Ohio parks, Lakeview Cemetery in Cleveland and Secrest Arboretum in Wooster. Three species were northeast Ohio natives: red maple, sugar maple, and black gum. One species was an Ohio native from further south, sweetgum. The final species was Callery pear, a widely planted, highly invasive ornamental tree from East Asia that has been banned from new plantings in Ohio since 2023. 

To understand how the trees were managing their water, they measured what’s called leaf hydraulic conductance. It’s a measure of a plant’s capacity to transport water or, put another way, how prepared its internal ‘plumbing’ is to keep up with increasing demand.

“Trees can respond to a changing environment, and some will respond better than others,” says Medeiros. “Plant physiology can give us insight into those responses, but physiology is very slow and difficult to measure, so we set out to identify quick-to-measure physiological proxies that could be helpful for selecting trees for urban environments.”

To measure leaf hydraulic conductance, branches are collected from each tree and carefully kept moist as they’re transported back to the lab. After spending the night to equilibrate to lab conditions, the researchers actually hook up an individual leaf to a tube of water. (Yes, really — just like your cut flowers can drink water from a vase, a leaf can ‘drink’ water from a tube.) A grow light encourages the leaf to photosynthesize, while a precise scale measures how much water moves into, then out of, the leaf in a given time frame. 

They paired this water transport rate with another measurement, turgor loss point, which is the water stress level at which a leaf’s cells begin to wilt. Together, these measurements provide good metrics for how well a plant is functioning under current conditions. 

Adjusting to Stress

From spring to summer, the trees sampled from all five species made measurable physiological adjustments, either in hydraulic conductance, turgor loss point, or both, as conditions changed. 

The native species each made an adjustment comparable to that of the non-native pear.

Black gum and sweetgum adjusted the most. Black gum is rarely planted in American cities, but these trees’ response to drought stress suggests they may be underrated candidates for urban planting. The two maples, which are commonly planted in cities, showed relatively less capacity to adjust. For sugar maple, this makes sense, since the species naturally occurs in cooler, shaded forests. But for red maple, this was more of a surprise, as it’s generally considered well suited for urban plantings — it’s planted in cities all over the U.S., even in locations where it’s not native.

The trees didn’t only differ by species; nearly every measurement differed when comparing trees from the two different parks. Lakeview trees outperformed Secrest trees across the board. A number of factors could be responsible for this — Secrest is more dry and compacted, while Lakeview’s soil more closely resembles a natural forest, for example. But in general, the differences highlight that the specific conditions of any given urban park, which are dependent on the park’s history, will also impact tree performance.

Together, the results have direct applications for urban planners deciding what to plant where. “At this point, the whole Earth is a managed garden, human decisions reach everywhere,” Medeiros explains. “How then can we thoughtfully manage our natural resources? Maybe the best place to keep a sugar maple is a really nice forest, and different species, with different physiology, should be prioritized depending on where you are in a city.”