In recent years, tree planting campaigns have been underway in the United States, especially in cities, as part of efforts to limit climate change.
Urban forests can help improve air quality, generate cooling effects and provide green spaces for outdoor recreation, while also serving as ecological habitat.
Last year, the US Forest Service announced a $1 billion campaign to expand access to trees and green spaces across the country, including in cities.
But a new study led by Dartmouth shows that some areas of urban forests in the US may be better able to adapt to a warmer climate than trees growing around the lawns of city homes.
The findings were published in Frontiers in ecology and environment.
The researchers were particularly interested in understanding the different tree species in cities and the correlations between native biodiversity and the ability of tree species to adapt to climate change in terms of water use and tolerating droughts.
These implications are relevant given that droughts and water shortages in the US are expected to become more common in the future.
Using previously published National Science Foundation (NSF) project data from six cities on how land use changes have made urban areas comparable – in Boston, Baltimore, Los Angeles, Miami, Minneapolis-St. Paul and Phoenix, Arizona – the researchers analyzed tree and shrub species in large public parks with relatively unmanaged nature reserves and residential areas. They also analyzed reference sites representing the region’s native ecosystems that have been replaced by urban landscapes. Each of the cities represents a different ecological biome, or community of plants and animals, in a particular climate.
When the researchers compared the tree species of the three land use types in each of the cities, they found that species in parks and natural areas were more drought tolerant and reflected greater native biodiversity than those in people’s yards.
Most tree species found on residential properties were not only non-native, which can lead to species invasions in natural ecosystems, but also had low drought tolerance and therefore may not be able to survive the warmer temperatures expected in the future.
Previous research has shown that approximately 30% of the land area in the US is residential, illustrating that trees planted in yards make up a large percentage of the trees in an urban forest.
“A lot of emphasis has been placed on planting trees in cities, but what people plant in their yards isn’t necessarily sustainable in our warming climate,” said lead author Giselle Mejía, a postdoctoral researcher in the Department of Environmental Studies at Dartmouth. “In terms of sustainability standards, we recommend that in the future we plant trees that are more diverse, native and also better adapted to the climate.”
“Planting trees that are drought tolerant and require less water than other species is something that should be taken into account,” says Mejía.
This is already the case for particularly dry cities like Phoenix and Los Angeles, which had the most drought-tolerant species in parks and reference sites than any other biome.
Trees in the hottest cities, Los Angeles, Miami and Phoenix, had the most species with low water use, in addition to drought tolerance. These include the widely planted native species Cornus florida (flowering dogwood) and Magnolia grandiflora (southern magnolia) and non-native species, also known as introduced species, Pyrus calleryana (callery pear) and Schinus terebinthifolius (Brazilian pepper).
“We need to understand the characteristics people are looking for when they choose a tree to plant in their garden so that educational platforms and incentives can be created to get people on board with planting trees that benefit us all and that may allow us to adapt. for future climate change,” says Mejía.
