Of course it matters. But that doesn't mean (very moderately) less dense softwoods are not fit for purpose. In home construction, you're either loading softwoods in compression along the grain or you're using thick material (or engineered ones, which are Lignins Georg and should not be counted) as beams.
Look at https://www.wood-database.com if you want numbers. Balsa is around 9 lb/ft^3 dense, with a crush strength of 1,690 lbf/in^2 and a modulus of rupture around 2840 lbf/in^2. White spruce--which is a pretty representative member of "SPF" in the USA--is about 27 lb/ft^3 even in its modern, "oh so weak" form, with a crush strength of 4,730 lbf/in^2 and a modulus of rupture around 8,640 lbf/in^2. That's, uh...a lot. Is it Good Old Hardwood? No, but who cares? The numbers we're talking about are so strong that if you need more, you need a big span or something, you throw a 2x12 at it or you go to engineered materials (which are basically wood alchemy, they're magic).
From a numbers perspective, the modern "oh they take so much off" 2x4 is overkill for things like external wall framing. We use them because of safety margins and because it allows us to box in more insulation.
Where it can matter is in terms of rot resistance. Sort of. Heartwoods do have accumulated waste products (they're basically dead while the tree is standing, if you ever see a tree desleeve it's because there's too much heartwood for the sapwood and it just peels off) that bugs don't like. And you can tell why; if you've ever bought a decent-sized board, you can taste the difference if you lick your finger, touch the wood, and lick your finger again. Heartwoods are bitter. There is value in that, and old growth does have more heartwood. But we have modern building practices that maintain a better moisture envelope not "because the wood is worse", but because it makes for a nicer house to live in. It also happens to compensate pretty well for that.
The people who fulminate about "the wood isn't as strong" are old and old at heart. It is not just "strong", but it is strong enough, and the fact that we can produce so much softwood for so cheap at such truly incredible strengths is worth celebrating.
Look at https://www.wood-database.com if you want numbers. Balsa is around 9 lb/ft^3 dense, with a crush strength of 1,690 lbf/in^2 and a modulus of rupture around 2840 lbf/in^2. White spruce--which is a pretty representative member of "SPF" in the USA--is about 27 lb/ft^3 even in its modern, "oh so weak" form, with a crush strength of 4,730 lbf/in^2 and a modulus of rupture around 8,640 lbf/in^2. That's, uh...a lot. Is it Good Old Hardwood? No, but who cares? The numbers we're talking about are so strong that if you need more, you need a big span or something, you throw a 2x12 at it or you go to engineered materials (which are basically wood alchemy, they're magic).
From a numbers perspective, the modern "oh they take so much off" 2x4 is overkill for things like external wall framing. We use them because of safety margins and because it allows us to box in more insulation.
Where it can matter is in terms of rot resistance. Sort of. Heartwoods do have accumulated waste products (they're basically dead while the tree is standing, if you ever see a tree desleeve it's because there's too much heartwood for the sapwood and it just peels off) that bugs don't like. And you can tell why; if you've ever bought a decent-sized board, you can taste the difference if you lick your finger, touch the wood, and lick your finger again. Heartwoods are bitter. There is value in that, and old growth does have more heartwood. But we have modern building practices that maintain a better moisture envelope not "because the wood is worse", but because it makes for a nicer house to live in. It also happens to compensate pretty well for that.
The people who fulminate about "the wood isn't as strong" are old and old at heart. It is not just "strong", but it is strong enough, and the fact that we can produce so much softwood for so cheap at such truly incredible strengths is worth celebrating.