Why Do People Paint the Bottom of Trees White?

Hmm:

http://www.ehow.com/about_5415717_do-paint-bottom-trees-white.html

Apparently:

1. Helps prevent sunscald
   
2. Protecting against sunscald can prevent openings for tree boring insects
3. Prevent splitting of the bark in cold weather
   

Probably other reasons. Some white mixtures seem to contain alkaline substances for insect and snail protection.