Primary source: Wood, E.M. and Esaian, S. (2020), The importance of street trees to urban avifauna. Ecological Applications, e02149. doi: 10.1002/eap.2149
As human populations grow and cities sprawl, wooded jungles increasingly yield to concrete jungles. In urban Los Angeles (LA), street trees are critical habitats for native birds, but new research shows that affluent neighborhoods boasted larger trees and more birds than poorer communities. These findings could help conserve urban biodiversity by informing city planners about the best ways to plant and maintain street trees.
Urban forests support wildlife and human health
Although urbanization and loss of wilderness are primary reasons for worldwide biodiversity declines, cities are not devoid of wildlife. City parks and forests, private yards, and street trees form “urban forests” that support native birds, mammals, and insects. These green spaces are important for people, too—natural areas are linked with important benefits such as improved air quality, reduced stress and anxiety, better physical health, and social engagement. However, urban biodiversity is not distributed equally. Research shows that urban tree cover is strongly associated with income in many parts of the US and other countries. This “luxury effect” means that people living in low income neighborhoods—who are disproportionately people of color—often have reduced access to the benefits of green spaces.
Trees (and birds) grow on money
In LA, street tree plantings are some of the most extensive and diverse worldwide. Punctuating sidewalk deserts, these leafy oases provide important habitat for many bird species. However, little is known about how street tree composition among LA neighborhoods affects city-dwelling birds. Researchers from California State University Los Angeles used census and real estate data to select 12 neighborhoods of low, medium, and high socioeconomic status. After identifying, counting, and measuring street trees within each neighborhood, the team strolled the streets and observed birds foraging in each tree. They focused on ten native bird species: five migratory birds that use LA as winter feeding grounds and five year-round resident bird species. The researchers selected this slice of avian diversity to include birds that feed on a range of things (such as seeds, flowers, and insects) and because several of these species have populations that are in decline.
Birds flocked to opulent neighborhoods where street trees were twice as numerous and almost five times larger than in poorer communities, according to the study recently published in the journal Ecological Applications. As such, wealthy areas supported five and two times more migratory and year-round birds, respectively, than low income communities. In other words, LA’s street trees and birds tend to be distributed according to affluence, in keeping with the luxury effect. This phenomenon is likely driven by a number of factors such as unequal distribution of public funds towards public green spaces, homeowners’ maintenance of private yards, historical patterns of land use, and high income earners choosing to live in greener areas. Street trees are public resources, but homeowners may be responsible for care of trees in front of their properties, and low income earners could be less able to pay for tree maintenance costs.
What are the best ways to manage urban forests?
The researchers suggest that planting and maintaining trees in poorer neighborhoods should provide the greatest boon to urban bird conservation. In general, greater bird density was associated with neighborhoods having more and larger trees. However, in wealthy neighborhoods with the greatest number of trees, bird density was not as high as in wealthy areas with intermediate tree numbers. This trend could be due to lush private yards that attract birds away from street trees and means that additional tree plantings in rich areas are unlikely to benefit birds.
Another important consideration for urban planners is whether native or exotic (non-native) trees are best. Exotics dominated treescapes in the study areas, but birds greatly preferred native trees. Coast live oaks and California sycamores were the most common native trees and were highly used by both migratory and year-round birds. Birds did not use most exotic trees, but some exotics such as Chinese elm, carrotwood, southern live oak, and American sweetgum were important resources. These findings show that both native and non-native trees can support urban birds, but tree species should be considered carefully to promote avian biodiversity. In arid cities such as LA that have few large native tree species, non-native trees may be especially important components of urban ecosystems. Foraging birds are likely to benefit most from streets lined with a variety of trees that produce seeds and fruits throughout the year to provide a continuous food supply.
Nature for all
The recent social media campaign #BlackBirdersWeek brought attention to the idea that birding (and enjoying the outdoors) is for everyone. However, unequal distribution of biodiversity in cities limits opportunities for people in poorer communities to access nature and its many benefits. In LA, projects such as the Million Trees Initiative and City Plants have helped plant trees in areas of the city most lacking in greenery. In addition to planting in low income neighborhoods, the authors suggest that cities improve management of established trees and provide incentives to homeowners and renters to care for trees outside their homes. Although better management of street trees in low income areas could be one way to help address biodiversity inequities in cities and enhance opportunities for people to engage with urban nature, greening programs can paradoxically have unintended negative consequences such as increasing property values and potential displacement of residents. Designing green cities that foster both biodiversity and social equality will likely require careful consideration, planning, and discussion that includes diverse voices.