That's one advantage of Southern Yellow Pine. It has a lot of resin in it that acts sort of like a self waterproofing agent.
The best is the stuff from out west, Ponderosa pine.
The key to southern yellow pine resiliency back when these trucks were made was the availability of old growth trees. These trees were slow growing due to stiff competition for sunlight in unthinned natural conditions. This resulted in many more growth rings per inch in diameter than what our modern plantation pines have. Tighter growth rings results in more resin in the boards thus less prone to rot.
