Vancouver residents have been able to go online for the past week and buy trees for $10 each through a city program designed to help boost the urban tree canopy.

But even as residents snap up thousands of trees, the city continues to lose them to development, disease or storms such as the one that swept through Stanley Park in 2006 that levelled thousands of trees and triggered a $10-million restoration effort. And despite Vancouver's reputation for parks and beauty, its tree canopy – which was 18 per cent when surveyed in 2013 – is lower than Toronto's, which was estimated to be between 26 per cent and 28 per cent the same year.

Experts say the challenge of maintaining or increasing the urban canopy is growing, even as cities launch ambitious programs to map existing trees and plant new ones.

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"Probably every major city in Canada has undertaken a program like the one in Vancouver, which is basically a subsidy to offset part of the cost of tree-planting for people on private land," Michael Rosen, president of Tree Canada, said on Thursday.

Such programs can boost community spirit and generate goodwill for municipal governments. But they are also practical, since the vast majority of city trees – typically 60 per cent to 80 per cent – exist on private property.

Given growing recognition of the benefits that trees provide – ranging from improving air quality to reducing the "urban heat island" effect that occurs when dark surfaces such as roads or parking lots absorb and radiate heat – it is in cities' interests to have more trees take root.

In general, the urban tree canopy has been declining in major cities over the past decade, Mr. Rosen said, citing increased densification, severe weather events such as last summer's drought in Vancouver and disease and pests such as the emerald ash borer, a beetle that is killing ash trees in Eastern Canada.

In Vancouver, the tree canopy fell from 22.5 per cent to 18 per cent between 1995 and 2013, according to the 2014 Urban Forest Strategy report. Over that period, nearly 24,000 healthy, mature trees were removed on private property, which includes residential, commercial and industrial land.

About half of those trees were removed because of development, while the rest were taken down under a city bylaw that allowed property owners to eliminate one tree a year, regardless of its health, size or species.

In 2014, hoping to halt or reverse the trend, Vancouver amended its bylaw to require a tree removal permit for every tree over a certain size. In a year-end update on the bylaw in December, 2015, staff said it had reviewed 1,200 applications since October, 2014. There had been no requests to waive the $65 application fee, and public response has been supportive, the update said.

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The report also noted that while city regulations require replacement trees on a one-to-one basis in most circumstances, "replacement trees are sometimes poor quality and do not survive the first year. … It is estimated that 25 to 35 per cent of new replacement trees die or are removed within the year."

Vancouver's inaugural Tree Week runs from April 2 to 10, and includes the 3,000-tree sale. The trees, including such species as cherry and lilac, have been chosen to suit a variety of conditions.

Historically, cities were developed largely without taking trees into account, Mr. Rosen said. Clear-cutting was seen as a sign of progress and decorative trees were an afterthought.

Even now, it is rare for developers to take trees into account when building on a site, he said. "The tendency is always to clear everything off the site, then develop it and then put trees back on."

Vancouver's Urban Forest Strategy calls for planting 150,000 new trees by 2020.