if FROZEN_MAP
base_terrain SNOW_FOREST
elseif TROPICAL_MAP
base_terrain JUNGLE
else
base_terrain FOREST
endif

or the final map looks *REALLY DUMB*

Palm trees in snow. Neh.

Also, to randomise between two maps,

start_random
percent_chance 50
#define TROPICAL_MAP
percent_chance 50
#define SNOW_FOREST
end_random