Rainforest

Rainforests are forest ecosystems distinguished by their closed and continuous tree canopy, high humidity, moisture-dependent vegetation, and the presence of epiphytes and climbing plants. They occur in regions where sufficient moisture supports luxuriant growth and where natural wildfire is notably absent. Although most often associated with the tropics, rainforests also occur in temperate zones, with numerous local variants described. They are among the most biodiverse habitats on Earth, containing an exceptionally high proportion of the world’s plant, animal and microbial species. Because of their ecological richness and the presence of countless bioactive compounds, tropical rainforests have been referred to as the “jewels of the Earth” and the planet’s “medicine chest”. Yet they are threatened by deforestation, habitat fragmentation and air pollution, leading to rapid loss of species and ecological functions.

Definitions and Characteristics

Rainforests are defined primarily by structural and climatic attributes. A hallmark feature is a dense, interlocking canopy that restricts light penetration to the lower layers of the forest. High year-round humidity supports an abundance of epiphytes and lianas, while a moist layer of leaf litter facilitates rapid nutrient cycling. Rainforests lack the fire-adapted traits common in drier ecosystems, as persistent moisture prevents the spread of wildfire.
Although the largest expanses occur in tropical and temperate regions, other rainforest types such as laurel forests, littoral rainforests, cloud forests, vine thickets and dry rainforests have also been identified. These reflect variations in altitude, rainfall, soil type and exposure.

Tropical Rainforests

Tropical rainforests flourish in warm, wet climates typically located within about ten degrees latitude of the equator. Mean monthly temperatures remain high throughout the year and rainfall exceeds at least 1,700 mm annually, with some regions receiving much more. The absence of a pronounced dry season distinguishes tropical rainforests from other moist tropical forest types.
Many major tropical rainforest regions are associated with the Intertropical Convergence Zone, where rising warm air and evaporation generate sustained precipitation. Tropical rainforests are found in Southeast Asia, the Pacific Islands, Sub-Saharan Africa, Central and South America and northern Australia. Notable examples include the Amazon Basin, the Congo Basin and the rainforests of Borneo and New Guinea.
Although once popularly described as the “lungs of the Earth”, tropical rainforests contribute little net oxygen to the atmosphere, as most oxygen produced is consumed through decomposition. Their global importance instead lies in carbon storage, hydrological regulation and unparalleled biodiversity.

Temperate Rainforests

Temperate rainforests occur far less widely than their tropical counterparts, developing in regions that combine temperate climates with high rainfall. These forests are present along the Pacific Northwest of North America, including Alaska, British Columbia, Washington, Oregon and California. In Europe, small pockets survive in parts of Ireland, Scotland, Norway, the western Balkans and Galicia. East Asian examples occur in Japan, Korea, Taiwan and the Russian Far East, while the Southern Hemisphere contains temperate rainforests in southern Chile, Tasmania, Victoria and New Zealand.
These forests often contain evergreen conifers or broadleaf species adapted to cooler climates, but they retain the structural complexity, thick mossy growth and rich epiphytic flora typical of rainforests.

Dry Rainforests

Dry rainforests represent a distinctive form occurring in areas of relatively low rainfall. They possess a more open canopy and typically show only two tiers of tree growth. Although less luxuriant than tropical or temperate rainforests, they sustain unique species assemblages adapted to intermittent moisture availability.

Vertical Structure and Forest Layers

Rainforests are vertically stratified, with each layer hosting organisms that have adapted to the distinct light, temperature and humidity conditions of that tier. Four principal layers are commonly recognised.

• Emergent layer: This uppermost layer comprises a small number of towering trees that rise above the main canopy, often reaching 45–55 metres and occasionally up to 70–80 metres. These trees withstand intense sunlight, strong winds and exposure. They provide habitat for eagles, bats, monkeys and other animals adapted to life at great heights.
• Canopy layer: The canopy forms a dense cover of interlacing branches and leaves, generally 30–45 metres high. It contains the greatest concentration of biodiversity, with approximately half of all plant species thought to occur in this layer. Epiphytes are abundant, obtaining moisture and nutrients from intercepted rain and organic debris. A large proportion of insect species—possibly a quarter of all insects—live within the canopy. Scientific exploration of this layer began in earnest only in the late twentieth century, giving rise to specialised techniques such as crane platforms, canopy walkways and aerial methods known collectively as dendronautics.
• Understorey layer: Situated between the canopy and the forest floor, the understorey receives only about five per cent of the sunlight that reaches the canopy. Many seedlings grow here in anticipation of future gaps in the canopy. Broad-leaved shrubs, small trees and shade-adapted plants dominate, alongside a rich assemblage of birds, snakes, lizards and larger predators such as jaguars or leopards in some regions.
• Forest floor: The bottom layer receives only about two per cent of incoming sunlight. Vegetation is sparse except where gaps or riverbanks create lighter conditions. Decomposition proceeds rapidly due to warmth and humidity, and fungi play a major role in breaking down organic material.

Biodiversity of Rainforest Ecosystems

Rainforests harbour extraordinarily rich assemblages of flora and fauna. A large proportion of the world’s vascular plant species are native to tropical rainforests, and many remain undescribed. Faunal diversity includes primates, felids, chameleons, turtles, frogs, insect families beyond number and numerous fungi. This high diversity is associated with the availability of physical refuges that reduce predation and herbivory, enabling species coexistence. The abundance of microhabitats also supports high total biomass.
Species interactions are sensitive to environmental change. For example, the timing of amphibian reproduction affects reptile species that depend on amphibians as prey. Specialised predators may decline where prey availability fluctuates or where habitat is altered. Continuous ecological monitoring is therefore crucial in assessing long-term trends.

Soil Characteristics

Despite the luxuriant vegetation, tropical rainforest soils are often poor in nutrients. Warm, moist conditions promote rapid microbial decay, preventing the accumulation of humus. Processes such as laterisation concentrate iron and aluminium oxides, producing characteristic reddish oxisols. Nutrients from decaying plant matter are quickly taken up by shallow root systems, making the rainforest’s fertility heavily dependent on continuous recycling rather than soil reserves.
Temperate rainforests, by contrast, often develop richer soils due to slower decomposition and greater accumulation of organic matter, although this varies by region and parent rock material.

Environmental Significance and Threats

Rainforests play vital roles in global ecological processes by regulating climate, storing carbon, maintaining hydrological cycles and supporting cultural and economic livelihoods. They are essential for indigenous communities and provide countless ecosystem services, from pollination to medicinal resources.
However, extensive deforestation, agricultural expansion, infrastructure development, mining and pollution are driving rapid loss of rainforest area and biodiversity. Habitat fragmentation reduces genetic diversity and disrupts ecological connectivity. Long-term impacts include heightened greenhouse gas emissions, altered rainfall patterns and increased vulnerability to natural disasters.