Bronzing of broadleaf evergreens

[Alex Downie]

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?

[PlantExplorer]

I bet you will find two schools of thought on this;

• 1.) The bronzing is defensive - whereby carotenoids are deployed to leaf surfaces exposed to the sun, in an effort to reduce winter burning. It is also possible that the carotenoids are better able to photosynthesize in cooler conditions.
  
  2.) The bronzing is passive - The natural chlorophyll breaks down in the winter sun and is not replaced as rapidly in the cool weather, exposing the underlying carotenoids, resulting in the characteristic bronzing effect.

My vote is for the latter explanation.

I am intrigued that your Umbellularia californica displays this characteristic, as the specimen that I am familiar with at VanDusen has not produced this effect in the years I’ve watched it – so it is reasonable to assume that it is not necessarily consistent for all specimens.
Possibly one of the finest examples of this effect can be seen in Cryptomeria japonica ‘Elegans’, which turns the most amazing shade of bronze-purple in the winter.