So after a whole day of retopoing ... (yes a whole fuckin day !) I have managed to unwrap the low poly and make a test bake. The Tree itself is at 5000 Tris atm but I can surely Decrease the ammount of tris by maximum of 1500 but then I will be spending the same ammount on additional roots, vines, vegetation etc etc. Here are some screenshots.
And the Uv Unwrap. You can see that there is quite few polys which I can get rid off easily.