When he tends to old trees on the Ashbridge estate, arborist Philip van Wassenaer has three H’s to weigh – heritage, habitat, or hazard?
Standing by “Emma’s willow,” a towering, mop-topped tree planted on the estate by Emma Rooney in 1919, van Wassenaer said too many arborists tip the scales toward “hazard.”
Emma’s willow has soft spots and a large hollow at its base, yet it has lived longer than most weeping willows do. It even survived the fire someone lit in its trunk a few years ago.
“There were rumblings then that this tree needed to be removed, that it was unsafe,” said van Wassenaer, who led a Sept. 9 tree tour on the Ashbridge estate, a two-acre heritage property at Queen and Connaught.
But using sonic tomography – a kind of MRI for trees – van Wassenaer was able to show that despite its defects, the willow was strong.
“Most of the time, when we call it a ‘defect,’ the tree couldn’t care less,” he said.
Of course, arborists do need to be mindful of the risk posed by unstable trees or branches. Over several years, van Wassenaer’s company has pruned the canopy on Emma’s willow to maybe a third of what it was, including a branch that was reaching the school yard next door.
But, where they could, they left the tree’s deadwood alone. However it looks to people, van Wassenaer said it makes great habitat for insects and birds.
“Show me a tree in the forest that ever had a piece of deadwood pruned out of it,” he said.
“Never seen one, in my experience. Because really, deadwood is not a problem for trees, it’s a problem for us.”
Settled in 1793, the two acres that remain of the original Ashbridge homestead have trees planted by generations of Ashbridge family members, who donated the land to the Ontario Heritage Trust in 1972.
Taller doesn’t always mean older – among the estate’s oldest trees are a lilac and a small Japanese maple.
Van Wassenaer showed a crook in the maple where its bark was worn smooth, likely by years of kids climbing on it.
“I still believe that climbing trees is an as-of-right for kids,” he said, smiling.
One of the most surprising trees on the estate is actually part stump. Right along Queen Street is a birch that seeded itself on top of a three-metre black locust stump. That “nurse tree” was cut maybe 50 or 60 years ago, van Wassenaer said, but its hardy wood still shows no sign of decay.
Another surprise on the Ashbridge estate is that the huge, beautifully rounded tree growing in front of the 1854 Jesse Ashbridge house is perhaps the most hated of invasive, non-native trees – a Norway maple.
Popular for its shady canopy, the Norway maple is now a problem in Toronto’s ravines and other natural areas, where it quickly overtakes other plants.
Even on city streets, van Wassenaer said the quick-growing Norway maple causes trouble because it ages just as fast – some of the worst-hit neighbourhoods in the December ice storm had streets lined with mature Norway maples, he said.
Yet the maple growing by the Ashbridge house has a beautiful, rounded crown that van Wassenaer has carefully pruned and even strengthened with steel cables.
“This is where we have to judge each tree by its merits, and in the place where we find it,” he said, noting the tree poses no risk to the big lawn around it.
The same is true when choosing new trees for Toronto streets, van Wassenaer said, which can’t always support native species.
“If it will grow out of concrete in a parking lot, isn’t it a valuable tree?” he said. “We’re getting to the point where there isn’t soil anymore, and our native trees – which are used to rich, deep forest soils with all kinds of organic inputs – won’t grow.”
In a far corner of the Ashbridge estate is the one native tree that used to line roads and highways across North America – an American elm.
The distinctive, fountain-shaped elms were nearly wiped out when Dutch elm disease swept the continent in the 1930s and 1940s (van Wassenaer, a Dutchman, pointed out that the disease didn’t come from Holland, but was discovered there).
“You can see the big wide, open and spreading canopy,” van Wassenaer said. “Before the elm disease came along they were one of the hardiest street trees – you could cut their roots, you could starve them, you could do anything and they would keep growing.”
Researchers at the University of Guelph are now trying to help the species recover by growing clones of the very few elms, like the Ashbridge tree, that seem to have genetic tolerance for the disease.
Whether by lab work or simple pruning, many trees can live far longer than people expect.
In Britain, where some oaks can grow a thousand years, van Wassenaer said there is a saying that an old oak will grow 300 years, rest 300 years, and expire gracefully for 300 years.
“I think one thing to remember is that when we start to see something dying in the top of a tree, it’s not the beginning of the end – it’s just the next stage.”