Which type of wood has the lowest resistance to decay?

Untreated sapwood has the lowest resistance to decay among the options listed. Sapwood is the younger, outer part of a tree's trunk, which is generally more vulnerable to rot and decay than the heartwood, the inner, older wood. The cellular structure of sapwood is less dense and contains more moisture, making it more susceptible to fungi, insects, and other decay-causing organisms.

In contrast, pressure-treated wood is specifically treated with preservatives to improve its decay resistance, making it suitable for outdoor and high-moisture environments. Hardwood, which typically comes from deciduous trees, often has a higher density and contains natural oils and resins that enhance its resistance to decay. Bamboo, while not a traditional wood, has a strong structure and can also be treated for greater durability and resistance to decay.

Thus, untreated sapwood's inherent characteristics make it the most prone to decay among these materials.