Outline The Factors That Are Responsible For

Vegetation Change Over Time Essay, Research Paper

Vegetation develops in many different and varied

environments with each environment having different effects on the vegetation

change over time. This essay will examine these different factors, in different

environments. It will examine different sereal stages in different environment,

the lithosere or rock environment, the psammosere or sand environment, the

halosere or saltwater environment and hydrosere or fresh water environment. A sere is a stage of events by which the vegetation of an

area develops over time. This begins with the pioneer community and end with

the climatic climax vegetation. The first plants to colonise an area are called

the pioneer community. The climatic climax vegetation is when the ultimate

vegetation development has taken place and the environment has reached

equilibrium e.g. when the natural vegetation has reached a stable balance with

the climate and soil of an area. As an environment goes through more sereal

stages the number of species of plants will increase as will their height. The

island of Krakatoa in the Sundra strait between Java and Sumatra is a volcanic

island, which erupted in 1883. Its development was rapid only taking

twenty-five years to develop from the pioneer community to climatic climax

vegetation because of the high humidity and temperatures and the rapid

weathering of the volcanic rock. The diagram below shows the way an environment

such as Krakatoa develops.There are two types of sereal succession primary and

secondary. Primary sereal succession occurs on a new or previously sterile

land. It occurs in four different types of environment lithosere, psammosere,

halosere and hydrosere. A lithosere is a rock environment. These are initially

colonised by blue-green bacteria, which are completely self-sufficient.

Therefore the pioneer community is the mosses and lichens which are capable of

living in areas which lack soil. The lichens and mosses help to weather the

rock along with other types of weathering. This along with the decayed matter

of mosses and lichens helps to form a veneer of soil, which can support more

advanced plant life. Seeds usually of grass then colonise this soil as time

passes the grasses will give way to fast growing shrubs which in turn will be

replaced by fast growing trees. Finally these will face competition from slow

growing trees such as ash and oak. Although each stage of the succession has

been superseded the vegetation from each previous sereal stage still remains

but in smaller amounts. Two examples of areas where the lithosereal development

has taken place are Krakatoa and Sutsey. Sutsey is like Krakatoa in the fact

that it is a newly formed island. The island was created after a volcanic

eruption on 14th November 1963. The island is located off the

south-western coast of Iceland. Unlike Krakatoa it has not developed quickly

because of the local climate which is extremely cold and very windy. These

conditions don?t lead to the quick establishment of life. The diagram below

shows a typical progression for a lithosere ecosystem. Another type of succession is one, which occurs on sand this

is known as a psammosere. The first coloniser of the sand are usually lyme

grass, sea twitch and maram grass these start dune formation known as embryo

dunes. The grasses slowly move back from the high tide mark on to different

dunes. These dunes are very arid and therefore are usually only populated by

maram grass which has adapted to the harsh conditions. Further back from the

main ridge are the older grey dunes, which are shelter from the wind and have

more humus in their soil due to the increased decomposition of the maram grass.

This humus in the soil supports different species and these start to break the

maram grasses dominance on the dunes. Finally around 400m away from the sea the

climatic climax is reached as oak and ash trees develop. One example of a

psammosere is Camber Sands, which has developed over time to have a full

diversity of plants on the different dunes. The diagram below shows the

development of the environment in a psammosere.Freshwater lakes and ponds are known as hydrosere

environments. These also develop through sereal stages. The pioneer coloniser

is usually algae and mosses whose spores have been blown on to the water

surface. These grow to form vegetation rafts, which then support other life

such as bacteria and insects. The next colonisers are the water loving plants,

which may grow on the water surface like lilies or may grow completely

submerged. Bacteria recycle nutrients from these plants encouraging the

encroachment of plants such as bulrushes. The increased sediment created by the

decomposition of these plant leads to increased sediment, which continues to in

fill the lake. Over time the marsh areas at the edge of the lake will be

colonised by small trees and shrubs. And finally the lake is completely in

filled as a result of the sediment build up. When this occurs the oak and ash

trees will be able to take root and the climatic climax vegetation will be

reached. An example of an area where this is occurring is the Norfolk Broads

where some of the broads are nearly completely in filled whereas others are

still at the first stage of development. The diagram below shows the different

stages in the development of a hydrosere.Haloseres are environment based in a salt-water environment

