Three mature elm trees that stood as sentinels over a majestic river valley vista were cut down by city crews on Tuesday.

The trees were felled because they had been poisoned — and now, city officials want help to find out who did it.

"We can't speculate why they've been poisoned ... but we want people to contact 311 if they have any information," said Crispin Wood, the city's management supervisor for urban forestry.

Three elm trees that stood on city land in the Valleyview neighbourhood were cut down by city workers after someone poisoned them with Roundup. (CBC) The trees stood on a long swath of city parkland that edged a river valley ravine in the area of Valleyview Drive and 90th Avenue. Stately homes stand across the street.

Last fall, city crews were investigating illegal tree cutting — which has happened previously in the area — when they discovered a large area of dead trees and brush.

They conducted soil tests and determined the trees had been poisoned by a heavy dose of glyphosate, also known by its commercial name, Roundup.

In front of a small crowd of cameras Tuesday, a city forester took about one minute to down one of the three dead elms with a chainsaw.

Wood said the city will replace the decorative trees, which had been planted by the city. Each of the three was valued at about $10,000.

"We want to replant because this site requires trees," said Wood.

A city worker uses a chainsaw to fell a mature elm tree that was intentionally poisoned near Valleyview Drive and 90th Avenue. (CBC) "Trees are here for the enjoyment of citizens. They provide shade for our park benches, they provide slope stability," he said, adding they also provided a habitat for wildlife and are part of a larger ecosystem.

The Roundup attack also affected a cluster of birch trees located slightly down the river bank. It's not yet known whether those trees will also have to be cut.

Wood said the city has not been able to identify people responsible for previous illegal cutting in the area.

This is the first time the urban forestry department has come across poisoning. Wood said he hopes the public will come forward with enough information to start a formal investigation.