"Also I think in most cases the slab is going to be covered by the planting material (Soil, Trees and some type of stable rim/ring around the out side edge."
This makes little difference. If you're fertilizing and watering the trees, that will also take a toll on the styrene. .
This is a re-invention of the wheel that is probably inferior to the wheel it's replacing. There are already adequate, tested alternatives out there. Auto body filler--bondo for you all in the states--is superior in durability and has been used for years for this. It can be colored, textured and modled onto wire frame pretty easily. It can be used outdoors and is lighter than a slab of polystyrene. It's also stronger.
Ciment fondue--which is alumina cement--is also available for this application. I have a couple of slabs made of it and they have worked very well for seven or eight years now.
http://www.btinternet.com/~colinlew...ing/Cement.html