Tropical Rainforest Biome (AQA A Level Geography)

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Jacque Cartwright

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Anatomy of a Tropical Rainforest

The tropical rainforest (TRF) biome is located within the equatorial climate belt, with a mean annual temperature of 27° C
- Between the tropics of Cancer and Capricorn - 23°27' north to 23°27' south of the Equator,
The TRF now covers less than 5% of the Earth's surface and supports 50% of global biotic organisms
The main regions of tropical rainforest ecosystem are in the following countries:
- Amazon which is the largest remaining rainforest on Earth usually associated with Brazil but covers parts of seven other countries
- Central America including parts of Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Panama
- Central Africa including Cameroon, the Central African Republic, the Republic of Congo, the Democratic Republic of Congo (DRC), Equatorial Guinea and Gabon
- Indo-Malaysia, including Malaysia, Indonesia and a number of other countries in South-East Asia

distribution-of-the-equatorial-climate

Distribution of the equatorial climate

Tropical rainforest climate characteristics

General Characteristics of Tropical Rainforests

Characteristics	Equatorial Climate
Location	Low latitudes Within the Tropics 15° north and south of the equator Amazon in South America, New Guinea, South-east Asia, Zaire Basin
Annual Precipitation	Convectional rainfall of over 2000mm
Temperature Range	Low range of mean monthly temperatures 26-28°C Constant high temperatures Diurnal range is about 5°C
Seasons	No seasons - hot and wet all year round
Humidity	High, usually between 77-88%
Soils	Many of the soils in the rainforest are latosols High concentrations of iron and aluminium due to high levels of leaching Nutrient cycling is fast, with nutrients concentrated in the very upper topsoil, and the bulk being stored as biomass. This makes the soil infertile for agriculture
Biodiversity	Tropical rainforests contain the highest biodiversity of plants and animals on Earth Estimates range from over 50% to 80% of the world's plant and animal species One 10km² area can contain up to 1,500 flowering plants, 750 species of trees, 400 species of birds and 150 species of butterflies

climate-graph-for-iquitos-peru

Climate graph for Iquitos, Peru, which shows the typical climate of the tropical rainforest: high annual temperatures and plenty of precipitation

Structure of the tropical rainforest

The tropical rainforest (TRF) has five distinct layers:

Stratified Layers of a TRF

Layer	Height (m)	Conditions
Emergent	35-55	Maximum sunlight, wind, and rain with lower temperatures at night Trees are enormous and exposed but hardy Straight, branchless trunks that receive the most light
Canopy	20-35	Trees compete for light and rain is mostly intercepted Most productive but heavily populated area, as each tree crown has a huge dark, leathery leafed, photosynthetic surface
Under canopy/storey	10-20	Hot and humid, with low light levels of 10% Slender, opportunistic trees with thin, smooth bark Climbing creepers such as lianas, form green corridors
Shrub	2-10	Just 5% of available light reaches this level which limits growth of vegetation to ferns, woody plants and younger trees Hot and humid throughout this layer, with drip through of precipitation
Ground	0-2	Less than 2% of light reaches this layer Temperature change is minimal (1-2°C max) Bacteria and fungi rapidly decompose litter from flora and fauna

trf-structure

Typical structure of a tropical rainforest

Tropical Rainforest Soil & Soil Moisture

Nutrient cycling

Tropical rainforests have a continual growing season with high rainfall and temperatures all year round
Nutrient cycling is rapid:
- Trees and plants lose their leaves all year round
- The high level of moisture and high temperatures leads to rapid decomposition and recycling of nutrients
- Plants grow quickly which leads to a rapid uptake of nutrients from the soil
Bulk of nutrient storage is as biomass
Few nutrients are stored in the soil and the litter
Net Primary Productivity (NPP) is higher in rainforests

trf-nutrient-cycle

Tropical rainforest nutrient cycling - note the size of the stores, the largest is biomass

Nutrient cycling is important in soil formation and condition

What is soil?

Soil is a complex mixture of sand, silt, clay and bits of decaying animal and plant tissue
Defined as:

The solid material on the Earth’s surface that results from the interaction of weathering and biological activity on the parent material or underlying hard rock

Humus is the organic component of the soil that helps retain most of its moisture and nutrients and is composed primarily of plant remains
The study of soils is called pedology (from the Greek word pedon, meaning soil or earth)

Soil profiles

A soil profile is a vertical cross-section which is divided into several distinct layers, referred to as horizons
The horizons are normally designated by symbols and letters
The presence or absence of some horizons allow pedologists (soil scientists) to classify the soil
In addition, the organic or O horizon can form above the mineral soil - commonly in forested areas, resulting from dead plant and animal remains