usually at the mouth of a river where there is a large amount of silt and mud

deposition. The pioneer colonisers are algae, which can stand being submerged

in water for the twelve-hour high tide period. These trap mud causing it to

accumulate. Spartina townsendii is

the next coloniser. It can tolerate saline conditions and grows on the slob

zone with only four hours of oxygen in every twelve-hour period. In contrast

the sward zone is inhabited by plants, which can only be submerged for four

hours in every twelve hours. The dominant species here are sea lavender and

grasses. The upper sward zone is only covered by the high spring tides allowing

different types of plants to establish. Further inland from here different

types of plants grow including grasses and shrubs followed by small trees and

ultimately ash and oak trees. The Medway estuary is one example of an area

where this type of progression is taking place. The diagram below shows the

primary succession of a typical halosere.When a climatic climax occurs this means that the ecosystem

or biome is in a state of equilibrium with a stability of transfers material

and energy. However the are a number of factors which can arrest the

development of the ecosystem before it has reached it dynamic equilibrium.

These include mudflow, landslide, disease or changing climate. One example is a

fire, which is likely to destroy the whole ecosystem. But it does allow for new

development as the ash created increase the nutrient quality of the soil

dramatically. But it does leave the soil more vulnerable to erosion. Some

ecosystems depend on a fire to clear the area and to allow the growth of new

seedlings, which will thrive in the good quality soil with little or no

competition for light. Some examples of ecosystems, which depend on a fire to

allow growth, are the forest of the Australian bush. There are three factors, which will dramatically effect the

vegetation change over time in an ecosystem. These are climate, soil quality

and water availability. If the climate that an ecosystem is developing in is

harsh such as that on Sutsey then vegetation change will be slow and very

difficult whereas if the climate is ideal then the ecosystem will develop

quickly such as that on Krakatoa. The soil quality is important because the

number of nutrients in a soil will affect the type of plants able to grow

there. A poor quality soil will only be able to support a small amount of small

plants whereas a good quality soil will support larger plants. Water

availability is key because it affects the number of plants which can grow in a

certain area. If there is little water available then only a few plants can

survive. In contrast if there is an abundance of water then the number of

plants able to be supported will be large. This can also be taken to another

extreme where there is too much water and the plants drown. Human factors are major arresting factors, which usually

lead to secondary. These can include deforestation, which leads to the removal

of many of the nutrients from the ecosystem and can lead to erosion. This has

occurred in the Gola forest in Sierra Leone. Sierra Leone has a climate, which

would allow a high proportion of it s landmass to be covered with rainforest.

But due to constant deforestation the rainforest has been dramatically reduced

in size and now only covers a small amount of the land. Pollution caused by humans can affect vegetation on three different

scales globally, nationally and locally. An example of a global pollution,

which affects vegetation change, is acid rain, which via air currents can

travel globally precipitating at any point of the world. Acid rain kills plants

and can change the balance of nutrients in the soil favouring certain types of

plants. An example of pollution on a national scale is global warming which can

affect whole countries and leads to the deaths of many plants and the increase

water levels around the world. Trampling is an example of pollution on a local

scale. Trampling kill plants and leads to the compacting of soils making water

absorption difficult and plant growth therefore difficult. Other examples of Human arresting factors, which affect

vegetation change over time, include desertification the progressive

downgrading of land usually caused by human mismanagement through things like

over grazing or via drought. Urbanisation and the gradual expansion of towns

into the countryside in Great Britain and other countries has led to the

destruction of ecosystems to allow space for expansion. This is particularly

true in less economically developed countries, which are in a stage of hyper

urbanisation and create space for this by destroying ecosystems. One example of

this is Mexico City, which has experienced massive expansion in recent years. Conservation is another way in which humans can affect the

natural vegetation change. The Norfolk Broads is a good example of this. The

Norfolk Broads area has been an area of conservation for a number of years the

area houses a number of rare species of plant and animal which conservationists

are trying to save. One thing they are trying to prevent is the gradual in

filling of the broads. This therefore disrupts the natural vegetation change. A

policy of aforestation has been applied in a number of areas to try and

repopulate certain areas with trees that were once there. This therefore has a

large effect on the vegetation change over time. In conclusion we can see that there are many different

things, which affect vegetation change over time. These are both physical

factors and human factors and both types have the power to completely change an

ecosystem affect its progression towards its climatic climax vegetation and its

establishment as a biome.