> From the above chart, apple/alder produce almost half the BTUs of oak.
Hugh, that chart has little to no bearing on pellets until you convert it into BTU/lb - it's made for cord wood which is a volume measurement. It may surprise many that species with the highest BTU's/lb are softwoods like pine, fir and spruce.
According to that chart, by the pound, apple wood actually has MORE BTU than white and red oak. I think many folks are confusing BTU with temperature. BTU is a unit of heat energy. You can have 2 different pellets; one that burns cooler and slower, and the other that burns hotter and faster, but both have the same BTU. In a pellet grill with a good controller, it doesn't really matter and both pellets will provide the same amount of energy. With grills that have a hard time maintaining high temps, or if you are running in a non temp controlled mode, then a pellet that burns hotter (but faster) may give better results.