Many people have the mistaken belief that naturally durable wood species will not rot. There are a handful of domestic wood species that are classified as naturally durable. Some of the more common in the US South are sassafras, live oak, Eastern red cedar, catalpa and black locust. In the West, redwood and Western red cedar offer natural decay resistance, particularly with heartwood lumber. The heartwood is the darker wood in the center of most trees. It is higher in chemical compounds that are responsible for decay resistance. Some species have a heartwood that easily visually detectable and others do not.
I purchased a swing set made of redwood about 15 years ago. Over the years, I have replaced many of the parts of the swing set with pressure-treated southern pine. My replacement pieces continue to perform fine. However, each year more pieces of the redwood continue to fail due to wood degrading fungi. Naturally durable wood species are NOT resistant to wood degrading insects and fungi. They will provide better protection than untreated wood. The US South has a very harsh environment for exterior wood in ground contact. Untreated pine field stakes can show failure in as soon as 1 to 2 years. My redwood deck began showing failure in 5 years for the pieces in ground contact and 7-8 for those above ground.
There is no substitute for pressure treated wood for exterior applications. This material is economical, safe, easy to work with and it will perform in excess of 50 years with minimal maintenance. In the US South there are numerous utility poles that are well in excess of 50 to 80 years of age that are showing no signs of degradation. For more information on naturally durable wood species please visit https://www.fs.usda.gov/treesearch/pubs/35536