Re: Why do leaves fall off trees when the temperatures drop.

The photoperiod, or length of the daily light period, is considered the 
environmental trigger for the processes of fall leaf senescence and leaf 
abscission. The pigment phytochrome senses the photoperiod, or actually 
nightlength. Hormone levels change in response to the photoperiod trigger and 
an abscission layer forms which allows the leaf to fall off. The 
main "abscission hormone" is now considered to be ethylene. Sprays of a 
chemical, Ethephon, that releases ethylene, are used commercially to promote 
leaf abscission prior to cotton harvest. Reduction in auxin and cytokinin 
levels also occurs and contributes to the process. Applying auxins or 
cytokinins to leaves can delay leaf abscission. The hormone abscisic acid also 
plays a role by triggering ethylene synthesis.

Deciduous leaves are a way for the plant to save energy by making "cheap" new 
leaves each year rather than invest more in "expensive" evergreen leaves and 
maintaining them yearround. However, the plant also has mechanisms to recycle a 
lot of the scarce mineral nutrients, such as nitrogen and phosphorus, prior to 
leaf drop.

Sometimes an early fall cold spell will kill deciduous leaves before the 
abscission layer has formed, and the dead leaves will hang on into winter. Some 
species, such as oaks, beeches and hornbean naturally retain some dead leaves 
because their abscission layers do not complete fromation until early spring. 
Retention of dead leaves, termed marcescent leaves, may be a defense mechanism 
against herbivores.

The webpages cited have more details.

References


Why and How Do Deciduous Species Shed Leaves?


Falling Tree Leaves: Leaf Abscission


Abscission Zone Photo


Problem: Leaves on Trees Don't Drop in Fall


Effects of Marcescent Leaves on Winter Forage Quality of Oak, Hornbeam and 
Beech and the Their Influence on Food Selection by Cervidae