How do you handle damage from a natural source, a fall or maybe a car crash. As there is no Trait to indicate success you cant roll it up like an attack. So, do you assign a damage modifier (like a weapon) and have the character attempt to Avoid it with a task roll with every success nullifying one damage point?

Guy falls from a 2nd story building. GM assigns +3 damage. The Character rolls his Dexterity to try and roll with the landing. He adds in a positive dice for his Athletic Feature. The GM then adds a negative dice for the debris below him and another as he was pushed without warning off the edge. The dice come up 1 success and 2 stress. So he takes the 2 points of stress and adds the +3 but then cancels 1 for the 1 Success he rolled. Total 4 Physical Stress.

Does that sound right?

Guy involved in a car crash. GM assigns +2 damage. Character rolls his Vitality to avoid injury. He adds a positive dice for being seat belted and another as he saw the crash coming and was prepared. GM adds a negative dice due to the speed of the crash. He rolls all three successes and a stress. The stress +2 damage = 3 but he cancels them with his 3 successes and is fine.

ok?

Both make sense to me.

Hmm, or should the severity of the damage simply be more negative dice?

Guy falls from a building. Check Dexterity to avoid getting hurt. 1+1 Athlete Feature. Negative 1 for debris, 2 for height of building and another 1 for being pushed.

Guy gets in a car crash. Check Vitality to avoid injury. 1+1 Seatbelted+ 1 Prepared for crash+1 for airbags. Negatives 3 for speed of collision and 1 for side impact.

That sounds better actually, kind of an attack in reverse.