To the casual observer, Toronto’s ravines are blazingly beautiful right now, but walking the trails with a forestry expert reveals that beauty is only skin deep.

The trees with flaming yellow leaves in the Park Drive ravine off Mount Pleasant Rd. are invasive Norway maples; the leafy shrubs with stalks that look like bamboo are rapacious Japanese knotweed, and the forest floor is virtually barren when it should be covered in seedlings and saplings.

“This is just a hill of destruction,” says forestry expert Eric Davies, sweeping his arm across a slope of the ravine, beneath Glen Rd. Bridge in Rosedale.

The slope is littered with trees toppled by soil erosion, disease and, in the case of the ash trees, a beetle native to Asia called the emerald ash borer.

The invasive Norway maples taking over the ravine inhibit undergrowth, so there is little to tether soil to the slope and erosion is exposing the roots of native trees — oaks and sugar maples — to freezing in winter.

“When you look at that slope, there is nothing on there — there are no herbaceous plants, there’s no shrubs, there are no seedlings,” says Davies.

“The soil — it’s getting hammered from all different sides.”

Davies is a PhD student in forest ecology at the University of Toronto and one of the co-authors of the Toronto Ravine Revitalization Study: 1977-2017, released last week. It found that over the past 40 years, the biodiversity and ecological health of Toronto’s ravines has declined to a critical level and they are now likely on the edge of ecological collapse.

It’s still possible to save them, says Davies, but action needs to be taken soon.

Toronto has essentially been carved out of a vast ravine system, which still covers 11,000 hectares (27,000 acres) of land, accounting for 17 per cent of the city.

Walking through a ravine with Davies brings the quiet struggle within it glaringly to life.

As he points to the problems on the slope of the ravine, two tiny birds begin foraging madly through the lower branches of nearby trees. They are ruby-crowned kinglets, so named because the males are crowned in brilliant red feathers. The birds are known for their frenetic energy.

Toronto is one of the migratory stops for ruby-crowned kinglets, but the declining health of the ravines could put that at risk, which could in turn affect biodiversity in farther-flung areas, according to Davies.

In fact the decline of native species of trees stands to affect many bird species and small wildlife.

Conifers that offer shelter to birds in winter, including owls, are in decline in the ravines, Davies says.

App readers, click here to launch the Toronto ravine species quiz.

There are few shagbark hickory trees, which produce nuts for small mammals like jumping mice and flying squirrels.

Large, old trees, the kind that offer nesting places to songbirds like the brown creeper, and pileated woodpeckers, one of the biggest birds on the continent, are few and far between.

The native butternut tree is now considered endangered in Ontario, due to butternut blight. Beech trees are dying from beech bark disease. Ash trees are on the brink of extinction.

These trees and the American beech and yellow birch — the original source of the wintergreen flavour used in products like Wrigley’s gum — are all being overtaken by the Norway maple.

“The Norway maples are driving out everything,” says Davies.

Planting them in public spaces was one of those things that seemed like a good idea at the time, Davies says. The trees grow quickly and are considered low maintenance because caterpillars and other pests won’t eat them. He estimates that about 25 per cent of Toronto’s street trees are Norway maples.

“Until quite recently, the goal of the horticultural industry was to ensure plants would grow fast and be pest-free,” Davies says.

“The Norway maple is very resilient … It gets into problems when it gets older because they have a weak structure, but they’re good for the first 20 to 40 years. They grow really rapidly.

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“They kind of give you that instant curb appeal, but then they kind of get more dangerous and more prone to falling down as they grow, whereas some of the native trees are a little bit slower off the get-go but then they mature into beautiful, functional trees.”

Even the autumnal colours of the ravines are changing, according to Sean Thomas, a forestry professor at the University of Toronto.

Unlike native maple trees, which can produce scarlet leaves, Norway maples produce only yellow leaves. It’s particularly noticeable this year because the weather has not been cold enough to trigger the process that results in flaming red leaves in native maples.

“If you don’t have periods of cold early enough, if you don’t have frost events, then you don’t get the colouration,” says Thomas. “The prediction in general would be that with global warming, there would be less early fall frost events and so less colour.”

An even greater problem in the ravines, according to Thomas, is Toronto’s “heat island effect.”

“Cities, especially cities with lots of tall buildings, tend to be a few degrees warmer than rural areas. That is quite pronounced in Toronto, as anyone who commutes from say, even Vaughan or Barrie would know. Any time in the winter that you go to the city, it’s a few degrees warmer. That also will tend to make the colours in ravines in Toronto much less than they would be in outlying areas.”

City Councillor Gord Perks agrees the ravines need more attention and investment.

“Toronto’s ravines are one of our greatest and most precious gifts. Unfortunately, we’re not putting anywhere near enough public resources into protecting and enhancing them,” says Perks, who will represent Ward 4, Parkdale—High Park, under the city’s new ward structure.

“Like so many other things at the city, eight years of austerity budgets have reduced our capacity to enhance the things that make Toronto great.”

Perks says what is needed is more ecosystem assessment to provide robust information that would allow the city to intervene.

But Toronto is facing more immediate challenges — including how to work within a new city government comprising 25 councillors instead of the previous 44.

Davies says that while the ravines are in trouble, saving them is not impossible. Similar efforts are being made in New York City, where the Natural Areas Conservancy, an independent organization, is advocating that the city budget $385 million (US) to bring forests citywide back to health.

The ravines report did not put a price tag on what it might cost to revitalize the Toronto ravine system, Davies said, adding that the first step it to pull experts together.

Davies says the work has to start by bringing experts to the table, including biologists, pathogen experts and invasive species experts. It would mean doing inventories of the ravines and creating a forest management plan, a prescription that lays out what to remove and when and how to optimize the regrowth of the native species.

“It would take a lot of time and expertise, but if you said: ‘We want to win this,’ if you got that level of expertise, it could be done, I think, no problem,” says Davies.

The report puts it this way: “Toronto could do the same as New York City and restore its ravines within 10 to 20 years, making it one of the world’s greatest urban ecosystems — an outdoor Louvre of wilderness and biodiversity.”

Correction — Nov. 19, 2018: This article was edited from a previous version that mistakenly said New York City recently budgeted $385 million (U.S.) over 25 years to restore and steward its natural areas. In fact, the city has not earmarked any money for the project. The Natural Areas Conservancy, an independent organization based in New York City, is advocating that the city budget $385 million (US) to bring forests citywide back to health. Incorrect information was provided to the Star.