Some other have said it already, but I've found that our oak lasts really well - all apart from the outer 2 inches of sap wood. Apparently the sapwood has higher sugar content/lower tannin content so in the damp rots away in about 12 months.
I bucked a log last year that had been down since the great storm in 1987, in contact with the soil. Not a trace of the outer sap wood left but the heart wood was totally perfect. Could have made sawmill timber.
Was that the Storm that took down 'The Seven Oaks'?