Aside from some spells and heat scales, it appears that trees change color with the seasons... possibly! You decide:
http://www.shrapnelgames.com/Illwint..._jotunheim.jpg
I kind of think that all the scales should be changed to +-4, with nations still limited to choosing +-3. That way, a +3 cold nation would be +3 cold in Spring and Fall, +2 cold in summer, and +4 cold in winter. Otherwise, the nation will not get colder in the winter, which is very strange. Also, if random events or spells temporarily change a scale of a province, the effect will be felt even if the nation has the scale at the normal max or min...
It's not very realistic, for example, for a hypothetical "Cause Massive Laziness" spell to have no effect on a province, just because the people are naturally +3 sloth. They should go to +4
-Cherry