Also some terrains might be covered by snow in particular time makes periods with more polar missions. If you put your first base in Europe you should have more night missions on snow thanks to that if you start game in January.
Harvesting would turn forest into grass, grass into plain, plain into desert... and other global effects. This makes no change other then feeling that this is global war and world is changing and you want to save it!! and change in balance of battlescape terrains.
Actually this is simple to implement. What we need is a new field for terrain. Normally there is "terrainID" for terrain. Lets add an array with 12 items that would simulate change of terrain per months. And when EndMonth trigger is run then change terrain behavior based on this array.
terrainID = 3
terrainMatrix = [3,3,3,3,3,3,5,5,5,5,5,5]
So this terrain is "3" in Jan-Jun but is 5 in Jul-Dec. This works both for visual effect and battlescape.
Tom