Focus on tree health preservation vital to address higher mortality of urban trees

Efforts to aid preservation of tree health are imperative for increasing urban tree cover say researchers who have found urban trees' faster growth is outweighed by their higher mortality,

Image: Cathy Smith (CC BY 2.0)
Image: Cathy Smith (CC BY 2.0)

The researchers from Boston University estimated tree growth, mortality and planting rates among the city's trees and compared these with forests in rural Massachusetts.

Rates of mean diameter growth, hence carbon cycling, among urban trees were found to be nearly four times faster in the city than the countryside. But this was outweighed by net loss of carbon storage through high mortality - more than double that of forest trees.

Co-author Ian Smith said: "We find that tree planting initiatives alone may not be sufficient to maintain urban canopies in older cities like Boston.

"Due to the age and size structure of the existing canopy, efforts to aid in the establishment and preservation of tree health are imperative for increasing urban tree cover and maximising ecosystem services."

The paper notes that in New York City, 26.2% of street trees died within nine years of planting, while an analysis of 11 previous international studies indicated the typical street tree population half-life - the time at which overall survaval rates dip below 50% - is only 13–20 years.

"Removal of large trees for (re)development is often justified by replanting several seedlings in its place, but high mortality rates and the time required for a seedling to reach full stature can result in a sustained canopy decline for many years," they conclude.

The findings are published in PLOS One.

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