Brachiosaurus had long forelimbs that caused its back to incline. This trait is not seen in most other sauropods. The neck’s exiting the body in a fairly straight line(the current theory) would have resulted in it pointing upwards. Debate continues on the exact angle of the neck and how flexible it was.
As I was watching Jurassic Park III last night, and wishing it would hurry up and end, I did pay a little attention to the one character I cared about other than the dinosaurs; Alan Grant. He mentioned something, while looking out of the plane window,about “You can see a herd of Brachiosaurus grazing”, and I remember thinking, “Hold it, Alan.”
( then I drew the above sketch.)
I am of the team of “Brachiosaurus ate from the treetops”, which is called “browsing”. Also, it had close cropping teeth adapted to eat the most elevated plant material such as conifer leaves and fruit.
To maneuver around in tall forests with a very long neck would be difficult, unless the neck was held vertically or at least semi-vertically. Extremely long tails would make movement in the forest difficult for high-browsers as well, and tails could not be pointed vertically . Brachiosaurus lost its long tail over time and it became quite short.
Hopefully, Alan Grant only meant that the Brachs were simply grazing from treetops, but it is very hard for us dino enthusiasts to imagine a poor Brachiosaur struggling to lean its neck way over to graze from the ground!