Why do people paint the bottom 1/3 or so of their trees?

Answer 1

Here is the web page along with the excerpt that explains sun scald that some trees are susceptible to and how to treat for it, which includes painting.

http://www.treepatch.com/SunScald.htm

Sun Scald
Sun scald is a tree injury that can result in cracks and splits on the trunk. It occurs in the winter usually on the south or west side of the trunk. The damage takes place when the cells in the living tissue beneath the bark break dormancy on warm, sunny days and then rupture and die when night temperatures drop below freezing. The tree is injured when enough cells in a given area are killed. The following spring these dead areas will appear discolored and sunken. In time the bark killed by sun-scald will split and peel. These areas also provide entry points for insects and diseases.
To reduce or eliminate sun scald injury, wrap the trunks of susceptible trees each fall with tree wrap paper. Do this for one or two seasons until the bark begins to roughen. Remove each spring so as not to provide a home for insects. Tree trunks can also be treated with white latex paint, wrapped with burlap, or a light colored tree guard.

Answer 2

To keep the sun from burning the tender young trunks of the trees.

Answer 3

Usually so people in cars see it better at night and so they won't run into it.
A lot of wreckless drivers and drunks hit trees.

Answer 4

To discourage pests and rodents from climbing and chewing the trees

Answer 5

Because they think it will deter parasites. It's just an old wives tale as many prarsites are larve from flying insects.