Toronto Hydro, which struggled to restore electricity to hundreds of thousands of people after ice-coated branches felled power lines last month, cut its tree-trimming budget by nearly a third in recent years, spending less to manage a potential hazard than Ottawa's smaller public utility.

A Globe and Mail analysis shows Toronto Hydro, the largest municipal electricity distribution company in Canada, allotted $2.5-million to pruning trees near overhead lines in 2012, down from $3.5-million in 2010. The utility said the decrease reflects a reduction in prices negotiated with a contractor and not a cut in the number of trees trimmed yearly.

Yet an examination of tree-maintenance budgets from 10 electric utilities across Canada reveals Hydro Ottawa, which has less than half the number of power customers as Toronto has and 12,200 fewer kilometres of overhead wires, spent more on trees in 2012 – $2.7-million – than Toronto Hydro, and nearly as much money the year before.

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The two utilities prune about the same number of trees annually: around 42,000 in Toronto and about 40,000 in the nation's capital.

Toronto Hydro's annual pruning figure appears low, said Sandy Smith, an urban-forestry professor at the University of Toronto and dean emeritus of the faculty of forestry.

"I think a budget should be proportional," Dr. Smith said. "It's clear a lot of the ice-storm issues were tied to not pruned trees or improperly pruned trees."

The practices and policies of Toronto Hydro and other Southern Ontario utilities affected by the pre-Christmas ice storm are under scrutiny as governments and an industry group probe one of the longest blackouts in provincial history. About 600,000 households, businesses and other power customers lost electricity, and some people were in dark, frigid homes for more than a week. A major cause of the outages was icy trees breaking apart and tearing down wires.

Ben LaPianta, the utility's vice-president of distribution grid management, said additional tree trimming would not have made a difference. He noted pruning near power lines is primarily to prevent branches from touching energized wires, particularly on windy days. According to the Ontario Electrical Safety Code, clearance between trees and overhead lines must be one metre for secondary lines and four metres for primary lines.

"Tree trimming is not designed … to prevent against the damage that arose from the ice storm," Mr. LaPianta said. "The only tree trimming that would have prevented the impact arising from the ice storm would have been to cut off the trees at 30 feet. In other words, there are no trees."

In reaction to the threat of ice storms, other utilities have boosted spending substantially. Hydro-Quebec, for instance, increased its budget for vegetation management after the ice storm of 1998. The government-owned utility, which has four million customers, spent $61-million trimming and clearing trees in 2012, compared with $42-million annually from 2003 to 2006, an increase of 45 per cent over six years. By contrast, Toronto Hydro spent more than $5-million in 1999, about double what it spent 2012. It says the drop largely reflects efficiencies from amalgamating contracts that predated its creation from several municipal utilities.

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Toronto Hydro has commissioned an independent audit of its response to the storm, during which about 42 per cent of its 719,000 customers lost electricity. A review of media releases and documents submitted to the province's energy regulator reveals that the company hired to perform the audit, Davies Consulting, helped Toronto Hydro develop a new tree-trimming program in 2007 and assessed the utility's resilience to severe weather in 2008.

Tanya Bruckmueller, a spokeswoman for the utility, said Davies Consulting has not worked for Toronto Hydro since 2008. She said the Maryland-based firm was chosen to perform the audit because of its expertise in evaluating large utilities and because it understands Toronto Hydro's territory and emergency preparedness plans.

The consulting firm will report to an expert panel created by the utility. The panel is chaired by former Conservative MPP David McFadden, who was part of a U.S.-Canada task force that investigated the massive 2003 blackout. The Ontario government is also conducting an examination of how utilities, municipalities and emergency organizations responded to the storm. Details of the probe are still being finalized, but a spokesman said the fire marshal's office and Emergency Management Ontario will co-ordinate the review.

Power was also severed in Quebec, the Atlantic provinces and other parts of the Eastern Seabord. In Newfoundland and Michigan, electricity regulators have decided to weigh in and examine whether utilities' maintenance practices, infrastructure and emergency response plans contributed to the December outages. In Ontario, however, the province's electricity and natural-gas regulator, the Ontario Energy Board, has not yet decided whether to launch an inquiry.

The storm cost Toronto an estimated $106-million in damages, of which $13-million are costs to Toronto Hydro. The utility dealt with 500 downed wires and 374,000 calls over 10 days. Many people complained about the utility's communications and the time it took to restore power. Electricity was reconnected to three-quarters of Toronto Hydro customers within 48 hours.

Toronto Hydro uses a different tree-maintenance program than other electric utilities in Canada. With the help of Davies Consulting, the utility designed a "reliability-based" model seven years ago, shifting away from a traditional fixed-trimming cycle.

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Several large utilities in the United States use this model. Mr. LaPianta contends reliability-based trimming, in which tree-related outages are analyzed to determine which areas require the most attention, is more efficient than Toronto Hydro's old program. Tree-related outages are down eight per cent since the switch, Mr. LaPianta said.

Tree-maintenance programs are difficult to compare. Factors such as terrain, the species and age of trees, and frequency of severe weather influence how often trees require trimming.

Enersource, which services about 200,000 customers in Mississauga, spent an estimated $900,000 on trimming in 2013. PowerStream, with about 350,000 customers north of Toronto, budgeted $1.4-million to deal with trees in 2013.