2D Tree cards have been a long time solution for trees in prelaxing matte shots. When stereo (3D) films came around it seemed that the 2D card gag would not hold up because 2D cards have no demention. It is true that you cannot use them totally like they used to be used but they still can be used in 3D films. Let me show you how.
Parallaxing Between and Within Objects.
To understand where someone can use tree cards and where you can't, I should go over the diffrence between parallax within an object and parallaxing between objects. Parallax within an object happens when objects are close to camera. In the case of a tree close to camera, the parallax you see are the branches and leaves within the tree passing in front and back if each other. At some point if you move the camera back far enough from that tree you start to loose the parallaxing that happens within the tree. If that tree is next to other trees the parallaxing that you see is the parallax that happens between each tree--in other words, you see whole trees passing in front of other trees. It is at this point that you can use 2D tree cards. If the shot requires you can get a little more tricky with your cards like make each tree a collection of cards with individual branches or sphears nested inside each other. For the closer trees you can still use tree cards but you will have to do a little something extra to make them work.
The two images you see below is the same tree but with a stereo offset. The left tree is what you would see from the left eye and the right tree is what you would see from the right eye. The parallax you see is within the tree--a shifting of the branches and leafs.
This is that same tree now populated across a field of tree cards. From the distance that you would see this field of trees the parallax you see is from one tree passing in front of another--in other words parallaxing between objects.
Using Tree Cards in the Foreground
The way to get tree cards to work as foreground trees with all the parallax you would expect to see within the tree is to prerender two offset images of the tree. Then in Nuke merge both tree renders with a "join views" node. Then pipe that into a card. When you switch between the left eye and the right eye, the card will actually switch between the images. For best results, transform the two tree renders before the join views so that the trunks are in the same place. The distance between these images will determine how much they come out of the screen in 3D. This method is best used for small or no camera moves otherwise it will break the 3D. And I should say that this is a big gag so you would have to do a lot of testing on a 3D projector to make sure it was working. Also make sure you give the Comper the controls so he/she can tweak it to fit in with the other elements. This method is not the silver bullet in trees for matte paintings. It is one of the many tree solutions one can use. The type of shot drives which method can be used.