How can tree branches grow upwards against gravity?
This review paper might help answer your question: http://www.plantphysiol.org/content/120/2/343.full

It seems that starch-filled organelles called amyloplasts sediment in the cell according to the pull of gravity, and in doing so this gives the cell some form of basic up-down polarisation.  If my memory serves correctly, this can then cause changes to the growth of the cell in the branches, meaning the side of the cell opposite the amyloplasts grows slower and the side closest the amyloplasts grows faster, causing the upwards curvature of the branches seen in your tree.