What mechanisms tell temperate trees when to drop leaves?
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.