You can use q10 trees (as lady V said) to change an entire grid, as it is shown here:
viewtopic.php?f=2&t=35295&p=465707#p465707 roll down to Terraforming. I think this would be faster than just planting random trees to start the terraforming process, even it still would be slow.
Trees terraform one tile for each stage of growth, but this tile will allways be inside the same grid they are planted, even if it is planted in the tile right next to the grids edge. So you need to find out the limits of the grids you have in your tree farm area and work each grid individually.
Another thing is that once you have one tile of the specific type of terrain you are willing to terraform to, this type of tile will spread to adjacent tiles (provided you have planted near some of the adecuate type of trees). So, it would be faster to grass all tiles inside the grid to be terraformed to allow the new type to spread faster. This I say because as the paved tile will not change its type they would not help the wanted type of terrain to spread.
I made a similar mistake and terraformed. It was so slow that I ended up planting in a place between my home wall and the sniper wall, because there the terrain was still forrest and I needed the wood. I was able to make this because someone taught me that it does not matter to gain Q which char plants the tree on the ground so I could use a disposable alt to plant and chop with no risk of my farmer being snipped (only the char that actually places the seed inside the pot has to have farming).
Now if you could switch the type of terrain from forrest to broadleaf, instead of using fir decide to use birch, the terraforming proccess would be a little faster, because birch grows a little faster, so high Q can be obtained with the same resources in less time.
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