Are my citrus trees dead?

We live in Tucson, AZ and we planted dwarf lemon, orange, and kumquat trees in our backyard this past June and the orange tree appears to be doing great, but the the lemon and kumquat have lost all of their leaves and the branches look brown and shriveled.
So is the tree dead and need to be replaced in the spring? Or will it come back?
Thanks, Dave
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David Johansen wrote:

Test the branches by bending them between your fingers, any living tissue will be below dead wood. You are looking to find the living tissue which will be flexible. You may also see a line where on the upper side the wood is dull, brown and shrunken and on the lower side fuller with some green, this is the line between alive and dead. Any live wood might shoot again, you will probably have to wait until the growing season starts to find out. If there is no live wood they are finished. If the only live wood is below the bud union the scion is dead. If the rootstock shoots again it will not be the desired cultivar that will bear good fruit. Before replacing them it would be good to find out why they died.
David
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