The tropical zone has the greatest diversity of plant life on Earth. The tropical zone centers on the equator and spreads outward to the Tropic of Cancer, about 23 degrees north of the equator, and the Tropic of Capricorn, about 23 degrees south of the equator. Scientists divide the tropical zone into sub-zones based on the amount and consistency of the rainfall it receives. Plants in each sub-zone have adapted to their environmental conditions.
Rain Forest Trees
Tropical rain forests have a high amount of rainfall with rain usually occurring at some point every day. The warm temperatures and high moisture levels create excellent growing conditions for trees, the dominant plant in the rain forest. Rain forest trees grow so tall and close together that they create a dense canopy that shields sunlight from the forest floor. The lack of sunlight makes it difficult for other types of plants to establish themselves. The rain forest has a great diversity of trees with no species dominating a particular location. The trees bear evergreen leaves and typically have a covering of thin bark, since they don't need thick bark to protect them from frost. The difficulty in receiving sunlight through a rain forest canopy has caused some trees to adapt by growing very rapidly when another tree falls and a break in the canopy appears. The plants remain in a stage of very low growth until an opening presents itself.
Epiphytes and Lianas
Two types of plants have successfully adopted strategies to compete in rain forest conditions: epiphytes and lianas. Epiphytes, plants that grow on other plants, can survive by placing themselves high on the branches of rain forest trees to obtain sunlight. Orchids and bromeliads stand out as two of the most successful rain forest epiphytes. The other type of plants to adapt to the rain forest environment, lianas grow as vines on trees. They have roots in the soil, but grow stems up to the canopy. Epiphytes and lianas rarely harm their host plant, but the strangler fig is a notable exception. This vine grows numerous thick stems that eventually join up and sometimes kill the host tree.
Another type of tropical environment, the savanna has a different climate from the rain forest. A tropical savanna goes through a dry season, followed by a wet season. The rainfall does not occur with enough consistency to support masses of forest trees. Grasses form the dominant plant life in savannas. Some savannas have no trees at all, while others have trees scattered throughout the landscape. The isolated trees in savannas often have long tap roots to reach water deep underground. The grasses that dominate savannas grow about 3 to 6 feet tall under natural conditions and often have underground storage organs, such as bulbs, to help them survive the long period of low rainfall.
Tropical deserts, such as the Sahara, develop when areas receive extremely low amounts of annual rainfall. Plants in these regions must adapt to the very dry conditions. One group of plants called ephemerals germinate very quickly from seed shortly after a rare occurrence of rainfall. Plants in the Euphorbia genus have adapted to the tropical desert by storing water in their tissues. This allows them to survive the the extended periods of time without moisture.