The OBJ import didn't map very well out of the box for me. The 3DS version looked a little better when rendered. Free models always need re-editing to make them work. That's why they're free. The lo-poly 3DS tree rendered here took awhile because it uses over-lapping transparency planes (that light goes through) with tree images applied to them. Carrara trees render much quicker and look much better. But they are hi-poly. Which makes me think that your scene is crashing Carrara, not the free lo-poly tree.

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