[5.01] Forest Preserve

A Forest Preserve is the only bonus Tile Improvement that is offer to you as 
a leader. It requires a large amount of movement up the technology tree to be
able to place this on a tile.

Requires   - Scientific Method Technology
Placed     - Anywhere with a Forest
Effects    - +1 Commerce when next to a River
             +2 Commerce with Environmentalism Civic
             +1 Happiness in nearby cities
             Gives the terrain a higher chance to spread forests.

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