For traits, is there a "set trait" effect?

Instead of having a trigger do something like "VampireThirst +3" or "VampireThirst -1", is there any way to make it be like "VampireThirst set 1", where it just goes to the first level of the trait? Could I do it by doing something like "VampireThirst -100" and also "VampireThirst +1" to set it to 0 and then add one? Or would that actually throw the trait into the negatives?

Scratch that - all I really want to know now is whether or not traits go into the negatives, or if they simply stay at 0.