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Characteristics of a Tropical Biome

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‘The characteristics of the vegetation of tropical biomes are more the outcome of continued human activity than a response to the climatic conditions’. With reference to one tropical biome, discuss the extent to which you agree with this view. (40 marks)
A tropical biome is a large community of plants and animals that occupy a distinct region. Tropical biomes are mostly located within the tropics, between the Tropic of Cancer and the Tropic of Capricorn and currently cover less than 5% of the Earth’s surface, but support 50% of all living organisms on Earth. The Amazon Rainforest is an example of a magnificent tropical biome found in the heart of Brazil. Its basin covers an area of 7 million square kilometres and it has an astonishing value in the natural world. The Amazon Rainforest is a key provider of ecosystem services, including carbon sequestration and freshwater resources, as well as being home to the most diverse and numerous arrays of species in the world. Several species found in it are endemic and endangered. Vegetation in tropical biomes is stratified with multiple layers and a high species density. A dense shrub layer forms the under layer of the rainforest and a range of tall trees emerge above the canopy.
Some people believe that these characteristics are more the outcome of continued human activity than a response to the climatic conditions, however others disagree. There are several reasons for this.

The vegetation of the Amazon Rainforest has adapted to survive in the characteristics of the biome, to make them well suited to their habitat. Year round, the rainforest has a low diurnal and annual temperature range and a high average annual rainfall (approximately 2000mm a year). The warm, moist and unstable air is forced to rise at the inter-tropical convergence zone, resulting in daily convectional storms that contribute to the rainfall each year. As a result of this high rainfall the Amazon River seasonally floods, creating seasonally flooded forests, which are locally known as igapo and varzea forests. Consequently, vegetation near the river banks has had to adapt to becoming inundated by water for several months of the year to survive. Terra firme forests on higher land, away from the river bank, do not seasonally flood and are therefore not adapted to waterlogged conditions in the soil but have shallow roots to maximise nutrient uptake from top layer of soil. Furthermore, to support the trees growth to obtain maximum sunlight for photosynthesis, these trees have developed buttress roots, which grow out from the base of the trunk sometimes as high as 15 foot above the ground. This also aids nutrient uptake by increasing surface area.

The heavy daily convectional rainfall also has the potential to saturate leaves in the canopy, dissolving and washing away nutrients from the surface. To prevent saturation, around 80% of canopy tree species grow drip tip leaves that help them shed water quickly and efficiently. Fungi, mosses and algae or epiphylls, may also grow in the puddles that form on the leaves, stealing the sunlight from the leaf surface.

There is also a year-round growing season due to the lack of defined seasons and a high humidity causing rapid evapotranspiration from swamps, trees and rivers creating a sticky and oppressive heat. The continuous opportunity for growth has resulted in a dense, diverse collection of vegetation with intense competition between species to compete for resources. As little as 1% of sunlight passes through the vegetation layers to the forest floor. The few plants that live here are mostly herbs with large flat leaves that capture as much of the dappled light as possible. As the Rainforest grows to the canopy, each tree is carefully adapted to take full advantage of maximum exposure to the sun but survive in the intense conditions of heat and wind. Lianas are an example of trees that have grown in response to the climatic conditions of the biome. They begin life on the forest floor but depend on trees for support as they climb upwards towards the sunlight needed for survival. In the canopy, they spread to other trees or wrap around themselves to produce a network of vines with thick, woody stems, to give support against the strong winds and heavy rainfall. The top emergent layer trees, such as the Balizia Elegans are particularly hardy to the elements, having to cope with the power of the sun, stormy winds and heavy rainfall. As any moisture is soon evaporated, one adaption to the climate is the development of thicker, non-drip tip leaves to aid moisture retention.
The vegetation in the Amazon Rainforest has adapted to survive these climatic conditions, therefore its vegetation characteristics are not the outcome of continued human activity.
However, increasingly, the characteristics of the vegetation found in the Amazon Rainforest have begun to be more representative of human activity than climatic conditions. Deforestation is further altering the characteristics of vegetation in the Amazon Rainforest and tropical biomes. Increased world demand for soya has resulted in the expansion of mechanised soya plantations, which has driven deforestation. In Santarém, it is estimated that 10% of the land deforested in recent years is now soya plantations. Soya bean cultivation is expanding in the Amazon Rainforest due to economics, including high prices for grains. These high prices are driven by increasing demand for meat in countries with a large and fast-growing population and the US government’s subsidies for corn-based ethanol products. Such subsidies mean that American farmers are planting corn instead of Soy and the reduction of soy production in the US leads to an increase in production in places like Brazil, which has large tracts of cheap land suitable for agriculture. Since 1990, the area of land planted with soybeans in Amazonian states has expanded at the rate of 14.1% a year and now covers more than 8 million hectares. The introduction of soya plantations wipes out biodiversity, destroys soil fertility, pollutes freshwater and displaces communities. Further pressure comes from the development of infrastructure, such as roads and ports, to support the soy expansion. This infrastructure attracts other developers into the area and causes increased destruction of the vegetation through direct destruction of vegetation and increased pollution, changing the global climate and forcing the vegetation to adapt as the climate changes due to human activity.
Furthermore, the vegetation characteristics in tropical biomes have been affected by the growth in cattle ranches. Extensive cattle farming is the number one cause of deforestation and, alone, the deforestation caused by cattle ranching is responsible for the release of 340 million tons of carbon to the atmosphere every year, equivalent to 3.4% of current global emissions. Beyond forest conversion, cattle pastures increase the risk of fire and are a significant degrader of riparian ecosystems, causing soil erosion, river siltation and contamination of organic matter. This leads to the increase risk of wider spread flooding and the death of trees adapted to living in terra firme forests.
The deforestation of vegetation in tropical biome such as the Amazon Rainforest has also resulted in the rainforest becoming fragmented. The natural characterised high vegetation density areas have been broken down, dividing habitats and altering the stratification of the rainforest created by the climatic conditions. Over time, the vegetation will be further shaped through the fragmentation of the forest, causing further biodiversity loss as species overpopulate the rainforest.
Over time, it has become more apparent that the characteristics of the vegetation of tropical biomes are more the outcome of continued human activity than a response to the climatic conditions. The initial growth of these biomes was as a result of the climate, as the vegetation has adapted to suit the conditions and thrive under the resources available. However, the destruction of these tropical biomes is changing the characteristics of the vegetation and the forest, resulting in the introduction of commercialised vegetation and the loss of natural biodiversity. It is therefore to a large extent that I agree that the characteristics of the vegetation of tropical biomes are more the outcome of continued human activity than a response to climatic conditions.

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