I was thinking of making a tileset specifically for world map exports, and I noticed something odd/cool with the world map display. I was looking around the embark map using Mayday's graphics pack, to see how major rivers looked as walls. I noticed that when a coniferous forest, tiles in the local map outside of the embark rectangle were the usual black-with-brown-background up arrows, but tiles inside the embark rectangle were tree tiles. This confuddled me, so I went to the tree raws and changed around some of the coniferous trees' tiles to levers and logs. This caused lever and log tiles to appear around the world map, and on the local map, tiles within the embark rectangle were levers and logs, but tiles outside were up arrows. I also tested changing the tile used by willows to a broken arrow. This caused some of the tiles inside of the embark rectangle on the local map of a marsh to change to broken arrows, but the tiles outside stayed willow-shaped. I did a third test in which I removed the biome tag from coniferous trees. This had no effect on the world I was using, but newly generated world contained "heavily forested" coniferous forests which contained no trees upon embark. All tiles within coniferous biomes on the region and local maps were up arrows.
The game seems to work like this: Tiles on the world, region, and local map for forest-type biomes are selected from the different types of trees that inhabit that biome, except in the case of tiles outside of the embark rectangle on the local map, which are the default for the biome. If there are no trees that live in that biome, it uses the region default. This all happens during world gen. If the tile is changed afterward, the tile on the map changes, but if the biome tag is removed from the tree, the biome still uses that tree for it's tiles.
Edit: Also, it seems that the game won't just use one tile for an entire biome if it can help it; if there's only one type of tree inhabiting that particular biome, it'll use both the tree and the biome default tile to represent the tile on the world map.