botany - What mechanisms tell temperate trees when to drop leaves?

I've been looking around and cannot find a definite answer on what it is that tells trees to change their metabolism and drop leaves. I see that such activity is called Deciduous.

What interests me are the specific mechanisms by which a tree knows that it's time to drop leaves. Is it photoperiod shortening (day duration?) Is it temperature changing metabolism? Is it build up of some toxic compound?

Thank you for your input!

Answer

Here is a good USDA link:

http://www.usna.usda.gov/PhotoGallery/FallFoliage/ScienceFallColor.html

It would appear that the shortening days are a main trigger, and there are other factors. I did a basic search using Google for "what triggers leaf fall in deciduous trees?", since I always thought temperature and changing seasons were major triggers and wanted to double check.