Regardless of what type of deck board is used, ignition can occur from an accumulation of combustible material (grass, leaves, needles) under the deck. Embers can readily ignite this debris during a fire. Enclosing the underside of the deck is one method to reduce the risk of ignition. There are two ways to enclose your deck. You can enclose it by applying sheathing or siding around the perimeter (a vertical enclosure), or by attaching- ing sheathing/panel materials to the underside of the structural support members (a horizontal enclosure). The more careful you are about not storing combustible materials near your home (and under your deck), maintaining near-home vegetation, and cleaning up windblown debris, the less important enclosing your deck becomes.
The closer the deck is to the ground, the harder it is to use as a storage area, but it also becomes more difficult to clean out debris that will accumulate. If you have a solid surface deck (e.g., one with a lightweight concrete walking surface), it is probably already enclosed horizontally. If you are going to store combustible materials under your deck, then vertically enclosing it with a noncombustible or ignition-resistant material would make sense. If you do this, make sure you avoid any potential moisture-related degradation issues by providing adequate venting or take other moisture control actions.
Deck enclosure will not guard against embers falling on top of the deck, so be aware of combustible material that you have on your deck. Firewood should be moved off of the deck during wildfire season. Combustible materials commonly found on decks (e.g., brooms, umbrellas, patio furniture, doormats, etc.) should be moved off the deck if possible, or moved as far from the building as possible during wildfires and spaced to avoid clustering items.