No. Treason is defined as "making war" against the United States. Nonviolent secession would not be treasonous, and its advocacy is protected by the First Amendment.
[WS:] I see your point. However, let's assume the following hypothetical scenario: citizens of a state or several states peacefully advocate secession and persuade their respective state legislatures to pass necessary state legislation to secede. Then what? How are they going to implement that legislation without "making war" against the feds - as it is extremely unlikely that the latter would simply pack and leave?
Wojtek