I've seen trees with the lower trunks painted white in a number of places around the world but have never found a definite answer to why this is done. Some suggestions that I've heard are to make them more visible at night time so people don't walk into them. Another is to make it easier for birds to see bugs on the trunk. Does anybody know why you would paint part of a tree white? What type of paint you would use? And why that paint doesn't damage the tree?
In "The Garden Primer" (Damrosch), p 422:
Apple borers can also kill the tree outright just by tenneling in through the trunk [...] Painting the trunk with white latex paint diluted to 50 percent will make it easier to spot the sawdust residue produced by the larvae's tunneling.
Similar advice is in the Fedco Trees catalog; they suggest mixing white latex paint with joint compound.
Also in "Primer" on p 659:
Another danger to the trunk in winter is sun scald [...]. You can protect younger trunks from winter sun and wind by wrapping them in burlap or painting the trunks with white paint.
This same advice is repeated in "The Backyard Orchardist" (Otto), p43. She actually says to paint not only the trunk, but scaffolds out to nine inches from the trunk.
I'm pretty sure I've seen a mixture of something involving white latex paint to apply to the trunk that is supposed to deter four-legged pests. (I want to say that the recipe had sand in it so that any deer/rodent tasting the bark would get an unpleasant mouth feeling and would avoid the tree, but I can't put my hands on a reference right now. I don't think bone sauce would show up as white.)
when trees are painted white from bottom up to a certain level of the trunk it is called white washing or winter washing. the practice is most common in countries where real winters occur but not exclusively there. it's an effective method of reducing reinfestation of borers etc that either hide in trunk bark to overwinter or overwintering in soil to crawl up the tree in spring. also assists a little against secondary fungal infection of trunk by insect or other primary injury. what is this space age paint substance? lime mixed with a bit of water. it's done mostly on fruit trees but also on ornamentals such as hard and expensive to replace rows of street trees in cities.
I've seen this a lot in Arizona and other southwestern states in the US. Mostly the white paint is on citrus trees. The white paint protects the young citrus plants from the sun, when their bark is still thin. Eventually the canopy of the leaves means that the white paint is unnecessary but some people will continue to paint them because they like the way it looks.
Here in Boston I've seen tree wraps (but I guess you could also use white paint) used on new tree plantings, I assume for a similar purpose and also to prevent winter sun heating / winter evening freezing cycles.
I remember my Mom would 'white wash' our trees in Kentucky in the spring with a mixture of lime and water. My brother and I would 'help.' She told us it would keep the bugs away. We had four maple trees in the front yard. We did the same with the apple trees in the small orchard we had. She would also use small pieces of lye soap and put them in the forks of the branches of some of the trees where she saw ants. The white washed trees really looked pretty in the summer.
In (some parts of) Germany trees have a white markers (in form of stripes or a big rectangle) when they enclose a road ("Allee"). This is (or was) to avoid cars crashing into them and to save the plastic marker which usually helps to identify the road or the metal protection rail.
In other places I realized that trees which are painted or marked with color are to be felled. I saw that in woods (to clean) and in city (to make space :-( ).
Here in Poland this is often done in late winter (February) to prevent sun scald.
I have some trees in Bulgaria. I did nothing until I lost some apple trees to the bugs. But when my big walnut tree started going the same way I found out what locals painted their trees with. It's called 'slow lime' as opposed to 'quick lime' It contains calcium oxide and magnesium oxide plus a few other goodies. I've now painted all my trees and will see what the result is next year. I was advised not to use latex paint.