Protect tree pits to boost rainwater infiltration, say US researchers

By preventing passers-by from trampling them, street tree pits protected by guardrails absorbed runoff water much more quickly than trees in unprotected pits, according to new US research.

Image: Lizzie Adkins, Columbia University
Image: Lizzie Adkins, Columbia University

Researchers at Columbia University, New York compared the infiltration rate of street trees with and without guardrails in a Manhattan neighbourhood.

They found water was absorbed in the protected pits six times faster than in those without guards, irrespective of whether mulch or vegetation were present - apparently due to the absence of soil compaction.

"Placing guards around tree pits allows urban trees to absorb more storm water runoff, taking pressure off the city sewer system," said the study’s senior author Professor Patricia Culligan.

Tree guards recommended by the city authorities cost up to $1,000 (£700), but an improvised $20 fence can work just as well, the study’s lead author Robert Elliott added.

"Only 14% of New York City trees have protective guards," he said. "Our results suggest street trees could manage six times as much storm water if every tree pit were enclosed."

In a separate, unpublished study, Elliott and colleagues compared the relative costs and benefits of guarded tree pits to "bioswales", or pits on pavements planted with shrubs to retain rainwater, of which around 3,000 have been installed in the city.

They found that bioswales substantially outperformed guarded tree-pits when comparing their initial cost plus maintenance over 10 years - though trees become more competitive if the cost of guards was reduced, or water flows are increased by removing the kerb by the pit, so allowing in water from the road as with a bioswale.

The results are published online in the journal Ecological Engineering.

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