Scientists warn of environmental threats rising from trend that is ‘likely to continue unless policies are altered’.
Cities in the United States are increasingly seeing concrete in place of greenery as urban areas lose an estimated 36m trees annually, according to a study from the Forest Service.
Tree cover in urban areas has declined at a rate of around 175,000 acres per year, while impervious cover – such as roads and buildings – has increased significantly across the country. An estimated 40% of new impervious surfaces were in areas where trees used to grow, the study found.
The total loss of tree cover reached 1% across cities and surrounding areas in the five years between 2009-2014. As four-fifths of Americans live in urban areas, it has serious environmental, social and economic ramifications, warned researchers.
“Understanding where these losses are occurring and the magnitude of change will hopefully facilitate informed discussions on how much tree cover communities want to have in the coming years, and on the roles of urban trees in sustaining environmental quality and human health and wellbeing,” said David Nowak, co-author of the study, published in the journal Urban Forestry and Urban Greening.
Urban forests moderate climate and reduce carbon emissions, improving the quality of air and water. Properly placed around buildings, trees can save energy by reducing the need for air conditioning by 30% and for heating by up to 50%. They also mitigate rainfall runoff, offering vital barriers in flood-prone cities. The estimated loss of these benefits – including carbon storage, pollution reduction, altered energy use in buildings – is valued at $96m (£71m) per year.
Urban trees also have social advantages, such as improving people’s mental and physical health.
“Trees in urban areas help ward off pollution, providing a long list of benefits for people and the planet,” said Rolf Skar, forest campaign director for Greenpeace USA. “This news proves once again that we need to prioritise adding more green spaces to our cities.”
Researchers Nowak and Eric Greenfield analysed tree cover in US cities and surrounding areas between 2009 and 2014. They looked specifically at urban areas in all 50 states and the District of Columbia, and a broader category named “urban/community areas”, which includes both urban land and politically defined community areas.
They found that 45 states showed a net decline in urban tree cover, with 23 states experiencing significant decreases – resulting in an overall annual net loss of 0.12%, which is comparable to 175,000 acres of tree cover.
Alabama, Florida and Georgia were the states with the greatest annual net loss in tree cover in urban/community areas. Georgia saw the worst results, losing 18,830 acres of tree cover per year, and Florida showed a loss of 18,060 acres. Wyoming, Minnesota and Alaska were the only states to have no recorded change in urban/community tree cover over the five years.
In solely urban areas, the state of Oklahoma recorded the largest tree cover losses at a rate of 0.92%, followed by Washington DC at 0.44% per year.
While there was no significant increase in tree cover in US urban areas over five years, impervious surfaces such as concrete increased by 167,000 acres per year across the states. This represented an overall growth of 1% in urban areas. There was no impervious cover loss.
“Trees improve city living,” said Paul de Zylva, senior nature campaigner for Friends of the Earth. “It’s worrying that trends show so many are being lost in US cities when leading cities worldwide are experiencing the many benefits of investing in trees as part of their plans to improve urban life.”
Nowak and Greenfield warn that tree loss will “likely continue into the future, unless forest management and/or urban development policies are altered, particularly given the threats to urban trees associated with development, climate change, insects and diseases, and fire”.