All tree leaves have 'thermostat' that maintains temperature
Deciduous - Latin for 'falling' - trees, have elaborate cellular mechanism to part company from their leaves, which act as "solar cells" in the summer but do not have a purpose in the darker winter months.
At the base of each leaf there is an abscission zone, which begins to swell in winter, slowing the transport of nutrients between the tree and leaf. In due course, a tear line forms and moves downwards, the leaf is blown away or falls off. A protective layer seals the wound, preventing water evaporating and bugs getting in.
Leaves naturally turn yellow as the green chlorophyl that generates energy from sunlight is drained from them. But the yellow colour attracts aphids. To conceal themselves from the invading insects, some species inject a bright red pigment.
Prof John Walker of the University of Missouri and his colleagues are the first to identify a pathway of genes involved in the process of abscission in Arabidopsis by using a combination of molecular genetics and imagine techniques.
The theory of production of red pigments helps to conceal the yellow colour that is attractive to the sap sucking insects was proposed by Dr Thomas Döring of Imperial College London.
Some trees stay yellow, if the costs entailed by the insects is lower (in areas where insect attack is lower)
http://www.telegraph.co.uk/news/science/science-news/3352225/Why-leaves-fall-off-trees-is-discovered.html
1999 explation of falling of leaves
http://warnell.forestry.uga.edu/service/library/for99-025/for99-025.pdf.
