Most of the time grass comes in the shortgras/moss style. 
But there is a small chance for grass to also become a large fern (upon germinating) if there is a birch tree nearby.
If there are birch trees AND other ferns then this chance increased further, creating lush fern patches in birch forests.


[just let birch trees turn grass into tall ferns once in a while]
[or let birch trees turn grass into tall ferns if there also is foxpoo nearby (as if fox poo and birch soil is the perfect habitat for ferns to florish)]
and fox poo is quite rare so let the fox poo call the action instead of the birch tree for better performance. if grass becomes tall fern it also "infects" neighboring grass]
