@ all: Thanks for the comments
@ Angel: the models were made in blender. So I didn´t use any special tool. The most interesting part was the creation of the small branches with the leaves: It´s based on just one single leaf (with transparent background), copied several times. Each copy was transformed and morphed in many ways (in PSP), and arranged with different effects (shadow, lighting, little color differences). Finally I painted the small branches, that connect the leaves.
The LOD models are simply based on rendered images (with alpha transparency) of the high poly version. So they match the shape of the tree. Unfortunately the lighting looks not so good on these models, so they should only be used at great distance. I might replace the second LOD by a version, that is more based on the construction of the detailed model ...