[Edi]

If they're added, yes. I don't have any reason to believe they wouldn't be. Remember, income bonuses from order and production are all added up, so why would supply bonuses not be? I've not seen any indication or mention that the bonuses from different scales would be multiplicative rather than additive.

Oh, and according to the manual it's 20% reduction per death scale, so Death 3 + Cold 2 for Heat 1 pref nation means 90% reduction for supplies. This kind of thing would be consistent with the numbers Cainehill provided if you take an average province with ~60-80 supplies plus two Farms of Plenty, then toss in Death 2 or 3 (from somebody's Dominion, undiscovered Well of Pestilence or whatever) and a 3 point difference bewteen local heat/cold scale vs national preferred heat/cold scale.

Edi