Why can timber piles last longer when they are kept continuously underwater?

Timber piles last longer when kept continuously underwater because the submerged environment prevents the growth of decay-causing fungi. For fungi to thrive and cause wood decay, they require specific conditions, including the presence of air (oxygen), moisture, and suitable temperatures.1

When timber is kept fully submerged, the lack of sufficient oxygen inhibits the growth and activity of these fungi, effectively preventing the primary cause of wood loss. In civil engineering, this principle is sometimes utilized by casting a reinforced-concrete slab around the heads of timber piles, with the underside positioned just below the lowest water level to ensure the timber remains continually submerged and protected from decay.12

However, it is important to note that while submersion protects against fungi, it does not provide immunity to all threats. In certain environments, such as tropical or semitropical waters, timber remains vulnerable to marine borers, which are organisms that can cause severe damage to submerged wooden structures.12