Actually trees growing close togeter usually don't have many branches down low, they are racing each other for the sun.
Also, does anyone have a pic of what Sedum looks like in bloom? All these threads about using dried plants for trees always show the dried plant, but they are much easier to locate when in bloom since they looks more disticntive.
There are many species of Sedum and they all have different characteristics. Try Googling Sedum and you will find a large array of pictures. Some species would be better for tree armatures than others.