6 BUREAU OF GEOLOGY
bare, rocky surfaces or sparse grass cover. Several factors combine to create a stressful environment for many types of plants. Top soil, in the usual sense, is almost non-existent on the islands. The "soil" consists of weathering byproducts of limestone or carbonate debris, which provides few nutrients and limited rooting material. The mean annual rainfall for the Keys is the lowest of any part of Florida, averaging as much as 50 percent less than the wettest areas of the State. Rainfall on the Keys decreases from about 50 inches per year at Key Largo to as little as 25 inches per year at Key West. In addition, the rainfall is rapidly lost through high evaporation or it readily percolates downward through the few feet of porous rock to the underlying brackish water table. Any plant attempting to colonize the islands must also be salt-tolerant, since the atmosphere is laden with salt spray. These conditions restrict the types of plants that can grow on the islands, and they curtail the growth of the plants that do gain footholds.
The foundations of the islands are ancient, dead coral reefs. However, more recent changes in the Keys are the result of natural and biological forces acting as geological agents, which are constantly at odds in adding to or eroding the islands. Perhaps the most important constructional biological agents are mangrove trees, which are ubiquitous in the Keys. Mangroves are salt-tolerant trees that thrive in the tidal zones along subtropical and tropical coastlines (Figure 3). In south Florida and the Keys mangroves are one of the most important components in the coastal marsh ecosystems. Figures 3, 4 and 5 illustrate the effects of mangroves along the islands' shorelines. A mangrove's thick tangle of aerial prop roots act as a baffle that catches and holds sediments, from both landward and seaward directions. Figure 4 shows a carbonate mud bank forming around a stand of mangroves. The lime mud accumulating in shallow water (lighter toned) is an ideal substrate for young mangrove seedlings, seen here in progressively younger generations offshore, reaching to join the main island to the small mangrove island in the right background. In this typical fashion, mangroves stabilize shorelines and add new land to the Keys. The labyrinthine mangrove islands and tidal channels shown in Figure 5 are the result of such mangrove growth and sediment accumulation. Conversely. mangrove roots also provide convenient supports for new colonies of oysters, which may provide rock foundations for future generations of mangroves.