All of life on Earth depends on plants, whether they are tall trees or tiny mosses on the forest floor. The plant life of an ecosystem is known as its vegetation. It can be seen from space as the green cover of an area. Vegetation plays many roles in the biosphere. It is a primary source of food and it controls the characteristics of soils. Vegetation is also part of the great biogeochemical cycles that provide water, carbon, and nitrogen to living things.
Vegetation varies in its composition and structure. Its nature characterizes a terrestrial biome, whether forest or grassland. The study of vegetation was originally concerned with identifying the species that made it up. Modern ecology is more concerned with the functions and interactions of vegetation with other environmental factors. A better understanding of vegetation cycles may help combat climate change and preserve biodiversity.
Historical Background and Scientific Foundations
Few regions of the world, even deserts, are completely free of vegetation. The term refers to the plant cover of a biome and includes trees, shrubs, wild flowers, and grasses. It can vary from dense tropical rain forest to wheat fields or patches of weeds in a garden. Vegetation varies over an area and over time. Vegetation has a three-dimensional structure with height, width, and length. Natural vegetation is not usually uniform. For instance, a forest consists of tall trees, shorter trees, flowering plants, and shrubs, maybe interspersed with clearings. Commercial tree plantations, however, may be uniformly spaced giving an even density of vegetation cover.
Vegetation also varies in a temporal way. There are the natural germination, growth, and death cycles, whose length varies with the nature of the plant. For areas with many deciduous species, there are dramatic changes in vegetation cover with the seasons. Human activities, such as deforestation and urban development, can also cause changes in vegetation over time. It is rare for vegetation to disappear completely. Changes in land use tend to lead to succession, which is the growth of a plant community that probably differs from the original one.
Vegetation plays an important role in the three main biogeochemical cycles, which are the hydrological or water cycle, the nitrogen cycle and the carbon cycle. The fact that vegetation itself has temporal cycles means that these interactions can be complex. They have an impact upon local and global climate and on soil characteristics, all of which have a feedback effect on the structure and productivity of the vegetation.
Vegetation takes part in the hydrologic or water cycle. It contributes about 10% of the water vapor in the atmosphere through transpiration, which is evaporation through leaves. The rest of the water vapor comes from evaporation from the seas and oceans. The water vapor forms clouds, then falls as precipitation onto the land, then runs through into bodies of water, eventually ending up back in the ocean.
Nitrogen fixation is the ability of the roots of leguminous plants, such as soybean or clover, to convert atmospheric elemental nitrogen into nitrate. These plants have nodules in their roots, which house nitrogen-fixing bacteria that carry out this conversion. The plant then converts nitrate into proteins and nucleic acids. These more complex nitrogen-containing molecules are supplied to animals when they eat plant foods. Eventually, these organisms die and decay, returning their nitrogen-containing compounds to the soil, where they are processed into nitrates by nitrifying bacteria. The final step in the nitrogen cycle is conversion of nitrate into nitrogen by denitrifying bacteria.
Finally, vegetation also plays a key role in the carbon cycle through sequestration, which is the absorption of carbon dioxide from the atmosphere through photosynthesis. Fixing carbon dioxide through photosynthesis makes glucose, which provides biochemical fuel for animals eating plants and for plants themselves. Carbon dioxide is thus released back into the atmosphere quite quickly, when animals respire. However, some of the carbon sequestered in plants becomes buried when plants die and decay, forming fossil fuels, thereby removing carbon dioxide from the atmosphere for a long period of time.
Impacts and Issues
Vegetation is a dynamic system whose interaction with biogeochemical cycles is complex and changing all the
WORDS TO KNOW
BIOGEOCHEMICAL CYCLE: The chemical interactions that take place among the atmosphere, biosphere, hydrosphere, and geosphere.
BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.
DECIDUOUS: Plants that shed leaves or other foliage after their growing season.
NITROGEN CYCLE: Biochemical cycling of nitrogen by plants, animals, and soil bacteria.
NITROGEN FIXATION: Conversion of atmospheric nitrogen into nitrate by the roots of leguminous plants.
time. Activities such as deforestation need to be understood in terms of how they will affect climate through these interactions. Loss of transpiration may lead to decreased precipitation and higher risk of drought. Reforestation, however, may provide a useful redress against global warming through carbon sequestration. Vegetation also protects against soil erosion and provides habitats for many species. A better understanding of the type and structure of vegetation which provides maximum benefit to the environment both globally and locally is therefore highly desirable.
Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.
NASA. “Image of the Day: Global Vegetation.” April 7, 2008. http://www.nasa.gov/multimedia/imagegallery/image_feature_l056.html (accessed May 2, 2008).