Sometimes it happens without any more warning than the sound of cracking.
A tree physiologist said that several years of repeated drought in British Columbia mixed with heat stress increased the likelihood of branches breaking off, and this could even happen on a "perfectly calm day" without any breeze.
The consequences can be tragic -- on July 31, a mother and her five-month-old son were killed by a falling tree on the beach at Cumberland Lake Park campground on Vancouver Island.
Then on Aug. 10, a woman was badly injured when part of a large tree fell on her at Pipers Lagoon in Nanaimo, also on the island.
Peter Constabel, a professor in the biology department at the University of Victoria, said he had seen photos of that tree, and he thought it looked like there was some rot inside