What causes the leaves to turn colors in fall? Is it the temperature, the shorter days or lack of or too much rain?
It's a combination of shorter days, weather and genetics. The shorter days of fall signal the plant it is time to prepare for winter. The leaves prepare for fall drop by developing a layer of corky cells between the leaf stem (petiole) and branch. This layer slows the intake of water and chemicals used by the leaves and export of sugars produced in the leaves. Thus the production of chlorophyll decreases and the green leaf color begins to fade. This exposes the carotenoid pigments once masked by the green chlorophyll giving us hues of yellow, brown and orange fall color. These pigments are responsible for the beautiful yellow fall color of ginkgo, redbud, witchhazel, larch and birch. The browns and tans often seen on oaks and beeches are due to an accumulation of tannins as the chlorophyll disappears. Now add in the often cool fall nights. If temperatures drop below 45 degrees the plant is unable to use all the sugars produced in the leaves during the day. To avoid a toxic buildup of sugar the plants use the excess sugar to produce anthocyanins. These give us the reds and purple colors seen in red maple, burning bush, and dogwood. Add to this the genetics of the plants that predisposes some to turn red, others yellow and some brown in fall. Now put it all together and you have the science behind that colorful show many of us enjoy each fall.