Question lowering alpha of an image

I'm working on a 3d game (well more like 2.5d) I have my flat images and using billboard. I'm trying to set trees alpha down when they are between the camera and the player but, When I use a material it's alpha only goes to invisible to full. Then when I use sprites the character image is in front of the tree. These images are what the sprite does. One shows the actual location, and one image shows what it looks like in gameplay.

 

I ended up just making two items one with the material and one with it as a sprite. Then go back and forth with the raycast. Probably not the best way but it works enough.