Trees placed using the terrain engine produce much higher number of draw calls when viewed from steep angles while the "Fade Length" is set to zero and the "Shadows Distance" in the project quality settings is set to higher value (e.g 120)
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To reproduce:
1. Open the project I've attached
2. Run the scene
3. Notice the high number of draw calls
4. Stop the scene
5. Change "Fade Length" in the terrain settings to "0.1" instead of "0.0"
6. Run the scene again
7. Notice that the number of draw calls has dramatically decreased.