Paint is applied to the base of the tree trunks to protect them from splitting in the winter and to protect them from herbicides. White paint is preferred as the color reflects sunlight the most.
Tree farmers often paint the bases of their fruit and pecan trees with white paint. This practice may also be used on other young trees from time to time. It may seem like a purely decorative addition to the bottom two or three feet of the trees, but painting the trunks is also practical.
In climates where daytime and nighttime temperatures vary quite a bit in the winter, tree bark heats up enough for liquid water to seep in or for liquid to start flowing up from the root system. As soon as night falls and temperatures drop, that water freezes and expands, which makes the bark split. This causes damage to the tree and can even kill it. Painting the base of the tree trunk with white paint helps protect it as the paint reflects the sunlight rather than allowing it to heat up the bark. A coating of latex paint also helps protect the base of the tree from post-emergence herbicides that affect already-emerged weeds.