Tropical rainforests contain the greatest species diversity of all biomes on Earth, and are thought to be the world’s oldest living ecosystems. Found near the equator in Africa, Asia and Central and South America, these are lush, dense forests characterised by consistently heavy rainfall and year-round high temperatures of between 18 and 25 degrees Celsius.
In a single year, a tropical rainforest can experience an impressive 254 centimetres of rain, often causing nutrients to leach from the soil. This, combined with the lack of light penetrating through the trees, means that very few plants grow on the forest floor, which is instead mostly covered with soil and dead plants. Decomposition occurs at an extremely rapid rate in tropical rainforests as a result of the warm temperatures and moist air. However, despite being somewhat deficient in living plant matter, the forest floor is home to some rather charismatic species such as the okapi (Okapia johnstoni), which roams the rainforests of the Democratic Republic of the Congo in search of the leaves, fruits and fungi on which it feeds.
The understory or ‘shrub layer’ of the tropical rainforest is the next level up from the forest floor, and typically contains small trees or bushes that rarely exceed 3.5 metres in height. These plants tend to have large leaves to trap as much sunlight as possible in the dingy lower regions of the forest. Many animal species inhabit the understory, from the powerful jaguar (Panthera onca) to tiny invertebrates and treefrogs, such as the large-eyed lemur leaf frog (Hylomantis lemur).
The primary layer of the forest which forms a dense roof over the ferns, mosses and vines of the understory and forest floor is known as the canopy. The canopy is typically formed of broadleaf trees which grow to impressive heights of 25 to 35 metres, such as the white seraya (Parashorea macrophylla), with a few trees growing even taller and forming the emergent layer, for example the Brazil-nut tree (Bertholletia excelsa). Many species have adapted to living in the trees of the canopy where food is abundant, including birds, bats, snakes and primates such as the white-handed gibbon (Hylobates lar).
Did you know? As well as tropical rainforests, there are also tropical dry forests. These are formed mostly of deciduous trees which shed their leaves to survive the long dry season. Although tropical dry forests typically support less biodiversity than rainforests, the fact that sunlight can reach the ground during the dry season means that a thick undergrowth layer can grow, providing plenty of food and shelter for ground-dwelling animals.
Found in north-eastern Asia, eastern North America and parts of Europe, temperate forests are characterised by having average temperatures of around 10 degrees Celsius and an average annual rainfall of between 76 and 152 centimetres. Unlike tropical rainforests, temperate forests go through four distinct seasons, with precipitation often falling as snow in the winter months. Temperate forests are many and varied, and while most contain a mixture of both deciduous and coniferous trees, one tree type typically tends to dominate.
In temperate deciduous forests, the dominant trees tend to be those with broad, flat leaves, such as maple (Acer species), birch (Betula species) and oak (Quercus species), and in the autumn the leaves of these trees change colour, creating a beautiful tableau of yellows, oranges and fiery reds. In the winter, the leaves drop to the nutrient-rich forest floor, which is covered in fertile soil and supports a range of shrubs, mosses, ferns and wildflowers. The presence of this abundant undergrowth in temperate forests means that, unlike in tropical rainforests, the majority of species can be found on the forest floor.
Temperate coniferous forests are typically found in coastal areas or inland mountainous zones, and have warm summers, relatively mild winters and tend to experience higher levels of precipitation than their deciduous counterparts. This moderate, moist climate allows for a long growing season, and so the dominant tree species in these forests, such as cedars (Cupressaceae family), pines (Pinus species) and firs (Abies species), can grow very tall, with enormous trees such as the giant sequoia (Sequoiadendron giganteum) of North America proving to be particularly impressive. The relatively slender, almost triangular shape of coniferous species makes the trees very strong and prevents the branches from snapping under the heavy weight of the winter snow. In contrast to the broad leaves of deciduous trees, the leaves of coniferous trees are either long, pointed needles or small, flat scales. These stay on the tree for several years, falling off gradually.
Animals living within temperate forest habitats have to adapt to the climate, with some species, such as the American black bear (Ursus americanus), coping with the cool winters by hibernating. Camouflage is another important adaptation for forest-dwelling species such as the adder (Vipera berus), which blends in well with its forest-floor environment. Although not as diverse as tropical rainforests, temperate forests still house an incredible array of species, from the industrious Eurasian beaver (Castor fiber) to the majestic bald eagle (Haliaeetus leucocephalus).
Did you know? You can tell the different types of coniferous trees apart by their needles - firs have short, blunt-tipped needles, while pines have needles that grow in bunches attached at the base, and spruces have very sharp, four-sided needles.
The northernmost forest type, boreal forests, or ‘taiga’, are characterised by short, moderately warm summers and long, cold winters, with a large proportion of the 40 to 100 centimetres of precipitation experienced each year falling as snow. The majority of boreal forests are found across Scandinavia and Russia, but this habitat type also occurs in Canada, Alaska and north-eastern Asia, typically between latitudes of 50 and 60 degrees North.
Boreal forest winters tend to be relatively dry, while summers are moist, and average annual temperatures are very low, ranging from about -5 to 5 degrees Celsius. Due to this cold climate, decomposition is very slow in boreal forests, resulting in thin, acidic and nutrient-poor soil.
The dominant trees in boreal forests tend to be coniferous species such as larch (Larix species), pine (Pinus species) and fir (Abies species) trees, although deciduous species such as white birch (Betula papyrifera) are also relatively common. The canopy of boreal forests tends to be rather dense, allowing little light to pass through, resulting in a limited understory and a fairly sparse forest floor covered in mosses and lichens. The growing season in boreal forests is short, typically lasting around three months, but the presence of moist ground in the summer as a result of snowmelt, in combination with long days, means that plant growth can be somewhat explosive.
Animal species found in boreal forests must be able to cope with extremely harsh conditions, and species such as the reindeer (Rangifer tarandus), snowshoe hare (Lepus americanus), grey wolf (Canis lupus) and bobcat (Lynx rufus) are all well adapted to their habitat, with thick fur helping them survive the long, cold winters. Boreal forests are also home to a number of fascinating bird species, including the spruce grouse (Dendragapus canadensis) and the great grey owl (Strix nebulosa).
Although sometimes considered to be a type of temperate forest, Mediterranean forests are typically found to the south of temperate regions, around the coasts of the Mediterranean, California, Chile, South Africa and Western Australia.
While the winters in Mediterranean forests are somewhat mild and wet, the summer months are typically hot and dry, and so many plant species living within these habitats have thick, leathery leaves to help them retain water in the intense heat. Special veins also help them to transport water when it rains, making the most of the little moisture available. Forest fires are a common occurrence in Mediterranean forest regions, and many plants living in these habitats are able to survive being scorched, with some even depending on fire to remove competitors and germinate their seeds. Mediterranean forests are characterised by short, broadleaf evergreen shrubs and oak trees, collectively known as ‘maquis’, and include the oak woodlands of California and the eucalyptus forests of Australia.
Many Mediterranean forest species are endemic, meaning that they are found nowhere else on Earth. A wide variety of charismatic species call Mediterranean forests home, including the Iberian lynx (Lynx pardinus), the Barbary macaque (Macaca sylvanus) and the Spanish imperial eagle (Aquila adalberti). These species have adapted to their habitat by being able to forage over large areas, whereas other Mediterranean forest species, such as the Mediterranean chameleon (Chamaeleo chamaeleon), have adapted by becoming agile climbers.