I also wonder why legos has black trees. Just a wonder
Brown on those years wasn't a very common color for bricks. I mean Berrels, Horses, Treasures Chests, Minifigs equipment, even 1x2 brick and 1x2 plate existed in brown by the end of the 80's, however qunitities of bricks in brown those years were not. It was change only in the mid-90's where 6020 and 6079 had even brown corner walls, and other sets had a lot of brown arches.