I apologize asking this but what exactly is heart wood? The inner side of the wood?
When you look at a log from the end, the outside rings are often lighter coloured. That's the "Sapwood", Basically it's still part of the tree's metabolism, and water / nutrients are transported up the tree in the area. Further in you find "Heart wood". It's usually darker coloured and isn't active any more. It gets plugged up with assorted chemicals and becomes purely structural, as in it holds the tree up, but doesn't do anything else.
It's more noticeable in some trees like Red Cedars and Walnut, because those chemicals darken the wood, and also help preserve it from bugs and rot. Other trees it's not so obvious, and in some the heartwood is not so durable.
So where it applies to this discussion is that the sapwood of a cedar log may start to rot after a couple of years, But the more durable wood further in is still good after 5 (or more). So you throw a nasty bug eaten log on the mill, slab off rotting sapwood, and the more desirable heartwood is still good.