In Beijing, I noticed quite a number of trees had their bases coloured white. Why is that so?
A lot of the trees are Plane trees - noted for their resilience to pollution.
It's as Wade, Jingyi and Christina said. Lime is used on tree trunks in my country as a kind of protection from bacteria and insects.
In Greece, people used to paint trees with lime (asbestos? Calcium?) for the bacteria. They also painted flowerpots and tall bushes.
Today, though, they just paint them with acrylics because they think it's a decorative Easter tradition.