Okay, this is my first Tut so bear with me...
I like making my trees in my maps viewable only from a distance. I was bored with the same old two dimensional layered trees, so i have my own way of doing it.
1. You must make a wall with the appropriate textures (make it as thin as possible without being without depth.) Make sure that it has the "invisible" texture on the ends of the walls. 2. Then press " alt+enter" to access the object properties, then press "tie to entity". You should now see three tabs at the top of the properties window. You need to use the "Flags"tab.
Set the Amounts for each variable as shown below
(if it's not shown, then down change it)


Class: Func_wall
Render Mode: Solid (this means that the object is impassible)
FX Amount: 255 (this tell the program to make the color 255, invisble in the game)


3. Now, you sould copy and paste the existing wall in exactly the same position as the first one. Then press "Ctrl+M" and check the rotate button. Then go to the "Rotate by Z coordinate" box, and type 90 degrees

Done!!! If you could even read that pathetic tutorial, you should have a decent 3-d to fringe your landscape.