Subtropical rainforests are rainforests that share the characteristics of tropical rainforests and temperate forests. Tropical forests cover a large part of the globe, but temperate rainforests only occur in few regions around the world. Temperate rainforests are rainforests in temperate regions. They occur in North America (in the Pacific Northwest in Alaska, British Columbia, Washington, Oregon and California), in Europe (parts of the British Isles such as the coastal areas of Ireland and Scotland, southern Norway, parts of the western Balkans along the Adriatic coast, as well as in Galicia and coastal areas of the eastern Black Sea, including Georgia and coastal Turkey), in East Asia (in southern China, Highlands of Taiwan, much of Japan and Korea, and on Sakhalin Island and the adjacent Russian Far East coast), in South America (southern Chile) and also in Australia and New Zealand.
The tropics region of the earth extends from a little over 20 degrees north to 20 degrees south and includes many of the world’s tropical rainforests. These tropical rainforests receive plenty of rain and high temperatures year-round, and there are no seasons. However, this changes as you travel away from the Equator. By the time you reach the edge of the Tropics, you can find deserts, but in some places you can find subtropical rainforests instead. Subtropical rainforests also extend outside the red zone to the north and south toward the orange dotted lines shown on the map.
Subtropical rainforests have a lot in common with tropical rainforests, and in places like Florida, a state in the United States where subtropical rainforests dominate, you can find palm trees; however, as you go north from Florida into the state of Georgia, the plants start to look more like what you would find in a temperate forest. However, the density of subtropical rainforests and tropical forests mark them out; in temperate forests, most of the vegetation is in the form of trees, and there isn’t thick vegetation along the ground level. Meanwhile, subtropical forests tend to have thick vegetation that extends to a couple feet above the ground, making navigation through the forest difficult.
Subtropical forests like those in the Southern United States (including the states Alabama, Florida, and Georgia) have summers similar to those in the tropics; however, they have cooler winters than tropical rainforests.
A tropical rainforest typically has a number of layers, each with different plants and animals adapted for life in that particular area. Examples include the emergent, canopy, understory and forest floor layers.
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. They need to be able to withstand the hot temperatures and strong winds that occur above the canopy in some areas. Eagles, butterflies, bats and certain monkeys inhabit this layer.
The canopy layer contains the majority of the largest trees, typically 30 metres (98 ft) to 45 metres (148 ft) tall. The densest areas of biodiversity are found in the forest canopy, a more or less continuous cover of foliage formed by adjacent treetops. The canopy, by some estimates, is home to 50 percent of all plant species. Epiphytic plants attach to trunks and branches, and obtain water and minerals from rain and debris that collects on the supporting plants. The fauna is similar to that found in the emergent layer, but more diverse. A quarter of all insect species are believed to exist in the rainforest canopy. Scientists have long suspected the richness of the canopy as a habitat, but have only recently developed practical methods of exploring it. As long ago as 1917, naturalist William Beebe declared that “another continent of life remains to be discovered, not upon the Earth, but one to two hundred feet above it, extending over thousands of square miles.” True exploration of this habitat only began in the 1980s, when scientists developed methods to reach the canopy, such as firing ropes into the trees using crossbows. Exploration of the canopy is still in its infancy, but other methods include the use of balloons and airships to float above the highest branches and the building of cranes and walkways planted on the forest floor. The science of accessing tropical forest canopy using airships or similar aerial platforms is called dendronautics.
The understory or understorey layer lies between the canopy and the forest floor. It is home to a number of birds, snakes and lizards, as well as predators such as jaguars, boa constrictors and leopards. The leaves are much larger at this level and insect life is abundant. Many seedlings that will grow to the canopy level are present in the understory. Only about 5% of the sunlight shining on the rainforest canopy reaches the understory. This layer can be called a shrub layer, although the shrub layer may also be considered a separate layer.
The forest floor, the bottom-most layer, receives only 2% of the sunlight. Only plants adapted to low light can grow in this region. Away from riverbanks, swamps and clearings, where dense undergrowth is found, the forest floor is relatively clear of vegetation because of the low sunlight penetration. It also contains decaying plant and animal matter, which disappears quickly, because the warm, humid conditions promote rapid decay. Many forms of fungi growing here help decay the animal and plant waste.
Flora and fauna
More than half of the world’s species of plants and animals are found in the rainforest. Rainforests support a very broad array of fauna, including mammals, reptiles, birds and invertebrates. Mammals may include primates, felids and other families. Reptiles include snakes, turtles, chameleons and other families; while birds include such families as vangidae and Cuculidae. Dozens of families of invertebrates are found in rainforests. Fungi are also very common in rainforest areas as they can feed on the decomposing remains of plants and animals.
The great diversity in rainforest species is in large part the result of diverse and numerous physical refuges, i.e. places in which plants are inaccessible to many herbivores, or in which animals can hide from predators. Having numerous refuges available also results in much higher total biomass than would otherwise be possible.
