With the advent of cooler weather the foliage of some broadleaved evergreens take on a subtle bronze-brownish cast. This is only noticeable where the sun strikes the foliage. In my garden this happens to Umbellularia californica, and several species of Asiatic evergreen oak (Q. glauca, acuta, and myrsiniolia). Freezing weather accentuates the bronzing, but the colour change is noticeable prior to hard freezes. The bronzing goes away in the late spring when the foliage regains its normal green colouration. Can anyone suggest why this occurs? Is it a protective mechanism? Why is the change only observed on the sun-exposed portions of the plant?