Hello, I have done some experiments and I was left more confused. Notice that I don't mean to remove the forest feature itself, but only some individual trees to make space for the improvement to be easily spotted.
First I tried doing what you told me, but I found some discrepancies:
The limbermill has forest as <FeatureMakesValid>, as mentioned above, but
the trees are not removed when placed (not the forest, some individual trees).
My Improvement also has forest as <FeatureMakesValid>,
but it gets lost under the treetops. This is very bad for gameplay. My improvement is not supose to remove th forest feature itself.
Quarries don't have forest as <FeatureMakesValid>, and yet, the
trees are removed when placed.
So, then, I hipotetized that the trees are only removed when the improvement adds eyecandies to the "floor" of the tile:
Quarries add some eyecandy to the "floor" itself when built over a Stone resourse. Here is the
quarry model without any eyecandy (according to Civilopedia). As you can see, there is extra eyecandy that comes from somewhere else. When you build a quarry over a forest (World Builder),
the tile removes some trees, but the forest remains.
Farms add some ayecandy to the "floor" too.
Here is the farm without eyecandy. When you build a farm, some trees are removed too,
but the forest remains (WB).
Lumbermills do not add eyecandy. When you build a lumbermill,
the trees remain, covering it under the treetops.
But I was proven wrong because forest preserves do not add eyecandy to the soil, and yet, the trees are removed when it is placed.
Here is the Forest Preserve model from the civilopedia, as you can see the little fence is part of the model itself, not extra eyecandy.
Before.
Trees being removed.
Placed.
The "Worked Tile" improvement, also removes some trees.
Not worked.
Worked. Adding to my confusion.
Look at this: pastures,
get lost under the treetops too.
The bRemove just removes the forest, but I need to keep the forest, just remove some trees to make the improvement visible.
I haven't been able to determine what governs the removal of individual trees. I couldn't find any relation between
eyecandy, model, FeatureMakesValid or being supposed to be built on forests with the
removal of trees when placed.
Any clue?