GW - the quick and dirty BTU calculation is based on the weight of the wood. If a chunk of poplar weighs half of what a chunk of oak the same size weighs, it has about half the BTU's. The one exception to that is gummy wood - it has more than you would think from the weight. A ton of oak and a ton of poplar should have about the same BTUs.
There's a pretty good BTU table. It shows poplar at about 2/3 of oak.
It does seem a bit odd elm falls halfway between the two. I'd think elm would be right up there with oak.
https://www.firewood-for-life.com/firewood-btu.html
I found this paragraph on another chart with similar values - more or less what I said above.
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All firewood has about the same BTU per pound. Non resinous wood has around 8000 to 8500 BTU per pound, resinous wood has around 8600 to 9700 BTU per pound. Less dense softwoods have less BTU per cord than more dense hardwood but they also weigh less per cord. Resinous wood has more BTU per pound because the resins have more BTU per pound than wood fiber has