Can you Carry a concealed weapon in a national forest?

It depends on what state you are in.

Open Carry is banned in Florida by state law (790.053) except while engaged in (or going to and from) a lawful fishing, hunting, or camping expedition 790.25(3)(h).

Open Carry in a state forest or park is no different from anywhere else in the state of Florida. 790.33 insures that and it was clarified by the state attorney general in 2009.