Urban trees grow fast and die young

Trees in urban streets grow more quickly but die faster than those in rural forests, resulting in a net loss of carbon storage from city planting initiatives, new research shows.

Researchers led by Ian Smith of Boston University, US, found that street trees in Boston grow nearly four times faster than those in forest stands nearby in rural Massachusetts.

However, mortality rates of street trees are more than double those in rural forests, with young and very large trees most at risk.

The findings have implications for urban greening programs, suggesting that planting initiatives alone are insufficient to meet municipal carbon storage, canopy cover and biomass goals.

Many cities are embracing greening initiatives to improve urban sustainability and reduce the environmental impacts of urbanisation, the researchers write. 

“However, cities have been dramatically understudied by ecologists,” they add. 

“Despite the widely espoused benefits of urban trees, the role of urban vegetation in the carbon cycle remains uncertain.”

Street trees take many years to pay their way on carbon costs. 

Emissions associated with nursery production, planting, irrigation, pruning, removal and disposal are high. Street trees must survive for between 26 and 33 years to attain carbon neutrality.

To find out whether Boston’s trees are helping the city to meet its environmental aims, the team counted and measured street trees, comparing their findings with a tree census carried out by the City of Boston in 2005-6. They then contrasted these findings with data collated over 25 years from nearby forest plantings, and modelled carbon storage changes. 

They found that young street trees grow much more rapidly than their forest counterparts, and attribute this to more available light, open growth conditions, elevated carbon dioxide, more nitrogen and a longer growing season due to warmer conditions in urban “heat islands”.

This accelerated growth creates huge potential for carbon storage. For example, a street tree with a diameter of 10 centimetres would grow to about 38 centimetres diameter in 35 years and store nearly 400 kilograms of carbon. The same tree in the forest would only grow to 16 centimetres diameter and store just 44 kilograms of carbon.

However, as the street trees grow, they encounter size-related risks such as limited root space, excessive pruning, and removal due to hazard risk and development.

“Through rapid growth rates, street trees have the ability to sequester carbon and potentially provide other ecosystem services, such as evaporative cooling, more efficiently than rural trees,” the researchers write. 

“Currently, these benefits are not fully realised due to the high mortality suffered by street trees.”

Action to lower the mortality rate would have a much larger impact than increasing planting on the total biomass, the researchers suggest.

“With cities at the forefront of implementing actionable climate mitigation policies to offset rising temperatures and atmospheric carbon dioxide concentrations, there is an urgent need to revise current strategies behind greening campaigns to capitalise on the unrealised, abundant ecosystem services provided by the urban canopy,” they conclude.

The research is published in the journal PLOS One.