Are these all new trees, planted within the past few months? If so, I would sure evaluate the planting procedures and after care methods. It appears that there is some misconception about the 'native' plants. They still need as much care upon planting or transplanting as any other plant species. They may be native to the US and even to your part of the country, but might not be native to that particular site or soil. For instance, some of those plants that you mentioned are happiest in very specific types of ecosystems and might very well struggle unless those needs are met.

Fertilizer spikes, by the way, can do root damage due to the concentration of salts in the vicinity of the spikes. Also, fertilizer is not medicine. Truthfully, when plants are in a stress mode (dieing back and yellowing), fertilization is the worst thing that can be done....unless a serious deficiency of nutrients is what is actually causing the problem in the first place.

yea - I dunno, lousy soil doesn't sound good. Also, how large are the transplants? The larger they are the more "shock" they will go through when transplanted. What time of year did you transplant? They should be transplanted while they are dormant in the winter - if they're moved while growing or budding this could be big problem.

PD

Has anyone used a strong herbicide in the area, to er, clear it? don't laugh, it happens. Even if it was used in the next yard, the strong ones, for killing trees.... can really travel, especially downhill, and persist, for maybe 2 to 5 years.
It's possible to kill even a well established plant, by cutting it back severely, in the fall. The bulk of living tissue is just under the bark, remember, and cutting it back, takes away it's fuel, It's dormant buds, it's ability to cope. Even light pruning triggers new tender young growth in the rose family, azaeas, etc..Got weed roses? well....
How's your drainage, are you in a swamp? What does the slope do with rainwater?
What about dog urine? Poochie isn't tied up there right? Is any one driving over the roots?
Is mulch piled up around the trunks in 'volcanos' where bugs can girdle the bark ? Way too much mulch, can deny oxygen to the roots, just like being overwatered.
What about other plants in the area, how are they doing?
Did you plant them in their packaging like some experts tell you? Please don't. Some burlap is soaked in growth retardant. Were the pots just white, with swirling roots? Curled up roots will strangle new root growth, and the trees will die in a couple of years. Trees too long in the pot are a must to avoid. Did you spread out the roots on a little mound of dirt, soak the hole, fill it in and soak it again? Is it planted to the same depth as it came from the nursery? Were the new plants left out on a hot day, or two, and the roots dried out? Does the top balance the bottom? Newly dug, or transplanted, plants need a bit off, above ground, to help balance up with roots below? Prune carefully.
What's the soil like? Is it a new yard, thoroughly cleared of topsoil? Pure sand or pure clay can't sustain most anything, not to mention trees,
What about the supplier? Maybe someone forgot to water them.
If you dug them up yourself, roots of trees, tend to go out, rather than down very much. In fact most roots are within the top 3 [to 12]inches of the soil. You can expect roots to go out at least as far as the drip line. Ornamentals are potted up young because roots are sensitive. Some like to spread out as wide as the tree is tall. So, Only very young trees are practical to dig, say less than 3 years, or 3 feet.
That's a bunch of stuff that often goes wrong. Maybe something will jump out at you. Sure hope it's anything but the herbicide. Your local county extension agent may be able to talk you through it, In some areas of some states, they'll visit.
Good luck, Florey