A <TreeType> is in fact a chunk of trees. You can either have a chunk that can be repeated multiple times or have a full tile of trees. So in the lines below, a "tree" is in fact a group of trees.
XML <ForestType>: The trees are included by the <ForestType>
Each tree is 4 size of leafs. Each tree will define properties regarding each of the leaf size. So don't be surprised to have 4 copies of each property. A "leaf" in elemental would be the equivalent of a tree minus the trunk in the real world.
<BigLeafSize> & <MidLeafSize> & <SmlLeafSize> & <FillerLeafSize>: This is a value that determines the scale of the leaf, the value seems to range between 24 for BigLeaf and 8 for Small Leaf. If you do not want certain types of leaf to appear, you could remove some leaf type. But if will only work for full tiles. Else, it creates a bug where a dark brown texture appears on the screen. Else, I suggest setting the value to 0.
<BigLeafAnimationOffset> & <MidLeafAnimationOffset> & <SmlLeafAnimationOffset> & <FillerLeafAnimationOffset>: It might not seem obvious, but the trees moves on the map and this value determines how much they moves. The default values seems to range between 2 and 5.
<BigLeafImage> & <MidLeafImage> & <SmlLeafImage> & <FillerLeafImage>: These variables define which picture is a Big, Mid, Sml or filler leaf. You can repeat this field as many times as you want, the pictures will be selected randomly. Here is an example:
<TrunkModel>: This is a link on the file which will determine how the tress will be spread on the tile. The files are in HKB, I do not know any way to edit these files but you can still change a lot to the forest without changing these files. The files are located in "Gfx/HKB/Trees".
<Type>: This is used to determine if it is a full tile or not. So there is only 1 value available right now which is "FullTile". If it's a chunk of trees, simply remove that parameter. After some research, I found that there is also the value "SingleTree" that could be used.
Tricks and Tips
Thinner Forest: There are various ways to make the forest have less trees. When working with full tiles, you can simply comment <…Size> parameter or set then to 0. Or for the chunk of trees, you can reduce the <MaxTreesPerTile> located in <ForestType> to reduce the amount of chunks that will be shown on each tile.
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