4 years later 
- anyway… I think its driven by the ‘Opacity Mask Clip Value’ of the material which by default is set to 0.333. You can increase the value to have more detail when you look at the material from the distance. You have to check the material of the LODs and change it there.
Another reason might be the automatic settings for the LOD distances in the tree meshes. For trees I always change the distances manually (actually any vegetation). Otherwise the LODs are kicking in too early and you see low quality trees everywhere.
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