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Thanks to helentr for helping me with this one. I had to place my surface layers within the internal node network for the object to get the transparency to apply properly. It may be possible to still do this outside of the internal network but I'll look at that after I tweak this a bit more.
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There are 3 layers here although only the 2 child layers add colour to the render. Whether that's supposed to work that way or not, I don't really care because I reckon these trees look much better with random coloured leaves.
Most of the tweaking is done via the last child surface layer and it's breakup fractal. fiddling with the coverage and fractal sizing can create different combinations including putting detailed patterns on each leaf.
The TGD is below. If you have the XFrog birch you may want to edit the path to your model in a text editor before you open it.
This is where saving a TGO comes in handy because it would be a pain to do this for every colour combination whenever you added a tree population. Now for some finetuning