I seem have gotten into a seam of black walnut in my stacks. Funny how you don't even remember where this wood came from, by the time it's ready to use. I likely felled this tree five years ago, as it's been split and stacked for 4 years. I mark them with a tag when I split and stack them, but there's no record kept of when or where they were felled.
Now what has me laughing is that while I've grown to despise black walnut as a terrible low-BTU wood, it actually scores substantially higher than Nealm's beloved doug fir, on our venerable Sweep's Library. In fact, according to this list, the fractional gap between black walnut and red oak is smaller than the gap between doug fir and black walnut:
https://hearth.com/articles/chimneysweep/howood.htm
I'll be honest, I think that list has walnut scored better than it really is. Having burned absolutely obscene amounts of red oak over the last ten years, with at least a dozen cords each of white oak and black walnut mixed in, I can say the net BTU gap between black walnut and ANY form of oak feels more like 30%, although that might be somewhat colored by my recent and sudden shift from white oak directly to black walnut.
What's this mean for Doug Fir? I don't know. I would like to understand the basis behind these BTU charts, and how much variability there can be within a species. I suspect any tree slowly growing in a dense forest will have much higher BTU value than the same volume of tree growing quickly in a sunny yard, maybe even roughly proportional to rings per inch count variation. Seeing how vast the data is in these charts, it's hard to believe they got a real wide sampling of each species tested, as the investment in testing would already be very substantial, having only one or a few samples per species.