سه شنبه 29 آذر 1390 06:54 ب.ظ
Warm and wet describes the tropical rain forest climate. The average annual temperature is above 20� C; there is never a frost. Rainfall varies widely from a low of about250cm of rain per year to about 450 cm/year. That means a range from about 8 to 14 feet of rain per year.
As you can see from the map to the right, the tropical rainforests are, indeed, located in the tropics, a band around the equatorfrom 23.5� N (the Tropic of Cancer) to23.5� S (the Tropic of Capricorn) (red lines on map, right). Because the Earth tilts 23.5 degrees on its axis as it travels around the sun, at some point in the year (the solstices, June 22nd in the north, December 22nd in the south) the sun will be directly overhead on one of these lines. At the equinoxes the sun is directly over the equator.
Within this band, solar radiation is most intense, and thus the surface of the planet warms the most. The warmth leads to a lot of evaporation, and as warm, moist air rises, it cools, the water condenses, and the water falls back to the earth as rain. Thus, the warmest areas of the planet also tend to be the wettest, and this sets the stage for the tropical rain forest.
Not all of the land in the tropics is tropical rainforest. Some areas are too cold (mountaintops), or are too dry (the far side of a mountain range from the ocean gets less rain). In some places there may be a lot of rain, but it falls seasonally and the long dry season prevents a tropical rainforest from developing.
Another biome similar to the tropical rain forest is the cloud forest. These forests form on mountaintops in the tropics; I have been to such forests in Jamaica and Costa Rica, and they exist in other mountainous areas as well. Because of their elevation, cloud forests are cooler than the tropical rain forests below them; much of the water there does not fall as rain but is instead wrested from the clouds by the plants living in the forest. These forests are critically endangered by global warming; as the planet warms tropical rainforest is able to move up the mountainsides and the cloud forests are displaced into smaller and smaller regions at the tips of the mountains - and if these mountaintops get too warm the entire cloud forest will be replaced by tropical rainforest. You can read more about cloud forests on this page from our trips to Costa Rica.
Tropical Rain Forest Distribution
Indicator Plant Species:
Because of the great diversity of plants in the tropics, naming indicator species is very difficult unless you are a botanist. Certainly, a number of plant groups reach their greatest diversity here, but picking out individual species would be difficult. I've put the chocolate tree here as a representative simply because just about everyone knows what chocolate is. The chocolate is harvested from the pod you see growing here; the pods develop from flowers which are borne directly on the trunk. This gives a good point of attachment for what will become a heavy pod. Chocolate is native to the new world tropics, but has been transplanted to tropical regions around the world.
While it's hard to pick indicator plant species there is something that many of the tropical plants share. That is adrip tip on the ends of the leaves (below). This pointed tip promotes drainage from the surface of the leaf, and thus helps keep the leaf surface clean of epiphytes and fungi in the humid forest.
|How diverse are the rainforests? A hectare in the Appalachians may have up to 30 species of trees; in the tropicsa range of 40 to 100 is common and over 300 species is not unknown. The entire La Selva site in Costa Rica has over 1,600 species of plants in 1,500 hectares; a station in the Amazon basin has over 1,800 species. As many frogs have been collected from a single site in the rainforest as there are in all of North America. One estimate of invertebrate diversity is for about 30 million species in tropical forests. Read more about these estimates in this book.|
One of the biggest challenges in biology is explaining the diversity of the rainforests. Certainly the high productivity permitted by the ready availability of water combined with the warm temperatures is a factor. The massive size of many of the trees provides a number of new habitats for animals to exploit. The extreme specialization to avoid competition is another factor. Some scientists propose that human interference, in the guise of clearing small areas for agriculture, may play a key role as well. And there is time; the rainforests are areas which have been relatively unaffected by the climactic shifts associated with the glaciation which ended about 10,000 years ago.
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