The white paint on the trunks helps reflect sunlight and prevents the tree’s bark from getting too much sun. In the winter, an overabundance of sunlight can cause thawing of critical tissues under the bark. They then can refreeze when temperatures drop, leading to trunk cracks and fissures. These wounds are perfect entry points for pests and diseases.

https://www.bobvila.com/articles/why-paint-trees-white/