There are several reasons why tree trunks in China are painted white. One common explanation is that the white paint serves to protect the trees. The white coating reflects sunlight and reduces the likelihood of sun damage to the trees, especially during hot summers. Additionally, the white paint helps prevent insects, fungi, and other diseases from harming the tree bark. It also aids in lowering the trunk temperature, enabling the trees to better withstand extreme weather conditions.

Furthermore, the use of white paint can serve as a form of marking. For instance, in rural areas, white paint is used to mark agricultural field boundaries or indicate private property rights. Additionally, white paint is applied to trees along road edges to enhance visibility and safety.

In conclusion, the practice of painting the lower parts of tree trunks white in China serves the purpose of protecting the trees, marking boundaries or property rights, and enhancing visibility and safety.