Generalised soil profile showing distinct horizons. Organic matter is concentrated in the O and A horizon, making them the critical horizons for vegetation growth

Soil forming factors

Soils develop as a result of 5 factors:
- Parent material - material that forms the soil, which can be solid rock to deposits such as alluvium and boulder clay
  - Parent material influences the soil in several ways:
    - Colour
    - Texture
    - Structure
    - Mineral composition
    - Permeability and drainage
- Climate - most important factor
  - Different climatic conditions produce different soils, even when the parent material is the same rock type
  - Moisture (precipitation) and temperature influences rates of evaporation
  - When precipitation exceeds evaporation, leaching of the soil will occur
  - Temperature determines the rate of chemical and biological decay, which influences weathering and humification (humus production)
- Available organisms - range from microscopic bacteria to large animals including humans
  - Micro organisms such as bacteria and fungi assist in the decomposition of plant litter
  - This litter is mixed into the soil by macro-organisms (soil animals) such as worms and beetles
  - Soil horizons are less distinct when there is a lot of soil organism activity
  - The composition of humus is determined by roots, vegetation cover and litter inputs
  - Roots bind soil particles together, and can redistribute and compress soil
- Relief influences soil formation in several ways:
  - Soil profile thickness i.e. as the angle of the slope increases, so does the erosion hazard
  - Relief affects local climate, which is also a soil forming factor e.g. orographic rainfall and rain shadow
  - Slope gradient affects run-off, percolation and mass movement
  - Aspect creates microclimate conditions - whether the aspect is sun facing or leeward facing etc.
- Time - soil develops very slowly
  - In the UK it takes approximately 400 years to develop 10mm of soil
  - Young soils keep many of the characteristics of its parent material
  - Over time, their features change, as organisms and the addition of organic matter increase
  - The soils of Britain are fairly young because they are largely post-glacial

Exam Tip

Remember that soils pass through several stages as they develop
This usually results in a deep profile, with many, well defined horizons

Soil colour

Determined by the amount of organic matter and the state of iron and aluminium oxide
Soil colour is also related to soil drainage:
- Free draining, well aerated soils (has pore spaces) have rich brown colours
- Poor draining soils, develop under anaerobic conditions (the pore spaces are water logged) and have a grey or blue-grey colour
- Intermittent waterlogged soils are mottled with blotches of contrasting colour. The mottles are either a rusty red colour and due to iron concentrations or yellow due to high aluminium concentrations

Soil texture

Soil texture determines the permeability of water, which generally decreases as mineral particle size gets smaller
Textures range from clay, sand, silt and loam - soil high in silt and clay will be compact and lack pore spaces
A soil with ample pore space will allow healthy root development for vegetation to grow

Soil moisture

The soil moisture budget is high, as precipitation exceeds potential evaporation
The water budget shows the balance between precipitation and potential evapotranspiration
High levels of precipitation ensures that soil stores fill rapidly
There is a short time lapse between rainfall peaks, meaning soil water never reaches a deficit
Water therefore, is available to flora and fauna all year round

trf-water-budget

Water budget for equatorial rainforest environment. Precipitation has two distinct peaks, but as temperature varies little, the potential evapotranspiration remains fairly constant

Soil importance

Soil is the important factor in the development of vegetation within the rainforest ecosystem, which has drives ecological change and succession
The climatic conditions of TRFs, result in deep, highly weathered and leached soils
These soils are either rich in iron oxides (red colour) or aluminium oxides (yellow colour)
The most common soils of TRFs are:
- Oxisols
  - Bright red or have layers of orange or yellowish colour
  - Highly permeable and very resistant to erosion
- Latosols
  - These are red or yellowish-red in colour throughout and do not have distinct horizons
  - They are deep soils, often 20-30m deep
  - A thin but very fertile layer of humus is followed by an infertile second layer (due to rapid leaching), with a third layer of weathered bedrock, which is common to almost all soil types
Soil is completely reliant on the rainforest to maintain fertility, as all nutrients leach away quickly when the forest is felled and the layer of humus is no longer being replaced
Intense weathering, causes these soils to be 'washed out' (strongly leached), and become nutrient poor and low in organic matter
However, tropical soils are among the deepest in the world, which have been protected under forest cover for millions of years
Over this time, deep tropical soils have formed from the underlying, chemically weathered parent rock but are relatively infertile

Soil profile of a tropical rainforest

Note that the organic (O) horizon is missing due to quick nutrient cycling

Exam Tip

Did you know:

On the high branches of the canopy (30m), thick soil with earthworms exists. This soil made from dust blown in by the wind, fallen leaves, decaying organic matter, and animal droppings etc. The mixture is bog like, and is especially thick where branches meet.

These are perfect conditions for orchids, ferns, violets and pepper plants, bromeliads etc to grow and lichens and mosses grow on the leaves and branches.

