The most common natural community in Florida, flatwoods are a type of savannah with scattered pines, few shrubs (generally palmetto, gallberry, tarflower and lyonias), and a diverse groundcover of grasses and wildflowers. It can vary from a dry scrubby flatwoods with shrubby oaks through mesic to wet flatwoods that are similar to wet prairie. Dry prairie is similar to flatwoods with few to no trees. Acid soils are sandy, low in organic material, and are usually underlain by an impervious hardpan or clay layer, one to three feet below the surface. Moisture levels are from dry to saturated, varying with the site and with yearly weather changes. Flatwoods plants tolerate a wide range of moisture and soil fertility. Many species may be suitable for only drier or wetter conditions. Plants are generally not tolerant of salt and shade. Under natural conditions fires occur at 1-5 year intervals, preventing natural succession to upland forest.