Montane grasslands and shrublands
Grasslands are characterized as lands dominated by grasses rather than large shrubs or trees. In the Miocene and Pliocene Epochs, which spanned a period of about 25 million years, mountains rose in western North America and created a continental climate favorable to grasslands. Ancient forests declined and grasslands became widespread. Following the Pleistocene Ice Ages, grasslands expanded in range as hotter and drier climates prevailed worldwide. There are two main divisions of grasslands:
(1) tropical grasslands, called savannas, and
(2) temperate grasslands.
The tropical rainforest is a hot, moist biome found near Earth’s equator. The world’s largest tropical rainforests are in South America, Africa, and Southeast Asia. Tropical rainforests receive from 60 to 160 inches of precipitation that is fairly evenly distributed throughout the year.
The combination of constant warmth and abundant moisture makes the tropical rainforest a suitable environment for many plants and animals. Tropical rainforests contain the greatest biodiversity in the world. Over 15 million species of plants and animals live within this biome.
Distribution of Tropical Rainforests
Most rainforests occur in tropical regions of the world, though temperate rainforest habitat is in certain higher latitude areas of every continent. Tropical rainforests have been defined as occurring in a band around Earth between the Tropic of Capricorn (about 23° S. latitude) and the Tropic of Cancer (about 23° N. latitude), with areas in equatorial regions generally being the most densely vegetated and having the highest biodiversity of both plants and animals.
A Rainforest can be described as a tall, dense jungle. The reason it is called a “rain” forest is because of the high amount of rainfall it gets per year. The climate of a rain forest is very hot and humid so the animals and plants that exist there must learn to adapt to this climate.
Layer of Trees
Tropical rainforests have four layers:
- Emergent Layer
These giant trees thrust above the dense canopy layer and have huge mushroom-shaped crowns. These trees enjoy the greatest amount of sunlight but also must endure high temperatures, low humidity, and strong winds.
- Canopy Layer
The broad, irregular crowns of these trees form a tight, continuous canopy 60 to 90 feet above the ground. The branches are often densely covered with other plants (epiphytes) and tied together with vines (lianas). The canopy is home to 90% of the organisms found in the rain forest; many seeking the brighter light in the treetops.
Receiving only 2-15% of the sunlight that falls on the canopy, the understory is a dark place. It is relatively open and contains young trees and leafy herbaceous plants that tolerate low light. Many popular house plants come from this layer. Only along rivers and roadways and in treefall and cut areas is sunlight sufficient to allow growth to become thick and impenetrable
- Forest Floor
The forest floor receives less than 2% of the sunlight and consequently, little grows here except plants adapted to very low light. On the floor is a thin layer of fallen leaves, seeds, fruits, and branches that very quickly decomposes. Only a thin layer of decaying organic matter is found, unlike in temperate deciduous forests.
Ecosystem of Rainforests
Rainforest biome is very complex. It includes a myriad of different species of plants and animals that are all adapted to rain, and lots of it.
There are different levels of plants in the rainforests. The highest are the tall trees that often, particularly in tropical rainforests, form a closed canopy. On the next level are vines, orchids and epiphytes that grow high up in trees to reach more sunlight. Lower down are tree ferns and similar short trees, and next there is the forest floor – dark but far from lifeless.
Animal life is also complex. Insects are deeming in what is very favourable conditions for them. Frogs and reptiles thrive. Bird life is noisy and colourful with many species of parrots and cockatoos. Since birds live mostly in the rainforest canopy, rainforest snakes are often tree snakes. Many mammals also live up in trees, whether they are herbivores or carnivores. And rivers and waterholes contain different water animals. All those plants and animals are dependent on each other in a complex food web, and/or benefit from each other in other ways in the ecosystem.
Most tropical rainforest soils are relatively poor in nutrients. Millions of years of weathering and torrential rains have washed most of the nutrients out of the soil. More recent volcanic soils, however, can be very fertile. Tropical rain forest soils contain less organic matter than temperate forests and most of the available nutrients are found in the living plant and animal material. Nutrients in the soil are often in forms that are not accessible by plants.
Constant warmth and moisture promote rapid decay of organic matter. When a tree dies in the rainforest, living organisms quickly absorb the nutrients before they have a chance to be washed away. When tropical forests are cut and burned, heavy rains can quickly wash the released nutrients away, leaving the soil even more impoverished. Even forests on poor soils may grow very well. The soils on which Brazilian forests grow are generally relatively infertile, but the leaves of the trees accumulate high concentrations of nutrients. Nevertheless, Brazilian forest litterfalls are just as nutrient-rich as those in forests at La Selva, Costa Rica, where soils have much higher nutrient levels.
