Quote: "Also, does anyone know how I can fix this problem?
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That is a common problem.
The solution is to ALWAYS make sure that any part of a model that is transparent or has some transparency on it is to be rendered last.
That way you are making sure the background parts are already rendered before the transparent/ semi-transparent part gets rendered.
In objects like *.x files, the layer at the start of the file is always rendered LAST, and the layer at the end of the file is always rendered FIRST. This has implications for things like vehicles that have transparent glass on it.
As far as terrain is concerned, You are basically out of luck (based on some rendering tests i did a few weeks ago), unless you are doing bill boarding and you build up the layers so that the inner most layer (the furthest away from the camera) is rendered first and the progressive outgoing layers get rendered in the order that they are going outwards. Individual trees as you have them now is impossible to do as you have done them, and not have a problem from at least one view direction.