If you set the layer that your trees are on to wireframe, you will only get the outlines of each surface. This toggle is in the third column on in you layers palette. This will only be useful in the 3D window and if you do a Sketch render.
If you are wanting your trees to be SEMI-TRANSPARENT, then you will have to assign materials that have a transparency component to them to each component of your trees.
If you are wanting to render with an engine other than the Sketch one, a Transparency channel will need to be created on the CineRender engine for those same materials.
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Do you mean transparent in plan - i.e. just an outline?
If so it depends on the object, but in the settings there should be control of the plan (2D) fill.
Set it to an empty fill with a transparent ( Ø ) background pen.
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