Poles shrink only when the moisture content drops below what percentage?

The correct choice regarding when poles shrink due to moisture content is based on understanding the behavior of wood and how it reacts to changes in humidity. Wood generally begins to shrink when the moisture content falls below the fiber saturation point, which is typically around 30%. At this threshold, wood fibers start to lose the water that is bound to them, leading to dimensional changes such as shrinkage.

With this in mind, the choice indicating 30% reflects the scientifically recognized point where significant shrinkage begins, making it the correct answer. The other percentages—20%, 25%, and 35%—do not represent the correct threshold for substantial shrinkage to occur in wooden materials.