[The Woodman]

Using your hypothesis valleyforge, it would be possible for mast years to run concurrently if optimum conditions prevail. It doesn't appear to work like that in nature though.

Genetically modified fruit cropping trees have lost their masting capability but the native species retain it. Not all species mast in the same year either, I've noticed a good sloe crop forming recently whilst hawthorn is poor.

Although I was always aware that this phenomenon occurs, I've never understood why, but it is cyclical.