Well I do use untreated pine wood, like Luka, and I'd say no more sophisticated than his in the pic above. It's not permanent, but lasts long enough.
Not surprising, but it is interesting that there is also a "sweet spot" in the soil level for wood decay in these boards. And, it is probably the same as for root development, too, as it is these same types of soil organisms that benefit both processes that thrive there. Due to optimal moisture/air/temperature ratios, for me it begins about 1 inch below the mulched soil surface, and goes down about 6 inches.
