Look at this, dude!!

[gitano_anton]

The speedtree leaves over the background hasn´t total opacity.
This happens with a matte-shadow object in the background?

[J_Bug]

Have you tried increasing the max value in the Global switches section?
Are you using any atmospherics?

--Jon

[gitano_anton]

To J_bug

Can you explain me that, please. The concept of Max global settings.

[J_Bug]

I meant to type Max transp levels...

VRay has a parameter to allow a maximum number of transparency levels. With your trees there are a large number of transparent objects one in front of the other. If this value is too low it will show the background color. I'm a little confused though. Is that blue your background color or something else? What's up with the matte objects your talking about?

Are you using fog?

--Jon

[Geddart]

-Check if "affect Background" in your color mapping settings is on.
-Do you have an environment override on?