Urban Forests in the US Show Varied Climate Adaptability

Urban Forests in the US Show Varied Climate Adaptability
by Clarence Oxford
Los Angeles CA (SPX) Jul 15, 2024

In recent years, numerous tree-planting initiatives have been launched across the United States, particularly in urban areas, as part of efforts to mitigate climate change.

Urban forests contribute to improved air quality, cooling effects, and provide recreational green spaces, while also serving as vital ecological habitats.

Last year, the U.S. Forest Service introduced a $1 billion campaign to increase access to trees and green spaces nationwide, focusing on urban areas.

However, a new study led by Dartmouth reveals that certain sections of urban forests may be better equipped to adapt to warmer climates compared to trees in residential lawns.

Researchers aimed to understand the diversity of tree species in cities and the relationship between native biodiversity and the ability of tree species to adapt to climate change, particularly regarding water usage and drought tolerance.

These findings are significant given the expected increase in droughts and water shortages in the U.S.

Utilizing previously published National Science Foundation (NSF) project data from six cities-Boston, Baltimore, Los Angeles, Miami, Minneapolis-St. Paul, and Phoenix-the researchers examined tree and shrub species in large public parks with relatively unmanaged natural areas and residential yards. They also analyzed reference sites representing the native ecosystems replaced by urban landscapes. Each city represents a distinct ecological biome, a community of plants and animals in a specific climate.

Comparing tree species across the three land-use types within each city, researchers found that species in parks and natural areas were more drought-tolerant and exhibited greater native biodiversity than those in residential yards.

Most tree species in residential yards were non-native, posing risks of species invasion in natural ecosystems, and exhibited low drought tolerance, making them less likely to survive future warmer temperatures.

Previous research indicates that around 30% of the U.S. land area is residential, showing that trees in yards constitute a large portion of urban forests.

"There has been a lot of emphasis on tree planting in cities but what people are planting in their yards may not necessarily be sustainable in our warming climate," says lead author Giselle Mejia, a postdoctoral fellow in the Department of Environmental Studies at Dartmouth. "In terms of sustainability standards, we recommend planting trees that are more diverse, more native, and also more climate adaptable in the future."

"Planting trees that are drought tolerant and require less water than other species is something that should be considered," says Mejia.

This is especially true for arid cities like Phoenix and Los Angeles, which had the most drought-tolerant species in parks and reference sites compared to other biomes.

Trees in the hottest cities-Los Angeles, Miami, and Phoenix-had the most species with low water-use capacity and drought tolerance. These included the widely planted native species Cornus florida (flowering dogwood) and Magnolia grandiflora (southern magnolia), as well as non-native species Pyrus calleryana (callery pear) and Schinus terebinthifolius (Brazilian pepper).

"We need to understand the characteristics that people are looking for when they choose a tree to plant in their yard, so that educational platforms and incentives can be created to help get people on board with planting trees that would benefit all of us and potentially adapt to future climate change," says Mejia.

Research Report:How do urban trees vary across the US? It depends on where and how you look