In cities across the United States, many more tree species are found in residential yards than in nearby parks or native landscapes outside the city, according to a new analysis. The findings suggest that residential yards make a major contribution to tree biodiversity in the urban forest – but whether that translates into greater ecosystem services or resilience against climate change is less certain, researchers argue.

Urban forests have a variety of benefits, providing improved air quality, green space for people to exercise in and enjoy, and habitat for other species as well. And in recent years, cities have been planting more trees as part of efforts to mitigate climate change and reduce the urban heat island effect.

But the urban forest is also a sprawling and somewhat messy concept, encompassing street trees, trees in large parks with relatively unmanaged areas, and trees in residential yards, the product of diffuse and perhaps contradictory planting decisions. And until now it hasn’t been clear how the trees actually present in cities stack up against the benefits people are expecting the urban forest as a whole to provide.

In the new study, researchers analyzed data on tree species found in residential yards, large public parks, and nearby native landscapes in or around six cities representing different climatic zones across the United States: Baltimore, Boston, Los Angeles, Miami, Minneapolis-St. Paul, and Phoenix.

Most tree species in found yards are non-native, the researchers report in the journal Frontiers in Ecology and the Environment. They also tend to be less drought-tolerant than trees in public parks and natural areas.

“The divergence between natural and human-managed areas was most dramatic in arid climates,” the researchers write. The driest cities, Phoenix and Los Angeles, had a lot of drought-tolerant species in parks and natural areas; in fact, all tree species found in Phoenix parks and natural areas were both native and drought tolerant.

But even in dry cities, residential yards host plenty of thirsty species. In a hotter, drier future with less water for residential irrigation available, these pockets of the urban forest might not fare so well, the researchers suggest.

A previous analysis of this data set suggested that human preferences for certain species and traits – such as showy flowers – lead to a homogenization of the urban forest, with tree communities in different cities more similar to each other than to nearby native landscapes.

The new analysis adds some nuance. “Here, we found that the effect of human preferences—as manifested by high numbers of introduced woody species—is most obvious in residential yards and in cities with warmer climates, such as Los Angeles, Miami, and Phoenix,” the researchers write. “For example, in the northern cities of Baltimore, Boston, and Minneapolis-St Paul, approximately 50% of tree species in residential yards were native, whereas in the southern cities of Los Angeles, Miami, and Phoenix, less than 25% of tree species in residential yards were native.”

Of the six cities, Miami had the highest percentage of non-native species in large public parks. “These results suggest that hot and wet cities may be especially vulnerable to declines in ecosystem services associated with native biodiversity (e.g., plants and wildlife) in response to increasing urbanization,” the researchers say.

Source: Mejía G.A. et al. How do urban trees vary across the US? It depends on where and how you look. Frontiers in Ecology and the Environment 2024.

Image: Adobe Stock By jamenpercy