Despite the growth of vegetation in a tropical rainforest, soil quality is often quite poor. Rapid bacterial decay prevents the accumulation of humus. The concentration of iron and aluminium oxides by the laterization process gives the oxisols a bright red colour and sometimes produces mineral deposits such as bauxite. Most trees have roots near the surface, because there are insufficient nutrients below the surface; most of the trees’ minerals come from the top layer of decomposing leaves and animals. On younger substrates, especially of volcanic origin, tropical soils may be quite fertile. If rainforest trees are cleared, rain can accumulate on the exposed soil surfaces, creating run-off and beginning a process of soil erosion. Eventually streams and rivers form and flooding becomes possible. There are several reasons for the poor soil quality: First is that the soil is highly acidic. The roots of plants rely on an acidity difference between the roots and the soil in order to absorb nutrients. When the soil is acidic, there is little difference, and therefore little absorption of nutrients from the soil. Second, The type of clay particles present in tropical rainforest soil has a poor ability to trap nutrients and stop them from washing away. Even if humans artificially add nutrients to the soil, the nutrients mostly wash away and are not absorbed by the plants. Thirdly, The type of clay particles present in tropical rainforest soil has a poor ability to trap nutrients and stop them from washing away. Even if humans artificially add nutrients to the soil, the nutrients mostly wash away and are not absorbed by the plants. Finally, these soils are poor due to the high volume of rain in tropical rainforests washes nutrients out of the soil more quickly than in other climates.
Effect on global climate
A natural rainforest emits and absorbs vast quantities of carbon dioxide. On a global scale, long-term fluxes are approximately in balance, so that an undisturbed rainforest would have a small net impact on atmospheric carbon dioxide levels, though they may have other climatic effects (on cloud formation, for example, by recycling water vapour). No rainforest today can be considered to be undisturbed. Human-induced deforestation plays a significant role in causing rainforests to release carbon dioxide, as do other factors, whether human-induced or natural, which result in tree death, such as burning and drought. Some climate models operating with interactive vegetation predict a large loss of Amazonian rainforest around 2050 due to drought, forest dieback and the subsequent release of more carbon dioxide. Five million years from now, the Amazon rainforest may long since have dried and transformed itself into savannah, killing itself in the progress (changes such as this may happen even if all human deforestation activity ceases overnight).
Tropical and temperate rainforests have been subjected to heavy legal and illegal logging for their valuable hardwoods and agricultural clearance (slash-and-burn, clearcutting) throughout the 20th century and the area covered by rainforests around the world is shrinking. Biologists have estimated that large numbers of species are being driven to extinction (possibly more than 50,000 a year; at that rate, says E. O. Wilson of Harvard University, a quarter or more of all species on Earth could be exterminated within 50 years) due to the removal of habitat with destruction of the rainforests.
Another factor causing the loss of rainforest is expanding urban areas. Littoral rainforest growing along coastal areas of eastern Australia is now rare due to ribbon development to accommodate the demand for seachange lifestyles.
Forests are being destroyed at a rapid pace. Almost 90% of West Africa’s rainforest has been destroyed. Since the arrival of humans, Madagascar has lost two thirds of its original rainforest. At present rates, tropical rainforests in Indonesia would be logged out in 10 years and Papua New Guinea in 13 to 16 years. According to Rainforest Rescue, an important reason for the increasing deforestation rate, especially in Indonesia, is the expansion of oil palm plantations to meet growing demand for cheap vegetable fats and biofuels. In Indonesia, palm oil is already cultivated on nine million hectares and, together with Malaysia, the island nation produces about 85 percent of the world’s palm oil.
Several countries, notably Brazil, have declared their deforestation a national emergency. Amazon deforestation jumped by 69% in 2008 compared to 2007’s twelve months, according to official government data.
However, a January 30, 2009 New York Times article stated, “By one estimate, for every acre of rain forest cut down each year, more than 50 acres of new forest are growing in the tropics…” The new forest includes secondary forest on former farmland and so-called degraded forest.
Subtropical forests can be found around the edges of the tropics; they can be found in the Southern United States, Mexico, Southeast Asia, and India.
Southern China has subtropical forest/rainforest in Guangdong, Guangxi, Yunnan, Hunan, and Fujian provinces, primarily, as well as on the island of Hainan. On Hainan the forest type transitions into tropical. In far southern Yunnan, tropical rainforests start to predominate, near the border with Myanmar/Vietnam.
Southern United States
The Southern United States is often associated with plantations and, for this reason, “the South” can easily imply farming land rather than dense forest. However, a visit to the South will prove otherwise; with the exception of rivers and the areas immediately surrounding them, thick forest is found throughout much of the South, particularly in the southern parts like Florida and Georgia. Subtropical forests predominate in the south, and encompass such types as mangrove forest, Subtropical pine forest and palm savannas, and Subtropical montane forest, such as those found in the southern Appalachians. Subtropical forests/rainforests predominate, and become significant, in the states of South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, and Texas, giving way to a variety of different Tropical rainforest and forest types found in southern Florida, far south Texas, and on the island of Hawaii.
The forests in this region of the world usually contain a significant amount of blackwater rivers and wet grassland, often called swamp.
Similar to tropical rainforests, getting around on foot is not easy in subtropical rainforests. As mentioned above, thick vegetation dominates in subtropical rainforests and means that getting through it is not always easy.
In places like Florida, there are hiking trails that solve this problem, but still have to deal with mosquitoes during the warmest parts of the year. While mosquitoes are not so bad in cities and on the shoreline of subtropical regions, as soon as you get into the subtropical rainforests themselves, you can encounter large numbers of mosquitoes, including around waterways.
In areas with decent road networks, getting around by car is a good way to see subtropical rainforests without having to actually be inside them, especially if there is a clearing on each side of the road.
In summer, especially, you’ll need to watch out for mosquitoes if you decide to hike through the rainforest. Even if you’re driving through the subtropical rainforest, a few seconds with the doors open while you get inside the vehicle is enough time for mosquitoes to get in the vehicle with you.