Answer: Unfortunately I think your trees may be dieing. These trees are difficult to overwinter successfully even in large containers. The winter stress (not only cold on the roots but also especially spring freeze thaw cycles) then accumulates along with the energy expended for the spring growth spurt and it is sometimes just too much. It is also possible they were overfertilized, and that would contribute too, in a container you need to be careful with that because the soil texture is different from soil in the ground and also the roots are restricted to such a limited area. There might also be a fungal or other problem happening. On the other hand, although evergreen these trees do shed some of their oldest needles each year so part of what you are seeing might be normal. So I would suggest you work with your local county extension to try to see if these can be saved, but sadly (although granted I have not seen them) I am not too encouraged. They may appreciate a selection of photos showing the overall plants and some closeups. I'm sorry.
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