"I don't understand how evergreen can acidify soil if they never drop their leaves. Those needles stay on the tree year round so how do they amend soil exactly?" They don't as such (see below, however...), but...trees do remove the resources from the soil around them, so it's not exactly true to say that plants and plant parts don't influence the soil pH or soil resources (same thing, really), around them. Evergreens are famous for drizzling dying needles in piles below them, although it depends on the evergreen (less than one might imagine, I've seen few that didn't, really, that were needle-based). Yes, we can argue for days about "needles" and the definition thereof, but they do age and are replaced. The full explanation follows. As they compost in place, they rot and generally get pretty close to neutral pH (most sources say 6-8. That seems about right from casual measurement). That compost has a lot of free binding points for resources (you can look into CEC--cation exchange capacity--if you want). Those can't just stay empty, it's a matter of energy potential. So they'll grab onto a hydrogen ion from water, letting the oxygen go back into the air. Hydrogen ions are weak acids. The overall result is a weakly acid pile once all is said and done and it's gone from compost to indistinguishable from dirt. How acid depends on the CEC, which depends on the initial composition and the admix into soils. This is not simple. But if you isolated it back out and measured it, let's just estimate it at pH 5.5-6, but a volume low enough that it's not having too terribly much impact on the soil overall. That's easy to fix with a little calcium, or whatever a soil test recommends. The hydrogen only weakly bonds. The calcium has a dual, and much stronger, bond. Any plant pulls calcium (in larger amounts), magnesium (smaller), and potassium (in the middle) from the soil, reducing pH. A soil test would be required to tell you how much, but it does tend to drift faster within a tree's root system than otherwise. Succession comes into play with other plants like dandelions, which don't mind absolutely awful soils. So the situation isn't simple here, as those will throw deep tap roots and mine resources from deeper in the soil, bring them up, and overall richen the topsoil beyond what it had previously. Deeper tree roots can do exactly the same thing. But those resources end up bound into the tree's wood, leaves (needles, in this case), and roots. Tapped from the soil, however, leaves space only for hydrogen again, as described above, or for something else entering soil solution from the solids (aluminum, a strong acid and also toxic, is another possibility, but you could get calcium if you have undissolved calcium carbonate in the soil too). As it relates to blueberries and cedar trees, the rough answer is, "probably not." Cedars like a 5.5 to 7.2 pH (depending on the species, I didn't track back that far as the sub-question didn't really apply). Mine do very well at 6.2, Thuja standishii x plicata. Blueberries would prefer 4.5 to 5.5, so there isn't much overlap there. Trying to grow them too closely together is not going to end well, creates a gradient in the soil that's awfully sharp, and one would be playing games with sulfates and calcium that I really don't want to think about. And, as always, mind the sulfur at the surface; it tends to turn to sulfur dioxide and nobody needs the atmospheric smog and greenhouse gasses. Dig it in, but mind the root systems and patches of extremely low pH.

I would agree that Treeguy has good suggestions. I also, despite the fact that you like and HAVE the Lacebark Elm, would agree with him on not planting it. If it isn't already invasive in your area, maybe you shouldn't give it a chance to become so. If you REALLY like it, you can keep it in the pot, like a large bonsai, and pick off all the seeds. However, since you will continue to need to drive through the area to get to the back, I would try for a tree that can be limbed up to allow you to drive under it, rather than plant a tree whose branches will be in the way. Not that there may not be a couple of years, while whatever tree is in its youth, when branches will be in the way, but a permanent "in the way" is NOT a good idea. I would go with the Tupelo from the list, or try for a Tulip Tree (maybe too tall?) - both of them will grow tall enough to look good after limbing up. The Redbud and the Dogwood will stay too small for driving under, especially with a pick-up truck, IMO, and the SBM will also not lend itself to driving by or under. I think you need a tree that will grow to 25-40', fairly quickly, and maybe more. Just plan for the eventual width as well. If you aren't needing to drive to the back frequently, then maybe another of the larger trees might do - most of the maples' roots tend to be surface, but oaks' are usually deeper, as are (to my knowledge) a thornless honeylocust, or a catalpa - although there you would possibly have to put up with the caterpillars. A single-trunk birch is another thought, although the roots might be a problem. Ironwwood, Katsura or Linden are also thoughts. A good mulch layer - no more than 4" - and possibly laying down sheets of plywood or OSB to spread the truck weight out might help also with soil compaction over the roots. And if it is only once or twice a year, then I wouldn't worry too much abut it.

Thanks all for your thoughts! Yes, this is the only place I can plant. It is a typically smallish downtown Toronto yard, with several trees shading the (Eastern) 2/3 of it. That leaves the back (Western) 1/3 open for planting. I've actually decided to do fewer beds because I have become less optimistic about my sunlight situation (and my budget for buying soil). The research I've done suggests that the highest concentration of Juglone is in the roots of the tree, and if I can keep the roots from coming in contact with the soil around my sensitive plants (as well as keeping the beds away from the dripline and clearing them of any Walnut tree debris), I am hoping that even the sensitive plants should be ok. The whole yard is pretty dense with tree roots (maple, cedar...) which could also creep into my beds and, while not poisoning my plants, they could crowd their roots and feed on their nutrients. I've been told that as I water my garden, the tree roots are likely to rise towards the surface of the yard where the moisture is, rather that growing down. I hope I don't sound too paranoid, but I figure I should try to do this right the first time.

Honey fungus, Armillaria mellea root rot, is present worldwide throughout temperate regions and the disease has been reported in all of the 50 states. It's there, whether you recognize it as a honey mushroom or not. There are many species of Armillaria and while A. mellea is generally the culprit, other closely related species have also been included in the generic application of 'armillaria root rot'.