What is “naturally durable” wood in reference to Code required “naturally durable wood or preservative-treated wood in contact with concrete?
Here is a study done in 1995 that looks like the heartwood of Douglass Fir, and the heartwood of redwood hold up the best for outdoor exposure. It may not give an indication of what you are looking for with regards to contact to concrete, but it at least gives insight. 

In my years in the trade and having a curiosity about how houses were built, how they last, or fail in areas always has intrigued me. I have noticed a great number of homes over time that have D. Fir plates, directly on concrete AT ground level, in a 60+ year old home and the wood is still viable.

I would imagine that there will be this "list" that the gov't relies on to determine the "naturally durable" wood specie somewhere.....

I read/heard that poplar, when super heated becomes surprisingly rot resistant...

None of this may not directly answer the question, but I would like to mention black locust has been used for fence posts that would last for decades. I am sure there is no milled lumber of black locust available, but it sure would make the list.