Creating mesh LODs within the editor


I have a nature asset from the marketplace that doesn’t have LODs for its trees, and I’m trying to create them within the Unreal Engine editor.

So, I go to the asset, choose the number of LODs, then I modify the screen size for each LOD, and reduction.
Problem is, when I try to go as low as 9% for the lowest LOD, it often removes the trunk, so I end up with just the braches.
Is there any way to control what exactly the editor removes, so that I still have some trunk left? Or, perhaps, I should look at other methods of creating LODs? Perhaps doing it directly in Simplygon?

Any advice/link/comment on the topic will be highly appreciated!
Thanks in advance!