|Variables such as climate, sunlight, rainfall, soil and elevation determine the character of a forest -- whether it consists of small needle-leaved juniper, pine, spruce, or dense tropical vegetation thick with vines and buttressed trees, or open, dry woodland dominated by giant Ponderosa pine.|
|There is no one point where a tropical forest suddenly becomes temperate, no definite line where a boreal forest switches to broadleaf. the various categories we have for different types of forests are of course useful, but there are also large transition areas which may fit into several categories, or none at all.||
An interactive walk through a temperate cedar-hemlock forest. See how the forest is put together and how many of the topics discussed in these articles are related -- everything from riparian zones and plant succession to the destruction of the forests.
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Vines and epiphytes ("air plants" which grow on the trunks and branches of trees) are most common in tropical rainforests where there is not a pronounced dry season. Photo by Maya Walters.
Around the equator, conditions are generally
favorable for moist tropical forests because of frequent rainstorms. Moving slightly away from the equator, less rainfall and different soil
types make it easier for seasonal "monsoon forests" and more open woodlands to grow, though some tropical forests do stretch farther
The rainforests are the most well-known type of forest in the tropics, but this region also contains dry forests and seasonal forests. The word "rainforest" is now being used to describe many moist forests in the tropics which are not "true" rainforests. A tropical forest of broad-leaved trees with a constant, abundant supply of rainfall is the traditional meaning of the term "rainforest". Moist tropical forests with deciduous trees are commonly called seasonal forests, due to the fact that they experience a definite dry season.
|Most transition zones between forest types are very gradual. However, when the forest changes because of altitude instead of latitude, the transition is much more sudden. This can be seen very graphically by the "treeline" on temperate mountains. In some tropical forests, there is quite a sudden gradation from rainforest to higher level "montane forest".|
|The transition zones where forests meet different types of habitats are called ecotones. These ecotones can be very broad, and contain species from both of the surrounding habitats. This means that the diversity of both plants and animals in ecotones is very high. These transition habitats are often overlooked when parks and nature reserves are created, and are simply dismissed as not being a true forest. However, ecotones are not only home to many species that are not found in either of the adjacent habitats, they may also be important to the evolution of species. For example, many rainforest animals will colonize an ecotone between a tropical rainforest and a savanna. While they continue to breed with those animals still in the rainforest, the conditions are so different in the two habitats that those animals in the ecotone will begin to develop differently.|
|The boreal forests extend the farthest north, meeting the arctic tundra at the northern end of their range. South of the boreal forests, where the climate is less harsh, are the warmer temperate forests. Some conifers grow here too, and in fact dry areas are dominated by pines. Most temperate areas, however, are occupied by the broad-leaved trees. They grow in large spreading forms in order to to expose as many of their wide, paper-thin leaves as possible to direct sunlight. This is necessary for photosynthesis, the process by which all green plants manufacture oxygen and sugars from carbon dioxide, water, and the energy from sunlight. These leaves also allow huge quantities of water vapor to escape, which means that broad-leaved trees require much more precipitation and is why they are replaced by conifers in drier locations.|
Where the climate is cooler, temperate
broad-leaved trees are deciduous, meaning that each fall, nutrients from the leaves are drawn back into the branches, and the leaves
die and then fall to the ground. In the spring, new leaves are grown. Some tropical trees are also deciduous, but they shed their leaves
for the dry season instead of for winter.
The broad-leaved trees put on an impressive display of fall colors in eastern North America. Photo courtesy Naomi Woods. Vast quantities of water are needed to grow and maintain leaves. For this reason, in dry temperate areas, coniferous forests manage to take over from the broadleaf trees. Temperate rainforests are also dominated by giant coniferous trees, but very different species. The northern boreal forests consist entirely of conifers.
|There are many obvious differences between these main types of forests, but there are more subtle differences between forests of the same type. Two forests at the same latitude and elevation can both be classified as broadleaf but consist of many different plants and animals. Some species have managed to spread through forests on several different continents. Some closely related plants are found in forests separated by thousands of miles of ocean.|
Every region has a specific type of "climax"
forest, depending on the temperatures, moisture levels, and soil types. The climax forest is the community of plants that will eventually
grow and remain dominant in an area. When some trees that can only grow in direct sunlight reach their full height, they shade out all the
new seedlings underneath them. Seedlings of shade-tolerant trees are then the only ones that can grow, and they eventually come to
dominate the area. When a forest is burned or cut down, grasses are the first plants to return, then the area
is taken over by shrubs and small trees until the climax forest grows again, and remains until the area is disturbed again.
A cedar/hemlock climax forest. In disturbed areas, trees including birches, aspens, Douglas-fir, or Ponderosa pine could be found, depending on the climate and precipitation levels in the specific habitat. Photo by Maya Walters.
[boreal forests] [temperate forests] [tropical forests]
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[climate] [soil] [plants] [water] [seasons] [forest life] [biodiversity] [leaves] [succession]
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