Dynamically Spawning Trees Based On The Viability of the Landscape?

Hi all, I'm working on a ecology simulator and I'm trying to come up with ways to have trees spawn and spread in areas with a similar ecosystem to what you would see in the real world.

This is something of an unfamiliar area to me in Unity. It seems like a fairly popular topic on the Unity questions board, (See http://goo.gl/iUbYii) but none of the questions have any answers.

Do you have any suggestions?

Also, does anyone know how to programmatically adjust values on trees made in Unity's Tree Creator?

Thanks