It works for me if I take the textures OUT of the folders and place them right next to the obj / mtl files. But you need to fix the texture paths in the mtl files too.
If you're using textures with an alpha channel, it makes no sense at all to use more than just 4 points for each polygon. I've seen the tree's 'outline' is traced as a polygon. I'd not recommend that.
Concerning the height of the trees. Make sure you have CONSISTENT attribute data for all vegetation that you can map the same. Use metric values, float values. Not strings lik '2/5m'. The easier you have the attributes on the GIS side of things, the easier you have the code.
I don't understand, i believe that it was better to have 3d model with low polygons because of the size of the file. My tree palm contains 4 polygons but many vertices, when i export from sketchup, polygons are not triangulated, is that right?
What do you advised for the representation of tree and for the export in other software in order to manage 15 000 trees : fbx, obj, collada, kml? What is the problem with alpha channel?