In amongst all of this are birds, insects, small mammals, worms, caterpillars, spiders, butterflies, hummingbirds, lizards and salamanders. Many never come down to the forest floor, spending all their lives high up in the canopy.

Tropical Rainforest Flora & Fauna Adaptations

Food web

As in all ecosystems, TRF's have complex food web that includes:
- Producers
- Primary consumers
- Secondary consumers
- Tertiary consumers

trf-food-web

Typical TRF food web

Adaptations

At every level, there are particular adaptations that allow plants and animals to survive and thrive within TRF's

Plants have adapted to this environment in unique ways. The canopy itself, the ceiling of the jungle, is a dense continuous layer of greenery some 6 or 7 meters deep. Each leaf is accurately angled to ensure that it will collect the maximum amount of light. Many have a special joint at the base of the stalk that enables them to twist and follow the sun as it swings overhead from east to west each day. All except the topmost layer is screened from the wind, so the around them is warm and humid”. The Living Planet, D. Attenborough

Adaptations of Plants and Animals within a TRF

Plants	Animals
Waxy leaves with drip tips - these ensure that rainwater runs off the leaf and does not remain which would encourage mould growth or break the leaf. Too much standing water on the leaves can reduce photosynthesis. Drip tips ensures that precipitation is funnelled to the forest floor	Nocturnality - animals active at night save energy by feeding at night when it is cooler, but they have also developed stronger senses of smell, better hearing, and other organs like heat-sensing ones that enables them to find prey or evade predators
Buttress roots - these prevent emergent trees from falling over, along with ensuring that nutrients can be gathered over a wider area. As nutrients are found in the upper layers, it is important that roots spread outwards rather than downwards	Limited diet - although full of resources there is competition for them. Adaptation is made to avoid the competition but this limits the choice of diet. Toucans - their bills have become long, narrow and strong as they only consume fruits that other animals and birds cannot access
Lianas - woody vines that have their roots in the ground and then climb rainforest trees to reach the sunlight. Lianas can form bridges across the forest canopy which allow animals to move around the forest	Speed, stealth or camouflage - such as the chameleon being able to blend to its surroundings. The Sloth has algae growing in the fur, which helps to camouflage it
Epiphytes - such as orchids, grow harmlessly on other plants – they differ from parasites as they don’t affect their hosts in a negative way. Approx. 90% of epiphytes are flowering and the remaining 10% are ferns. They get moisture and nutrients from their surroundings and alter the environment of TRFs, as they hold water in the canopy, which decreases leaching of the soil	Gripping - as the trees of the rainforest are smooth, getting a hold to climb is difficult. Many animals have adaptations to allow this. Geckos have large, flattened toe pads with sticky scales which allow them to grip the smooth tree trunks. Spider monkeys use their tails to swing between trees. Sloths have long claws that allow them to hang upside down from high branches.
Thin, smooth bark - prevention of evaporation is not an issue in TRFs and trees have thin bark. The smooth nature of the bark helps hinder plant growth on the tree's surface which would reduce available sunlight for photosynthesis	Size - because the understorey is dense it makes it difficult for large animals to move around. Therefore, animals have adapted by reducing body size and stature. Rainforest jaguars are smaller than their savanna counterparts. However, snakes actually grow larger than anywhere else as they can fit into spaces between the trees and undergrowth, meaning they can grow to lengths of 10m
Carnivorous plants - are a group of plants that have cleverly adapted to living in poor soil – attracting, trapping and digesting animals for the nutrients they contain. Most consume insects, but larger plants are able to digest reptiles and small mammals. Carnivory is such an efficient adaptation that it has evolved independently several times and occurs in unrelated plant families. Carnivory doesn't replace the need for photosynthesis and root systems, it simply helps the plants make the most of all available resources	Changing of habitats - allows animals to maximize their protection from predators and limit the competition for food. Primates have prehensile tails to help them climb trees and avoid the understorey competition for resources. Many animals can swim, allowing them to cross rivers and cope with flooding
Sugary goodness - Fruits hold the seeds of a plant and in order to be dispersed across the rainforest, plants have adapted to become laden with sugar, making them a sweet temptation to birds and animals	Poison - apart from the production of poison, these animals also tend to be brightly coloured and patterned to warn other animals of their danger. Poison dart frog with very bright colours or yellow, red, green and blue spots and swirls.
Leaf angling - Leaves are often arranged at different angles so that a plant avoids shading its own leaves – important in rainforests where competition for light is intense	Mimicry - when animals imitate another species in the hope of being left alone - some dart frogs are not poisonous, but look like their deadly relatives, and are left alone. Stick insects have adapted to be stick or leave shaped, allowing them to look like a small tree branch

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