Biscayne National Park - Worlds of Green

Biscayne is predominantly an underwater park, but it has islands too, lumps that loom up from the horizon like the backs of enormous sea turtles, full of mystery and intrigue. Some, like the Arsenicker Keys, barely count as islands: they contain no dry land, and their greenery consists only of saltwater-loving mangroves. Throughout the Florida Keys, forests of these small trees perch like spiders above the silty mud and limestone rocks, supported by a crisscrossing web of arching roots. Unlike more conventional trees, with their subterranean roots, mangroves have their roots for the most part exposed to the air. They sprout out and downward from branches and trunks, forming a tangled maze and anchoring themselves in the mud. The trees' oval leaves make a dense green canopy, shielding fish and others in the shallows from the blazing sun. At high tide brown-spiraled periwinkle snails climb the roots, ten creep back down as the water recedes. Male fiddler crabs wave their hefty claws from exposed mudbanks, trying to attract females into their lairs. With their graceful pink necks and long spindly legs, roseate spoonbills and an occasional flamingo stalk the fringes of the dense thickets; silvery bonefish search for worms, crabs, and other prey along the muddy roots.

Their tangled roots blocking access and acting like a natural fence, mangroves by and large keep bigger forms of life from the islands. Protected from nonflying creatures that might do them harm, thousands of birds flock about. Brown pelicans with their ungainly bodies and huge sacklike bills cruise along, diving headfirst into the sea when they spot a fish. Poised atop their nests, such birds of sea and shore as herons and frigate birds fill the air with screams and calls, their collective voices so loud that the noise is almost deafening.

Mangrove stands, once thought to be mere mosquito-infested wastelands, are now known to make two very different, and two very valuable, contributions to life on earth. One concerns the ocean's food chain; the other, building up land. As mangrove leaves fall and pile up among the roots, bacteria and fungi attack them, turning them into nutrients needed by tiny forms of aquatic life. When storms surge into the tidal swamps, these nutrients are washed out to sea, nourishing plankton, the first link in the oceanic food chain. The roots, too, play a role in building land, trapping silt and debris. As the sediments settle out of the water, they build up the bottom. Though the increase in elevation is slight, it does make a difference (it holds back the tides), and so does the greater amount of soil. Plants less tolerant of saltwater, such as buttonwood, begin to take root. Over time the land will reclaim the swamp.

But not all of Biscayne's islands are amphibious places, swept throughout their entirety by the tides. Elliot and Adams keys are far more substantial, and they are among the ones with tropical hardwood forests. The most northern of such forests in the United States, they cover what is nearly the last of the undeveloped Florida Keys. Trees here tend to have crooked trunks, very hard wood, and evergreen leaves, which create a dense shade all year. Vines abound, twining their way toward the light. Spanish moss, among other plants, gets the sunlight it needs by growing perched on trees rather than in the ground. In the middle of the islands, hundreds of hermit crabs drag the snail shells in which they live over mossy green rocks, picking among the detritus for food. Giant land crabs dig deep, intricate tunnels through the soft stone.

Biscayne's forested islands, so vastly different from underwater reefs, are linked with them by time. Forming a loose chain five to seven miles from the mainland, the islands were once part of a line of coral reefs stretching from what is now Miami to Key West and beyond. But that was about 100,000 years ago; since then the sea level has dropped about 25 feet, leaving the reefs high and dry. Trees on these former reefs do not stand securely in the ground, like mainland trees, but instead wrap their roots tightly around the ancient coral rock, seeking scarce patches of soil. Remarkably, the same types of coral that today flourish a mile or two offshore also sit in the forest, whitened and stony, supporting plant growth. A brain coral occupies precisely the same position as when it was alive and under water many thousands of years ago, but instead of fish, ferns and flowers are its modern-day companions.