Keep On Cooking

I would guess it got colder, the outer rings seem more tightly packed than the inner rings

Edit: I was way off

A hard up-tick in exposure to cosmic radiation?

There are several types of wood that has this feature--mostly species of the Yew. Yew is the wood of choice for traditional longbow making because of the different properties of the wood. The heartwood resists compression and the sapwood resists expansion. So the bows are constructed with the darker heartwood on the inside of the bow while the lighter sapwood is on the outside. Thousands of men armed with Yew longbows changed the course of history. Remember Henry V at Agincourt?

Wow! Those are Scots pine logs? They're not one of those trees with the deliniated heartwood/sapwood. And I just noticed the colors are reversed. Heartwood is darker than sapwood. Those trees are all messed up
I wonder if there has been a change in the wood's reaction to compression and expansion?