Hi Wayne, and welcome to the forum.
I think the question of how much weight will always be subjective. There are a tremendous amount of variables that can affect it. If the tree is fresh cut, it has a lot of water inside it that will make it easier to sink it. But if it dry, it will definately be more bouyant. I recently weighted a dry pole 10' long and 4" dia with a broken cinder block on each end. As the water rose, the one end still floated. Had to add another piece of block. Another example would be in the old days when they floated logs down rivers and across lakes, bringing them to the mills. These were fresh cut logs and the guys would stand on top of these to move them around. So by that example, they were holding a man of +/-200#. Kind of sounds like what you have. If you can leave a root ball on one end, you can almost be gauranteed that end will sink, but the other end may not.
I have seen pics on here where guys have staked trees down instead of using weight. Check out this link for possible help. Also use the search feature for help. Good luck..........
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92463#Post92463 
