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A makeshift tribute seen at Trinity Bellwoods Park on Sunday, next to the tree where a 30-year-old man died Friday after being hit by a falling branch.Eduardo Lima

Toronto parks officials investigating how a massive branch plunged from a tree and killed a man in a west-end park on Friday say a mysterious phenomenon known as "summer branch drop" may be responsible.

At about 7:30 p.m., witnesses said, a tree limb fell to the ground after a loud pop, killing a 30-year-old French citizen sitting with his pregnant wife near the front gates of popular Trinity Bellwoods Park, near Queen Street West and Strachan Avenue.

The city's probe of the incident continued over the weekend, as parks department officials and arborists inspected both the tree and the fallen branch, which has been moved off-site.

Matthew Cutler, a parks department spokesman, said on Sunday that the tree was in good health, although some smaller dead branches were pruned from it by city workers on Saturday. But he said like all other trees in high-traffic parks or along streets, it had been subject to a special inspection in the wake of the 2013 ice storm, and in a 2014 report was deemed healthy.

However, the tree, a Siberian elm, is one of 19 species known to be susceptible to a bizarre phenomenon known as summer branch drop, which sees otherwise healthy trees suddenly drop a large branch, even in the absence of high winds.

While not fully understood, arborists believe that when a heavy rain and a cooling follows a prolonged dry, hot period, large amounts of water that have moved into a tree branch can no longer evaporate as quickly through its leaves, rendering the branch heavier than normal.

"It's possible. It's something we're looking at," Mr. Cutler said. "Basically, the tree gets really stressed out in the weather and at some point you hear a loud popping sound. There's somewhat of an internal explosion in the branch, and the branch just falls. That's consistent with not only what we have seen so far, in terms of our inspection, but also the witness accounts."

But others have been quick to blame the city, accusing it of failing to properly maintain the trees at Trinity Bellwoods – even failing to heed warnings from an arborist about the dangers they pose.

Andrew Baughn, an arborist certified with the International Society of Arboriculture, said he urged the city to carry out more tree inspections in the park after he and his son were nearly hit by a falling tree near a playground there last June. His mother, Anne, sent a similar warning directly to Mayor John Tory. The mayor's office says the information was forwarded to parks officials, who note they inspected Trinity Bellwoods again as a result.

Mr. Baughn accused the city of only doing "reactive" tree maintenance, rather than actively trying to prevent problems: "For some reason the City of Toronto is too cheap to do it."

But the city says since 2009, it has shifted its tree maintenance program toward preventive care, performing about 150,000 tree inspections last year. And its budget for tree care has actually risen dramatically, as part of a long-term plan to increase the tree canopy – increasing from $3.9-million in 2009 to $22.3-million in 2016. Toronto estimates that it currently has 4.35 million trees on city property.

Mr. Cutler of the parks department dismissed the criticism, saying it came from "an arborist who hasn't seen the tree, hasn't seen the limb, hasn't done an inspection.

"It's unfortunate, particularly as folks are grieving and we're dealing with a particularly tragic incident, that we're dealing in speculation," he added.

In 2007, Toronto's parks committee was warned by lawyers in a closed-door session about the potential liability it faced from injuries caused by falling tree branches if it did not increase tree maintenance. The warnings were issued in the wake of recommendations for more stringent tree care that came out of a coroner's inquest into the 2004 death of a 10-year-old boy from a falling branch at Royal Botanical Gardens in Burlington, Ont.

While rare, deaths attributed to summer branch drop are not unheard of. In 2012, after a lightning-like crack, a thick tree branch fell and killed a 31-year-old London woman in the British capital's Kew Gardens. While a coroner's inquest concluded that there was "insufficient evidence" to determine why the branch fell, some experts argued summer branch drop was to blame, and the victim's family called for more research into the issue.

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