Is it permissible to plant trees in a vast, empty plot of land whose owner is unknown, and which is likely to be state-owned or usurped from the state?

You are permitted to plant fruit trees on the aforementioned land if it is state-owned and permission is granted. However, if you are certain or highly suspect that it is usurped, then you are not permitted to do so, even if the state grants permission. This is because whoever takes from a usurper is like him in sin, and usurpation is forbidden by the Qur'an, Sunnah, and scholarly consensus. The Prophet, peace and blessings be upon him, said: "Whoever takes a hand span of land unjustly, it will be a collar around his neck from seven earths on the Day of Resurrection." Mere rumors are not to be relied upon, but it is better to leave that which causes doubt, due to the Prophet's saying, peace and blessings be upon him: "Leave that which makes you doubt for that which does not make you doubt."