Willie, yes what I wrote was close to the obvious and more likely common knowledge ... Mostly, I tried to convey that I'm not convinced there are easy rules of thumb, and that density is the only really reliable proxy left.
Sorry I can't help you with the remaining questions, no experience on those subjects. Only a biologist's logic. So here's a try.
Nutrient poverty will lead to slower growth, but it doesn't necessitate larger or more transport vessels: the roots don't suck up water randomly like a straw and filter out later the required nutrients. Most of the transport of minerals and nutrients happens selectively through specific ion channels along the cell membranes of the hair roots, so the plant concentrates the nutrients by itself at the desired concentration.
likewise, drier environments will require a lower crown to root ratio: more roots are needed to find and suck up water. But I don't think the plant needs more or larger vessels to transport the water in the stem and branches. The amount of crown that can be produced is then a product of the amount of water the tree can gather, as a larger crown (~larger leaf surface) loses more water through evaporation.
Now that I think of it, it could even be the opposite of what you suggest: a fast growing tree with plenty supplies of light, water and nutrients needs lots and large vessels to transport all that water to a crown that is large (because fast growing) compared to the diameter of the stem. so its density must be low. But since the crown is large, the girth can also increase rapidly. But at the expense of having large vessels, so low density.
I'm freewheeling here, so stop me if you think it doesn't make sense.
Ring-porous wood is still a bit different. there the amount of early ring growth seems (to me at least) determined by the crown size and the concomitant water uptake required for bud burst in spring. For a same crown size, A tree strongly limited by nutrients and water and not light will require the same amount of early growth for this bud burst as a tree having access to the same amount of light but a surplus of nutrients and water. The former tree needs all the water/ nutrients it can get just for the maintenance if its machinery (the leaves, roots etc) and will have little summer growth (late wood), hence narrow growth rings. The density of the summer growth may be good, but the porous spring growth (early wood) may be disadvantageous. The well-watered and well-fed tree with plenty of light will have much wider growth rings with plenty of late wood. The summer wood may be a bit less dense than that of its poor-soil neighbor, but, the overall density may be higher because it has less early wood.
Like I said, just freewheeling
