How much would an inventory of trees in Toronto's ravine system cost? City will find out

Toronto city council has decided to find out how much it would cost to take an inventory of trees in every ravine in the city.

The move is part of a larger strategy that council adopted unanimously this week to protect ravines.

In a decision at its Monday meeting, council directed its staff to determine the funding that would be required to carry out a "biological inventory" and an "ecological integrity" report on the city's 10,500 hectares of ravines. The motion was moved by Coun. Glenn De Baeremaeker.

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Eric Davies, a University of Toronto PhD candidate in forestry, told CBC's Metro Morning on Tuesday that the decision is great news.

"That's a really good idea," Davies said of the proposed inventory. "There are a lot of unknowns." 

Davies said council has not yet indicated how the inventory would be done, but he said foresters can do inventories by using drones to identify areas overrun by invasive species. Then they can do ground surveys to identify individual species and obtain a closer view, he said.

"It will give us a much better idea of the status of those ravines and where the problems are," he said.

Davies recently updated a 40-year-old study on biodiversity in Toronto's ravines in the Rosedale area. In 1977, the original study found that 10 per cent of the trees in four Rosedale ravines were Norway maples, a European tree species.

The 2017 study found that, four decades later, the number of Norway maples in those ravines had jumped to 40 per cent.

Davies said the problem is the Norway maples choke out native species, and such species as Oak, Black Cherry, Hickory and Eastern White Pine are not regenerating to the extent they should. 

"By leafing out really early, it actually shades out all the green cover. It shades everything out," he said.

Norway maples can create a green desert, where the ravine looks healthy but it is actually not, he said.

"In ecology, they talk about tipping points. Once you get to a certain abundance, the ecosystem can shift into a different state. Then it becomes very difficult to bring it back," he said.

"With Norway maple at 40 per cent, in a lot of areas it's at over 50 per cent. If we don't get on it soon, it's going to be a lot more difficult problem to address down the road."

Davies described Toronto's ravine system as "massive." He said it's important to monitor the health of Toronto's ravines and nurture native species.

"It will take a lot of work," he said. "If we were to really get on it, we could make significant changes in the next five to 10 years."

According to the city, ravines make up 17 per cent of Toronto's total area.