Thousands of ash trees infested by the tiny but destructive emerald ash borer will be cut down in Mount Royal Park by March 2019.

There are more than 10,000 ash trees in the park, and two-thirds have been treated against the invasive bug. The rest are getting axed.

"We'll remove dead trees and plant 40,000 small trees, 50 per cent of which will grow," said Luc Ferrandez, the executive committee member in charge of parks and the environment.

Many of the trees could not be vaccinated against the ash borer because, on top of being an expensive process, a tree has to be healthy enough to respond, the director of the Morgan Arboretum, Jim Fyles, told CBC Montreal's Daybreak.

"Even if trees are surviving, they may be losing branches," Fyles said. "We really can't be in a situation where we have falling branches on windy days in places where there are lots of people."

He said that cutting down thousands of trees may seem devastating for the mountain, but as recently as the 1998 ice storm, more than 5,000 were cut down without having a serious impact on the forest.

The next step will be planting land that's been cleared with tree species which are beneficial to the mountain. Otherwise, there's a risk that non-native species will take hold on the open terrain.

In 2016, Montreal committed $18 million to fight the spread of the emerald ash borer and to plant saplings to replace lost trees.