In the last 10,000 years, our ancestors have altered the natural ecosystems only slightly. However, scientific and technological development have paved the way to drastic changes in contemporary ecosystems. As a result, much land has been converted into agricultural and cattle farms. Vegetation that was originally propagated only by falling seeds is now cultivated by humans as crops in agricultural fields.
In this lesson we continue to investigate the biosphere, but with our focus on natural vegetation and agricultural crops of the lands.
The plants are stationary on the Earth's surface, as the other abotic components such as landforms, soils, streams, and lakes. Plants are also consumable and renewable sources of food, medicines, fuels, clothing, shelter, and a host of other life essentials. There are many persistent themes in the writings of geographers about the ways that humans have used plant sources to their advantage, or have been hindered by the plants in their progress. A plant geographer classifies plants in terms of its lifeform, which is the physical structure, size, and the shape of the plant. These lifeforms are principally trees, shrubs, lianas and herbs. Most of them have life spans of many years.
As we travel through a hilly area, we observe that the vegetation is strongly influenced by landforms and soil. Landforms refers to the configuration of the land surface, including features such as hills, valleys, ridges, or cliffs. Vegetation on an upland, relatively high ground with thick soil and good drainage is quite different from that on an adjacent valley floor. It is because of the water that lies near the surface much of the time. Vegetation is often different in form on rocky ridges and on steep cliff, where water drains away rapidly and soil is thin or largely absent.
Similarly, these plant communities have an optimum temperature associated with each of its functions, such as photosynthesis, flowering, fruiting, or seeding germination. A plant's growth depends on overall optimum yearly temperature conditions. In general, the colder the climate, the fewer are the species that are capable of surviving. A large number of plant species cannot survive below freezing temperatures. In the severely cold Arctic and alpine environments of high latitudes and high altitudes, only a few species can survive. This explains why a forest in the equatorial has many species of trees, where as forest of the subarctic zone may be dominated by just a few.
Based on the dominant plant species, the following are the principal biomes, listed in order of availability of soil water and heat. 1. Forests (ample soil, water and heat), 2. Grasslands (moderate soil and water, adequate heat) 3. Deserts (extreme shortage of soil, water and adequate heat) and 4. Tundra (insufficient heat).
About 420 million years ago, ancient plants began to occupy the land. Over the millions of years that followed, these plants developed and adapted to their new habitat. They were the first forests. In these forests, giant horsetails, club mosses, and ferns that stood up to 12 meters tall dominated. Later, gymnosperms developed and the first flowering plants appeared 144-165 million year ago. They evolved together with insects, birds, and mammals and flourished. Today, forests occupy approximately one-third of Earth's land area. There are three major types of forests, classified according to climatic conditions such as equatorial forests deciduous forests and tundra forests.
a. Equatorial forests: These are found around the Equator. This region has lots of sunshine and heavy rainfall. So the plants grow in large numbers, both quickly and densely. There are no seasons here, and so the trees remain green all through the year. Hundreds of species of trees and plants are found in evergreen forests. Some species include orchids, thorny plants, and creepers. Each tree has several roots and is broad-based, with an average height of 25-35 meters . Herbs, bushes, and other kinds of plants also thrive in these forests. The forests are densely packed and do not let much sunlight through the foliage.
b. Decidous forests: These are located in subtropical and temperate areas. Hot summers and cold winters exist in these areas. There is rainfall during some months of the year in both of these zones. Therefore these trees shed their leaves during summer in subtropical zones, and during winter in temperate zones. In countries like India, trees shed their leaves during the long, dry summer season. These subtropical forests are sometimes called monsoon forests.
c. Taiga forests: These are largest biome located between 50 and 60 degrees north latitudes. Such forests can be found in the broad belt of Eurasia and North America: two-thirds are in Siberia with the rest in Scandinavia, Alaska, and Canada. Seasons are divided into short, moist, and moderately warm summers and long, cold, and dry winters. The length of the growing season in these forests is 130 days. These regions receive snow in the winter, and depending on their latitude, may have some in the summer. The trees of these forests have needle shaped leaves. More than 1700 species of trees are found in these forests. These species all have the ability to withstand severe cold.
Grasslands are big open spaces. About one quarter of the Earth's land is grasslands. There are not many bushes in the grasslands. We usually find trees only where there are rivers or streams. The grassland seems like an endless ocean of grass. Grasslands receive about 20 to 65 centimeter of rain per year. If they received more rain, the grasslands would become a forest. If they received less, they would become a desert.
Grasslands are often located between deserts and forests. Grassland soil tends to be deep and fertile. The roots of perennial grasses usually penetrate far into the soil. In South America, grasslands are called 'pampas', in Europe, 'steppes', in Africa, 'savannas' and 'prairies' in Canada and the USA. The grass lands are divided into two types. They are: Savanna Grasslands and Temperate Grasslands.
Savanna Grasslands: Savanna is a grassland with scattered individual trees. Savannas of one sort or another cover almost half the surface of Africa and large areas of Australia, South America, and India. Climate is the most important factor in creating a savanna. Savannas are always found in warm or hot climates where the annual rainfall is from about 50 to 127 cm per year. It is crucial that the rainfall is concentrated in six or eight months of the year, followed by a long period of drought when fires can occur. If the rains were well distributed throughout the year, many such areas would become tropical forest.
