What makes leaves on trees shrivel and turn brown only to have them fall off trees in november

Question

I notice that the leaves have started falling off some trees but not all what is it that deprives the trees of the green chlorophyll in the leaves making them turn browns, oranges, reds, and yellows?

Answer 1

Shorter days, cool nights, decreasing auxin levels leads to the breakdown of chlorophyll revealing the secondary pigments normally masked by the chlorophyll.  As the leaves die, the secondary pigments break down and the leaf turns brown.

Leaves turning brown at this time of the year can be from disease, drought stress, over-fertilization, or insect damage.