<P> The emergent layer contains a small number of very large trees, called emergents, which grow above the general canopy, reaching heights of 45--55 m, although on occasion a few species will grow to 70--80 m tall . Some examples of emergents include: Balizia elegans, Dipteryx panamensis, Hieronyma alchorneoides, Hymenolobium mesoamericanum, Lecythis ampla and Terminalia oblonga . These trees need to be able to withstand the hot temperatures and strong winds that occur above the canopy in some areas . Several unique faunal species inhabit this layer such as the crowned eagle (Stephanoaetus coronatus), the king colobus (Colobus polykomos), and the large flying fox (Pteropus vampyrus). </P> <P> However, stratification is not always clear . Rainforests are dynamic and many changes affect the structure of the forest . Emergent or canopy trees collapse, for example, causing gaps to form . Openings in the forest canopy are widely recognized as important for the establishment and growth of rainforest trees . It's estimated that perhaps 75% of the tree species at La Selva Biological Station, Costa Rica are dependent on canopy opening for seed germination or for growth beyond sapling size, for example . </P> <P> Tropical rainforests are located around and near the equator, therefore having what is called an equatorial climate characterized by three major climatic parameters: temperature, rainfall, and dry season intensity . Other parameters that affect tropical rainforests are carbon dioxide concentrations, solar radiation, and nitrogen availability . In general, climatic patterns consist of warm temperatures and high annual rainfall . However, the abundance of rainfall changes throughout the year creating distinct moist and dry seasons . Tropical forests are classified by the amount of rainfall received each year, which has allowed ecologists to define differences in these forests that look so similar in structure . According to Holdridge's classification of tropical ecosystems, true tropical rainforests have an annual rainfall greater than 2 m and annual temperature greater than 24 degrees Celsius, with a potential evapotranspiration ratio (PET) value of <0.25 . However, most lowland tropical forests can be classified as tropical moist or wet forests, which differ in regards to rainfall . Tropical forest ecology - dynamics, composition, and function - are sensitive to changes in climate especially changes in rainfall . </P> <P> Soil types are highly variable in the tropics and are the result of a combination of several variables such as climate, vegetation, topographic position, parent material, and soil age . Most tropical soils are characterized by significant leaching and poor nutrients, however there are some areas that contain fertile soils . Soils throughout the tropical rainforests fall into two classifications which include the ultisols and oxisols . Ultisols are known as well weathered, acidic red clay soils, deficient in major nutrients such as calcium and potassium . Similarly, oxisols are acidic, old, typically reddish, highly weathered and leached, however are well drained compared to ultisols . The clay content of ultisols is high, making it difficult for water to penetrate and flow through . The reddish color of both soils is the result of heavy heat and moisture forming oxides of iron and aluminium, which are insoluble in water and not taken up readily by plants . </P>

Tropical rainforests occur in equatorial regions. this is primarily due to the effect of
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