Is it permissible to take branches from an apricot tree whose owner is unknown, planted on land that may be owned by someone who exploits government subsidies? And will what is taken of branches, and what they produce in fruit in my tree, be lawful or unlawful?

If the trees have an owner, then if what was taken from the saplings is customarily overlooked, there is no harm, for customary permission is equivalent to verbal permission, especially if the saplings are in a public place. However, if it is not customarily overlooked, then it is not permissible to take it, and one must guarantee its value to its owner. The possibility of tree owners defrauding support agencies does not affect their ownership.