Reduce drawcalls when instantiate many objects like in Cities Skylines

Hello!

I've made a forest fire simulator (playable here: http://www.habrador.com/labs/forest-fire/). The forest is random so I have to instantiate all the trees one by one. I also need to change the individual trees to tress that have burned down. I'm doing that by deactivating the green tree and activate the tree that has burned down, which I have already instantiated.
The problem now is performance, the game is running slower thanks to all drawcalls. My question is how a game like Cities Skylines can instantiate so many trees and buildings, while my little forest can't. What is their secret? I can't combine all trees to one mesh because I have to remove individual trees...

 

Try using Graphics.DrawMesh and mesh combining. Im using it to display forects and grass on procedural planet.

 

Omg, it worked! From 5000 batches to 30

Update! It worked, but is really slow when the trees are changing, so I have to think of a smarter version of it...

Update 2.0! Ok, After 3 days of ups and down I figured it out. I began by combining the trees into chunks of 20000 vertices. To remove trees from a large chunk, I had to find the tree in the combined mesh and then just remove the vertices and the triangles. To the add a burned tree, I find a chunk with vertices left and then added the tree to that chunk with CombineMeshes(). And yes, this was so messy and Unity's documentation wasn't the best so I will write a tutorial.

 

If anyone is interested, the tutorial is finished and can be found here: Dynamic Mesh Combining, and you will learn how to make a Tree Brush tool like in Cities: Skylines.