These hammocks form on slightly elevated areas unharmed by deep peat fires or limestone plateaus rising several inches above the surrounding peat. Hardwood hammocks exhibit a mixture of subtropical and hardwood trees that grow in very dense clumps, such as southern live oak (Quercus virginiana), gumbo limbo (Bursera simaruba), royal palm (Roystonea regia), Florida poisonwood (Metopium toxiferum), and willow bustic (Dipholis salicifolia).[40] Near the bases of hammocks sharp saw palmettos (Serenoa repens) flourish, making the hammocks very difficult to penetrate. Water in sloughs flow around the islands creating moats. Though some ecosystems are maintained and promoted by fire, hammocks may take decades or centuries to recover; the moats are therefore essential for protection.[41] Islands vary in size, but most range between 1 and 10 acres (0.40 and 4.05 ha); the water slowly flowing around them limits their size and gives them a teardrop appearance from above.[42] The height of the trees is limited by factors such as frost, lightning, and wind: the majority of trees in hammocks grow no higher than 55 feet (17 m).