Why is it ...

That some Oregon grape leaves turn bright red in the winter, but not others?

Oregon grape, Mahonia nervosa*, Miracle Beach.

I always thought it was because of the cold. But here's a red Oregon grape plant; less than a metre away is a clump of green plants. No red to be seen. An effect of a mini-climate, with freezing winds blowing just here, and by-passing the cluster of plants a step away?

*I identify this as Mahonia nervosa, "Dull" Oregon grape, so called because it's not quite as shiny as the tall Oregon grape, Mahonia aquifolium. Both are common in this area, and at first glance look alike, but the "Dull" grows to about waist height, whereas the "tall" grape can be a couple of metres high.

And the leaves are different.** M. aquifolium's leaves have one central vein, with other veins branching out from it. M. nervosa, this one, has a strong central vein, and then fainter veins branching out from the base of the leaf. Look closely at the photo above; the secondary veins are visible. (See E-Flora's photo.)

**I didn't know this until I was poking around to see if I could find an explanation for the difference in colours.