Savanna has both a dry and a rainy season. Seasonal Fires play a vital role in the savanna's biodiversity. In October, a series of violent thunderstorms, followed by a strong drying wind, signals the beginning of the dry season. Fire is prevalent around January, at the height of the dry, season. Poachers who want to clear away dead grass to make it easier to see their prey often cause fires in savannas. The fires do not devastate the community. Most of the animals killed by the fires are insects with short life spans.
A fire is a feast for some animals, such as birds that come to sites of fires to eat grasshoppers, stick insects, beetles, mice, and lizards that are killed or driven out by the fire. Underground holes and crevices provide a safe refuge for small creatures. Larger animals are usually able to run fast enough to escape the fire. Although the dry stems and leaves of grasses are consumed by fire, the grasses- deep roots remain unharmed. These roots, with all their starch reserves, are ready to send up new growth when the soil becomes more moist. The scattered shrubs can also subsist on food reserves in their roots while they await the time to venture above the soil again.
A fire leaves scorched earth covered with a fine layer of powdery black ash in its wake. During March, violent thunderstorms occur again, this time heralding the rainy season. When the rains come, savanna bunch grasses grow vigorously. Some of the larger grasses grow an inch or more in 24 hours. The savannas experiences a surge of new life at this time.
Temperate Grasslands: Temperate grasslands are characterized as having grasses as the dominant vegetation. Trees and large shrubs are absent. Temperatures vary more from summer to winter, and the amount of rainfall is less in temperate grasslands than in savannas. The major temperate grasslands are the veldts of South Africa, the puszta of Hungary, the pampas of Argentina and Uruguay, the steppes of the former Soviet Union, and the plains and prairies of central North America. Temperate grasslands have hot summers and cold winters. Rainfall is moderate. The amount of annual rainfall influences the height of grassland vegetation, with taller grasses in wetter regions.
As in the savanna, seasonal drought and occasional fires are very important to biodiversity. However, their effects aren't as dramatic in temperate grasslands as they are in savannas. The soil of the temperate grasslands is deep and dark, with fertile upper layers. It is nutrient-rich from the growth and decay of deep, many-branched grass roots. The rotted roots hold the soil together and provide a food source for living plants.
Deserts cover about the fifth of Earth's surface and occur where rainfalls in less than 50 cm/year. Most deserts, such as the Sahara of North Africa and the deserts of the southwestern U.S., Mexico, and Australia, occur at low latitudes. Cold deserts, occur in the basin and range area of Utah and Nevada, and parts of western Asia.
The seasons are generally warm throughout the year and very hot in the summer. The winters usually bring little rainfall. Temperatures exhibit daily extremes because the atmosphere contains little humidity to block the Sun's rays. Desert surfaces receive a little more than twice the solar radiation received by humid regions and lose almost twice as much heat at night. Rainfall is usually very low and/or concentrated in short bursts between long rainless periods. Evaporation rates regularly exceed rainfall rates. Rainfall is lowest on the Atacama Desert of Chile, where it averages less than 1.5 cm. Some years are even rainless. Inland Sahara also receives than 1.5 cm a year. Rainfall in American deserts is higher, almost 28 cm a year.
Most deserts have a considerable amount of specialized vegetation. Soils often have abundant nutrients because they need only water to become very productive and have little or no organic matter. Soils are course-textured, shallow, rocky or gravely with good drainage and have no subsurface water. They are coarse because there is less chemical weathering. The finer dust and sand particles are blown elsewhere, leaving heavier pieces behind.
The Tundra is the coldest of all biomes. 'Tundra' comes from the Finnish word tunturia, meaning treeless plain. This biome has extremely cold climate, low biotic diversity, simple vegetation structures, a short season of growth and reproduction. Energy and nutrients are found in the form of dead organic material.
The Arctic tundra is located in the Northern Hemisphere, encircling the North Pole and extending south to the coniferous forests of the taiga. The Arctic is known for its cold, desert-like conditions. The growing season ranges from 50 to 60 days. The average winter temperature is 34'C but the average summer temperature is 3-12'C which enables this biome to sustain life. Rainfall may vary in different regions of the Arctic. Yearly precipitation, including melting snow, is 15 to 25 centimeters.
Soil is formed slowly. A layer of permanently frozen subsoil called permafrost exists, consisting mostly of gravel and finer material. When water saturates the upper surface, bogs and ponds may form, providing moisture for plants. There are no deep root systems in the vegetation of the Arctic tundra, however, seasons are short and most plants reproduce by budding and division rather than sexually by flowering. The fauna in the Arctic is also diverse. Still a wide variety of plants that are able to resist the cold climate. There are about 1,700 kinds of plants in the Arctic and sub arctic region.
Alpine tundra is located on mountains throughout the world at high altitude where trees cannot grow. The growing season is approximately 180 days. The nighttime temperature is usually below freezing. Unlike the Arctic tundra, the soil in the alpine is well drained. The plants are very similar to those of the Arctic.
Water is the common link among the five biomes and it makes up the largest part of the biosphere. Aquatic regions house numerous species of plants and animals, both large and small. In fact, this is where life began billions of years ago, when amino acids first started to come together. Without water, most lifeforms would be unable to sustain themselves and the Earth would be a barren, desert-like place.