I need to make a site plan with the top view of my project (including shadows) but the trees show a horrible mesh from this view.
I was wondering if it's possible to change the tree 3d view from the top from their 2d representation... i think that it is done selecting the object and going to open object on the libraries, is that correct? but then I got lost .... can anyone help me?
It may be possible, but if the tree is changed from its 3D model to a flat object for the 3D view, then it will not cast a correct shadow. It may be easier to create 2 views: one of the model with its cast shadows, and a second one of just the trees in plan view showing the tree symbol you desire, then overlay the 2 views in a layout with the plan view brought to front.
Yes, that would be perfect...
But how to change the object image of the 3d view to show the 2d reference?
Is it possible to make a copy of the object and remais one of them as the objet to cast the shadow and the other object with the 2d representation, right?
The short answer is no. Each library part has a 2D Script and a 3D Script (along with other scripts) and the commands that work in one do not work in the other. It is similar to drawing a symbol with lines in the 2D Window, these lines will not appear in the 3D Window. If you don't want to use my earlier suggestion of overlaying 2 Views in a Layout, you could place the 3D View in a layout and just place the trees as 2D Symbols on top of the View over the trees in the Layout. You can place Objects on Layouts and use their 2D Symbol (the 3D component will be ignored), the object will need to be rescaled, as you have moved from Model Space to Paper Space.