How can I remove darkness from behind objects? I am getting problem like below image. First trees row getting darkness of behind second trees row. you can understand clearly using below image.

I am using z-transparency to hide my plan background. 

How can remove darkness of behind row from front row trees.? any one can help me.?



** Edited **



This is working for me. It give output like above image.
Your image alpha channel must be used for the mix factor between the transparent area and the color information (diffuse). So, just connect the Alpha output of the image node to the factor input of your Mix Shader node. This makes it so that only the parts of the image that are transparent are actually appear as transparent.

Just a proposal. Unfortunately we don't know you node tree for your tree planes.