Small islands of trees growing on land raised between 1 foot (0.30 m) and 3 feet (0.91 m) above sloughs and prairies are called tropical hardwood hammocks.[58] They may range from one (4,000 m2) to ten acres (40,000 m2) in area, and appear in freshwater sloughs, sawgrass prairies, or pineland. Hammocks are slightly elevated on limestone plateaus risen several inches above the surrounding peat, or they may grow on land that has been unharmed by deep peat fires. Hardwood hammocks exhibit a mixture of subtropical and hardwood trees, such as Southern live oak (Quercus virginiana), gumbo limbo (Bursera simaruba), royal palm (Roystonea), and bustic (Dipholis salicifolia) that grow in very dense clumps.[59] Near the base, sharp saw palmettos (Serenoa repens) flourish, making the hammocks very difficult for people to penetrate, though small mammals, reptiles and amphibians find these islands an ideal habitat. Water in sloughs flows around the islands, creating moats. Although some ecosystems are maintained and promoted by fire, hammocks may take decades or centuries to recover. The moats around the hammocks protect the trees.[60] The trees are limited in height by weather factors such as frost, lightning, and wind; the majority of trees in hammocks grow no higher than 55 feet (17 m).