It may seem like a hassle to use a POV tree maker and go through the conversion process, but POVTree is one of the only tree making apps, free or commercial, that makes realistic roots as part of the tree.  Many of the presets (9 in all) generate huge files, but they can be very detailed "hero"-type trees. You can have quite a bit of control over how complex the output will be as well. The "olive tree" preset shown above is over 90mb without leaves!   Luckily Carrara5 can use instancing as well as POV-Ray can so such huge trees can still be useful by means of the replicators.