Slightly increase tree growth speed on grass

[Karokan]

I don't think it should be equal to native terrain by any means, however I have a set of mulberry trees that are still at about 35% on grass/dirt tile, whereas I've fully grown two sets on forest tile in the same amount of time. (They are not stunted). It makes things kind of painful for people who want to grow trees in their walls post-terraforming.

I would think specifically on grass it would be a bit better, since there ARE grassland biomes and trees spawn naturally on them.. and the grass you plant functions the same as grassland biome for the purposes of foraging as well.