The roots of most forest plants lie fairly shallow, within a foot of the surface. Because of this, any disruption to the soil surface will have serious consequences for root structure. In rainforests on deeper soils, as in river valleys, many nutrients seep into the soil and nutrient cycles are mainly closed systems, with few nutrients entering from outside. In forests which lie on shallow soils, such as hillsides, there may be some nutrient input from the decomposition and weathering of rock.
Recycling of nutrients in tropical rainforest
Most tropical forests live “on the edge.” The input of nutrients into a tropical rainforest is generally lower than the demand, so the plants must recycle a high percentage of their nutrients in order to survive. This occurs through the decomposition of dead leaves, plants, and animals by soil microbes and the uptake by plants of chemicals released during decomposition.
Some additional nutrients may be derived from leaves leached by rainfall; others may drift in from smoke or aerosols. But the rainforest is to a great extent a closed system. It modifies its own supply of resources and thus growing conditions through its influences on the soil and on nutrient cycling. The cycling of nutrients in rainforests is a very important conservation mechanism, as, since nutrients are rapidly taken up from the soil into plants, nutrient leaching by high rainfall is minimized. High biodiversity is beneficial, because different species of plants have varied chemical compositions and nutrient-cycling patterns, and so are able to exploit many opportunities in the environment.
Of course the forest must have ample supplies of oxygen, hydrogen and carbon (supplied from the atmosphere or water or decomposing organisms; normally these are not limiting because of the recycling of these elements). Carbon and nitrogen enter the soil when plants die or shed leaves, and these elements augment the fertility and water-holding capacity of the soil. Calcium, phosphorus, potassium, magnesium and selenium are also essential minerals which are scarce in certain soils, and may become limiting elements (i.e., compounds or elements which are essential to plants, and, when scarce, restrict growth) for rainforest growth and productivity.
Micronutrients such as iron, boron, manganese, copper, zinc, molybdenum and chlorine are necessary in tiny quantities, and may also act as limiting elements. In some soils of the tropics (certain oxysols and ultisols), minerals such as calcium, potassium and magnesium, which, since they are derived from the weathering of rock, have been exhausted by leaching during centuries of heavy rainfall, and may be limiting factors. In other tropical soils, either phosphorus or nitrogen may be an important limiting factor. Phosphorus is often a limiting factor in lowland forests, and nitrogen in montane forests with shallow soils. Forests on nitrogen-impoverished ultisols in lowland Amazonia are often filled with leguminous trees, which have nitrogen-fixing microbes associated with their roots.
Plant growth is also regulated by a complex of interactions among nutrients. One element may limit the cycling or accumulation of other elements, as, for example, nitrogen accumulation may be limited by the low availability of water or other nutrients. Little is known of these interactions
Biodiversity in Rainforests
No one knows exactly how many species live in the world’s tropical rainforests — estimates range from 3 to 50 million species — rainforests are the undisputed champions of biodiversity among the world’s ecosystems, containing far higher numbers of species on a per-area basis relative to sub-tropical, temperate, and boreal ecosystems.
For example, whereas temperate forests are often dominated by a half dozen tree species or fewer that make up 90 percent of the trees in the forest, a tropical rainforest may have more than 480 tree species in a single hectare (2.5 acres). A single bush in the Amazon may have more species of ants than the entire British Isles. This diversity of rainforests is not a haphazard event, but is the result of a series of unique circumstances.
Why a Tropical Rain Forest cannot be replaced very quickly?
In this case nutrients will be lost from the ecosystem, but the outputs cannot exceed inputs for very long because the stock of nutrient capital in the system will be depleted.
When forests are burned, or the cut timber is removed as in logging, the nutrients that were in the tree biomass are either washed out in the case of burning or simply removed from the system. Because there was only a small stock of nutrients in the soil and most of the nutrients were in the biomass, there is little nutrient stock remaining to support regrowth of the forest. This is why slash and burn agriculture does not work for more than a few years after burning, and why the land is made very infertile and growing new vegetation is difficult.
We can’t simply “regrow” tropical rainforests once they are burned — once they are lost they are gone forever (or at least for 1000s of years, and even then the species that regrow will be different from the original forest species).