This is the story of my game of Civilization IV Epic Twenty-two from Realms Beyond Civilization.
So we start with Environmentalism and Forest Preserves accessible. What this means is GROWTH. With sufficient forests (and the map would certainly bring that), we'll have essentially limitless health and happiness, as fast as the workers can preserve. And the Khmer have cheap granaries to boot.
The tiles that we grow onto might not seem all that exciting. 2-1-3 production seems fairly marginal compared to a 3-hammer hill tile or eventual 2-0-7 town. But in the early game, we get just as much commerce as a hamlet combined with all the hammer productivity of a forest landscape. Fifteen 2-1-3 tiles sure do add up to some serious productivity. So it's all about the food.
I mentioned in the game info thread how this concept reminded me of forests in Sid Meier's Alpha Centauri. For those who haven't played it, forests started at 1-2-1 production and could be improved to 3-2-3. "Forest and forget" was the terraforming strategy. Blanket the landscape in trees and collect balanced production from every tile, just as we'll do here.