notice in the picture that the roots of the world tree look unrealisticly FAT.
Try changing the Z,X,and Y ratios of the model so that the roots(and the entire tree) look a bit more realistic. Good models to use as leaves are the Ashenvale trees, loads of bush models, or the other tree model which is extra thin and high quality (all in the UTM, sorry if u dont know what i speak of >.<)
i can elaborate more if you need to.