Chlorophyll is green (reflects mainly green light) and is strongly selective in absorbing light in the red and violet regions to drive photosynthesis. But obviously the Copper Beech leaves reflect almost all the wavelengths that the chlorophyll is able to absorb.

The Copper Beech (or Purple Beech) tree has been selectively bred from the European Beech, so there is no evolutionary benefit to the colour.

So how does a Copper Beech thrive? It should be almost incapable of growth!