[LChoy]

http://www.edmontonjournal.com/healt...431/story.html

Three key weather events over the growing season led to this unusual circumstance of green dead leaves being retained on trees, said Janice Cooke, a University of Alberta tree biologist.
The first is the summer drought. That reduced the amount of yellow and red pigments produced in leaves. So even if the other events hadn't happened, the fall colours would still have been weak, she said. The second event was the prolonged warm spell in September. Right up until Sept. 24, both daytime and nighttime temperatures were warm. Normally, many plants would have naturally been undergoing their winter preparations, taking their cues from the lessening hours of daylight. But temperature can be an overriding factor, Cooke said.
"We had these merrily happy trees --as happy as they could be in a drought year--doing their summer thing in September."
That was followed really closely by the third event--a dramatic cold temperature and hard freeze.