How can I change erosion parameters for a custom map?

[willzeraxdz]

So, i'm creating a map that is located on a desert city, and I have some cells that are filled with sand texture in WorldEd, my plan is that after a while in game, starting the erosion process, grass starts to grow in these empty cells filled with sand.

 

I did a test and a found a problem: the erosion system also makes trees grow, and I just want specifically grass to grow only (maybe just normal light grass or set up other vegetation grass tile).

Is there a way to do this? change the erosion parameters in a custom map, remove trees from the erosion system and specify what I want to grow when erosion starts in the game?

 

I know its something very specific and I shouldn't give so much importance, but I want to make my map as true to real life as possible to make the game immersive.