- The tropical rainforest biomes are between latitudes 10°N and 10°S of the equator.
Climate
- The equatorial climate has little variation, temperatures remain high throughout the year.
- High diurnal temperature range, high during day and low during the night due to no insulating clouds to keep the heat in.
- Annual precipitation is high, often in excess of 2000mm, rain falls all year round at the equator because of the inter-tropical convergence zone dominates the atmospheric conditions.
- Evapotranspiration is rapid, due to sun heats the humid forest.
- Low pressure conditions allow air to be rapidly uplifted.
- As the air rises it cools and water vapour condenses into clouds, the clouds continue to build into the afternoon leading to heavy rain and thunder, returning the previously uplifted moisture back to ground level.
- Humidity is high throughout the year; continuous evapotranspiration adds water vapour to the air.
- On the forest floor there is little breeze as the trade winds converge here.
Soils
- Underlying soil will have developed naturally over a long period of time and be in balance with its environment, this is known as zonal soil.
- The zonal soil associated with the tropical rainforest is a latosolyh.
Characteristic Features
- A latosol can be more than 40m deep.
- The constant hot wet climate provides perfect conditions for chemical weathering of the bedrock.
- Ferrallitisation is the process where bedrock is broken down by chemical weathering into clay minerals and sesquioxides.
- Red colour due to the presence of iron and aluminium minerals.
- As there is a moisture surplus in the equatorial climate (rainfall exceeds evapotranspiration), there is a downward movement of water through the soil.
- Silica materials are washed out of the A horizon and transported downwards by water by a process called leaching.
- Iron and aluminium compounds are less soluble and are left behind.
- The latosol is nutrient poor, plant uptake of nutrients is roughly equal to the input from decomposed litter.
- Soil moisture utilisation occurs throughout this period, as evaporation and transpiration exceed precipitation.
Vegetation
- Rainforests are the most diverse and productive biome, also the most fragile.
- The vegetation is in harmony with its environment and is a climatic climax community where the dominant species are the hardwood trees.
- The net primary production of the rainforest is 2,200g M² yr⁻¹, the figure is high because the growing season lasts all year and the litter is rapidly decomposed, replacing nutrients taken up by the vegetation.
- There can be up to 300 species of trees in every Km².
- The forest has a layered appearance, with the tallest trees(emergent’s), standing up to 45m, above the canopy, the canopy absorbs most of the sunlight and intercepts most of the precipitation.
- When a tree dies it brings down others as it falls, new trees grow quickly taking advantage of the light, it decomposes quickly assisted by detrivores and the hot and humid conditions.
- Fungi that grows on trees and has an important role in decomposing litter.
- The vegetation has developed and adapted to physical conditions of the rainforest:
- The trees grow rapidly, the leaves at the tops of the trees absorb light and photosynthesise, the bark is thin as the trees don’t need protection from harsh winter conditions.
- The top layer of soil contains the minerals the trees need, there is also an abundant supply of water, roots do not need to be deep e.g. buttress roots, also help to stabilise the trees.
- The leaves have adapted to the regular heavy rainfall by developing drip-trips, which allows excess water to be easily shed, the leaves are also thick and leathery to withstand strong sunlight and reduces the loss of water from the plant.
- Epiphytes grow on the trees, as the forest floor is dark.
- Rainforests form the habitat for a huge number of species.
The Effects of Human Activity on Plant Succession
- Deforestation is the deliberate clearance of woodland by cutting, burning or the application of a defoliant.
- Climatic climax vegetation has been destroyed, this has resulted in both secondary succession and plagioclimax.
- The vegetation that replaces the original rainforest tends to be smaller in height and less diverse, with a reduction in the general biomass.
Causes of Deforestation
- Demand for hardwood such as teak, its demand for building and furniture is increasing.
- Many developing countries rely on export earnings from timber to help pay their debts and finance major development projects.
- To provide land for rubber plantations, cattle ranches for beef farming, soya plantations, mining, roads and railways.
- Population pressure has also led to an increase in the clearance of rainforests.
Impacts of Deforestation
- As habitats shrink, plant species become endangered and the food chain within the forest is disrupted.
- Some animal species are threatened by extinction.
- The vegetation protects the latosol soils from the regular heavy tropical downpours.
- Once the trees are removed the topsoil is open to erosion and to leaching of nutrients and minerals.
- Runoff causes sediment to block river channels and increases flooding.
- The microclimate of the forest is disturbed by deforestation-the daily water cycle of rapid evapotranspiration followed by afternoon precipitation cannot occur, there is less cloud cover and a greater temperature range.
- The use of burning leads to local air pollution and contributes to climate change.
- Economic benefits in terms of income from mining, farming and exports from hardwood.
- Culture of the indigenous people is destroyed, and they may be forced to move from their land. People are one of the arresting factors that interfere with plants succession.
- They can often stop a plant succession from growing and then when this is maintained it is known as a plagioclimax. Sheep grazing on moorland is an example.
- A secondary succession is a plant succession that takes place on land that has already been colonised. Secondary succession can follow a natural disaster such as a volcanic eruption or a period of human intervention such as colonising a former quarry site.
- Much of the British Isles was covered in deciduous woodland with the climax species being either Oak or Ash depending on the soil and the underlying parent rock. This was largely cleared for agriculture and settlement which has started a plagioclimax in most areas of Britain.