To answer Gary Madore: A Balsa tree is a hardwood species but a very very soft one!
I've also seen it classified as both, because it behaves differently based on the amount of moisture available (dry season vs. rainy season) ... It's sometimes classed as one of the ones you mentioned, like the Tamarac.
As has been said, the distinction between Hardwoods and Non-Hardwoods is based on their being Conifer or Deciduous. Deciduous refers to trees that shed their leaves in the winter while Conifers are considered a Needle bearing (evergreen) tree. Althought, as usual, there are exceptions to the rules. Holly, some Magnolia, etc keep their leaves but are considered Deciduous (0r hardwoods) Then there is the Bald Cypress which has needles but sheds them in the winter and it is classified as a Dediduous tree. (actually the "needles" are very thin leaves) .
Tis all corn-fussion but the classification of Hardwoods vs Non-hardwoods (should not be considered as a Softwood) Some of the Non-hardwoods are harder than some of the Hardwoods and some Hardwoods are softer than some non-hardwoods.... Still confused?
As a rule... Deciduous trees are harder than Conifers and so they are called Hardwood.
The Spring/Summer growth rings are a different matter all together. Both (non tropical) tree species exibit growth rings based on the growth cycles, In the spring when growing conditions are great, there is rapid cell development as a result they are larger and more porous (being softer in texture) While later in the summer the growth slows and becomes dense and smaller cells resulting in harder wood. Tropical trees do not express this growth patterns as drasticly although they too have seasonal rest periods and will develop slight differeances in their patterns.
There is not cut and dried rule to follow as each is enfluenced by location, weather, and Al Gore's Global warming, or the reality of the approaching ice age. Grown on the east side of the hill or the west, Near a creek or in urban concrete, all these things change the Annual Rings.
Bizarre, Iíve ask my wood physic course teacher the same question when I studied woodworking in Victoriaville. His answer was that Leaf trees such as maple, oak and ash are all considered hardwood trees. On the other hand, evergreens such as pine cedar and spruce are softwoods. Even if poplar is part of hardwood family, it his to soft to be consider as a hardwood.
This as nothing to do with the way it grows but more with the nature of this tree in general. Trees actually grows faster in the spring time, when the snow melts, then diring any other season of the year. The moisture contained in the ground from the melted snow eventually diminishes and the growing process will slow down. Coming fall, trees will slow down their growing process even more and coming winter time the gorwing process will stop completely. This is why we see annual rings in the end grain of the wood. Popular in general is the hardwood that grows the fastest in the spring time. This is why the wood is so soft.