B.C. storm: Why did the wind and rain do so much damage?

This weekend's storm hit B.C. hard, bringing trees crashing down and leaving power lines sparking — knocking out power to half a million BC Hydro customers at its peak.

• B.C.'s coastal windstorm by the numbers

The Vancouver Park Board alone says it lost approximately 500 trees in the storm. So why did the fierce wind and driving rain do so much damage?

Drought stressed trees

Brian Quinn, director of park operations for the Vancouver Park Board, says the drought-like conditions around southern B.C. contributed to the severity of the damage by weakening trees.

"Typically the younger trees are going to show more drought stress, because they don't have the same root capacity as a larger tree," said Quinn.

Not extreme enough

The tree species may also have been a factor. In fact, Vancouver is currently in the process of reassessing the types of trees it plants, looking for species that can take extreme heat and drought.

"Extreme weather is the new normal. And every emergency like this is a test," said Mayor Gregor Robertson. "We have lots to learn now, from this emergency and the response that happened."

Rain softened soil

B.C. Tree Service arborist Milan Pecaric says the rain softened the soil, making it easier for trees to be uprooted. 

"We did have a fair bit of precipitation," said Pecaric. "So we had a major saturation of a lot of soil as well. So that softened things up quite a bit."

Leaves in the wind

The combination of foliage and wind direction was also a factor, Pecaric said.

"I think the main reason this happened is the wind came from the south, and these trees, fully foliated, got subjected to winds they weren't used to and they don't have anchoring roots in that area."

Tree removal ... or not

Pecaric believes the city should rethink its tree removal bylaw, which requires owners to get a permit before chopping down a tree on their property.

He says homeowners may now find it costly and difficult to remove trees that may legitimately be dangerous in bad weather.

"If you have 120-foot hemlock or poplar in your property ... and developments change ... these trees are being subjected to winds they've never experienced before ... they become great risks."

Vancouver deputy city manager Sadhu Johnston says a change to the policy isn't planned. 

"The last major storm event we had like this was in Stanley Park. Those larger trees actually buffer the wind coming in, just like a windbreak you see at a farm."