Rainforests
Tropical rainforests are the most diverse ecosystem (生态系统) on Earth, and also the oldest. Today, tropical rainforests cover only 6 percent of the Earth's ground surface, but they are home to over half of the planet's plant and animal species.
What Is a Rainforest？
Generally speaking, a rainforest is an environment that receives high rainfall and is dominated by tall tress. A wide range of ecosystems fall into this category, of course. But most of the time when people talk about rainforests, they mean the tropical rainforests located neat the equator.
These forests raceive between 160 and 400 inches of rain per year. The total annual rainfall is spread pretty evenly throughout the year, and the temperature rarely dips below 60 degrees Fahrenheit.
This steady climate is due to the position of rainforests on the golbe. Because of the orientation of the Earth's axis, the Northern and Southern hemispheres each spend part of the year tilted away from the sun. Since rainforests are at the middle of the globe, located near the equator, they are not especially affected by this change. They receive nearly the same amount of sunlight, and therefore heat, all year. Consequently, the weather in these regions remains fairly constant.
The consistently wet, warm weather and ample sunlight give plant life everything it needs to thrive. Trees have the resources to grow to tremendous heights, and they live for hundreds, even thousands, of years. These giants, which reach 60 to 150 It in the air, form. the basic structure of the rainforest. Their top branches spread wide in order to capture maximum sunlight. This creates a thick canopy (树冠) level at the top of the forest, with thinner greenery levels underneath. Some large trees grow so tall that they even tower over the canopy layer.
As you go lower, down into the rainforest, you find less and less greenery. The forest floor is made up of moss, fungi, and decaying plant matter that has fallen from the upper layers. The reason for this decrease in greenery is very simple: The overabundance of plandts gathering sunlight at the top of the forest blocks most sunlight from reaching the bottom of the forest, making it difficult for robust plants to thrive.
The Forest for the Trees
The ample sunlight and extremely wet climate of many tropical areas encourage the growth of towering trees with wide canopies. This thick top layer of the rainforest dictates the lives of all other plants in the forest. New tree seedlings rarely survive to make in to the top unless some older trees die, creating a "hole" in the canopy. When this happens, all of the seedlings on the ground level compete intensely to reach the sunlight.
Many plant species reach the top of the forest by climbing the tall trees. It is much easier to ascend this way, because the plant does's have to form. its own supporting structure.
Some plant species, called epiphytes, grow directly on the surface of the giant tress. These plants, which include a variety of orchids and ferns, make up much of the understory, the layer of the rainforest right below the canopy. Epiphytes are close, enough to the top to receive adequate light, and the runoff from the canopy layer provides all the water and nutrients (养分) they need, which is important since they don't have access to the nutrients in the ground.
tranglers and Buttresses
Same epiphytes eventually develop into stranglers. They grow long, thick roots that extend down the tree trunk into the

A. Y
B. N
C. NG