Now I am confused (more than usual).

Don't cedar/Christmas trees become water-logged and eventually sink? Depending on conditions, the trees only act like buoys for a limited period of time?

I thought that if you wanted them upright, you had to cement them standing up in a bucket (or some similar arrangement).

If you wanted a brush pile, you could tie several trees together and anchor them to the bottom where they would subsequently assume their final configuration.