Wooden 6X6 in ground

I would stay away from concrete and wrapping with plastic. Both will retain moisture. I would go with gravel.

Concrete base/foundation/column, Simpson Post base.

Dry the posts, take them to a sawmill and get them high pressure saturation treated before putting them in. That will go a loooong way towards longevity. I've been ramming pressure treated pine posts into bare dirt for the last 20 years and they're all still there (I have a farm for a main source of income).

If you bed them in concrete, make sure that the concrete gently slopes away from the base of the post on all sides.

-P.

The _best_ way is to use a concrete footing. If the wood is going to be buried then the species counts for more than the method of burial and Tamerac is not a good choice for such use. Red cedar (also sometimes called "juniper") or pressure treated pine would be a better choice but if you can get hold of some bois d'arc (aka "osage orange" and "bodark") or ipe I think you'd be in better shape.

I would agree with the gravel(crushed stone), 1/2 to 3/4 inch aggregate size. If the posts are cut from the heartwood of tamarak grown in wet areas, all the better.