The tax code allows the credit for everything needed to generate the solar power. That would probably include removal of any trees that prevent installation, but would not include removal of trees that block the sun. Blocking the sun would hurt the efficiency of the panels, but not prevent them from working. Imagine if the trees were smaller and weren't in the way. Once they grew, they would block the sun light, but removing them would not allow any credit (for this example ignore the time lapse).
