Slwere growing trees are generally more dense. For example madrone is very, very dense and very high in Btu's content. Not because it has more per pound but a pound of madrone is the size of a baseball bat handle and about 16" long! Hardwoods being more dense take much longer to dry and become seasoned...again due to density. While you may only be able to fit 60 lbs of cottonwood into a King's massive 4.27 cu ft firebox, you can fix over 125 lbs in madroe into the same stove...all things being equal, IE moisture content, length etc. I burn mostly western larch (Tamarack) and a local guy gave me several cords of walnut. (Yeah I know it makes great furniture and wood stocks, grips etc...) but I wouldn't have time to address these comments if I had to go cut trees!