1. Here's why
You might have noticed that there are no Orientation sliders for trees and plants as there are for other objects in Lumion 10.5 and older versions:
The reason is that trees and plants in the Lumion Model Library in Lumion 10.5 and older versions were made with a third party tree/plant creation software called SpeedTree. SpeedTree does not allow its models to be orientated for Pitch or Bank rotations, only for Heading.
This is a technical constraint.
The exceptions to this are for the Leaves and also Forest Wood sub-category plants where you can use the Rotation sliders for Orientation, Pich, and Bank.
Lumion 11 and newer versions are unaffected by this technical constraint.
If you cannot upgrade to Lumion 11 or newer (which is unaffected by this technical constraint), you could consider using Imported Models of trees and plants where you can change the Pitch and Bank rotation values. Those models can be sourced from other online 3D model resources, for example 3D Warehouse or Xfrog.
However, not all trees and plants are suitable and work well in real-time in Lumion. Some might only have single-sided leaves. Others might be extremely heavy in terms of textures and 3D Points and can cause a slow down of the onscreen redraws and renderings.
For that reason, you may notice that you can only add a few trees and plants from external sources before Lumion will get slower, so please use imported tree and plant models sparingly. Please also note that imported trees and plants are static and will not sway in the wind.