Covering much of the landscape and forming the primary trophic level, vegetation is a fundamentally important component of terrestrial ecosystems. Plant canopies and root systems interact with the atmosphere and other Earth realms, playing significant roles in the flow of heat, water, nutrients and gases. The photosynthetic processes in plants take in atmospheric carbon dioxide and produce life-giving oxygen and food. Plant growth and decay contribute to soil development.
Plants rarely grow in solitary fashion, except in extreme environments where good sites for plants are limited. In most places, different plant species grow together as members of a community. Variations in the types of plants found in different habitats can be readily observed. For example, think of differences in vegetation found in a marsh and on a rocky hilltop. The plants living in these two places will largely be of different species as well as having different physical structures.
Looking over large areas, patterns in the distribution of vegetation types can be seen. Each plant species is distributed according to its unique tolerance of many interacting factors related to the physical environment, climate, historical events, migrational (dispersal) ability, and adaptability. Plants with similar tolerances develop into discernable plant groupings with distinct floristic (species composition) and structural features. Broad groupings of distinctive plant formations are referred to as "biomes."
Regional or global extents of various biomes depend on climate and weather conditions including temperature and precipitation patterns, and physiographic barriers such as mountains and oceans. Local variations in vegetation patterns are additionally influenced by soils, competition, browsing by animals, and microenvironments. Vegetation types can be described in a number of different ways, two of which will be considered here: by the species found in an area, and by the physical characteristics, functions or structures of the plants.
Vegetation in a localized area can be described by the assembly of plant species occurring there. Though the term "diversity" is often used broadly to simply indicate the number or variety of different species in a community, it may also be technically defined by an assortment of simple diversity indices formulated by ecologists over the years. Some terminology will be discussed here but this is not an exhaustive compilation of all possible diversity measures.
Species richness, and representation (abundance or population density), are important to consider, and they relate to species diversity. Species richness refers to the number of different species found in a given area. However, some of those species may only have a few individuals or cover a small area, while other species may be represented by hundreds or thousands of individual plants or cover a much larger area. Regions of the northern boreal forest covering many square kilometers may be dominated by a single species of black spruce, while in the tropical rain forests a small plot may contain dozens of different tree species.
The representation (or abundance or importance or population values) of different species can be measured several ways: by percent frequency (how often does the plant occur), percent cover (how much ground does the plant material cover), or biomass (how much plant material by weight or mass is there). Species "evenness" or "equitability" is the apportionment or balance of individuals or representation among the species. Evenness is largest when all species are represented about equally.
One measure of species diversity is richness weighted by evenness. The weighting by evenness is important because though the richness of an area may look high by a large number of species present, if a species is represented by only a few individuals or a very small amount of cover or biomass, then it may not persist in an area for very long.
Generally, species diversity and richness increase going from the poles to the equator, and from high elevations to low elevations. Diversity and community stability have been correlated (linked), but this is not a consistent rule. Floristic diversity in a habitat or community indicates the degree of resource specialization among member species. Since competition is greatest between organisms having the same ecological requirements, it can be reduced if member species have somewhat differing needs that partition a habitat's resources.
When creating vegetation maps often what is used to define the vegetation type is the dominant species -- typically a small number of species that are abundantly represented -having the largest number of individuals, the greatest canopy cover, or the greatest biomass. For example, in a forest described as "oak," there may be other tree species, as well as shrub and herbaceous species in the understory. But because oak trees dominate the overstory canopy cover and biomass, the vegetation type is categorized as oak forest.
Another way that vegetation types may be defined is by the presence of an indicator species. This species may not be dominant, but its environmental requirements define a specific vegetation type. For example, the region referred to as the Sonoran desert in the southwest U.S.A. is defined by the presence of the tall saguaro cactus.
Unfortunately, the accurate discrimination of individual plant species using remote sensing data has been difficult, especially with data having low (coarse) spatial resolution. Even higher spatial resolution datasets can be inadequate for species identification, so currently taxonomic detail is most accurately determined by ground survey.
Vegetation can also be characterized by the structure and size of the dominant plants, using terms such as deciduous forest, savanna, wetland, desert, or tundra, rather than looking at the species composition. In a distinctive plant community or vegetation type, a functional relationship exists among member plant species. Even though the species of a particular vegetation type may be different in different places, similarities in structure and function allow them to be grouped together. For example, different grass species are found in the prairies of North America, the pampas of Argentina, and the steppes of Russia, but these areas are all considered grasslands. Some species in one grassland fill the same functional roles (ecological niches) as species in other grasslands.
Vegetation structure and its canopy (crown or overstory) reveal something about a plant's history and interaction with its environment. There are a variety of environmental factors that affect the growth and structure of plants, such as heat, light, moisture, wind, fire, soils and nutrients. Interaction with other organisms through competition and browsing (grazing) also affects vegetation structure.
In describing vegetation, some key characteristics are stature (size of dominants and codominants), growth form (e.g., trees, shrubs, herbs), canopy coverage of the plants, number of canopy layers, leaf type (e.g,. broadleaf or needleleaf), and phenology (persistence of leaves, i.e. deciduous or evergreen). These aspects of vegetation structure determine the rates of interception of solar radiation, as well as the transfer of heat and moisture to the atmosphere, which, in turn, control photosynthetic rates, respiration, and transpiration of the plants. This sets climatic limits on plant growth and thus limits to a plant's distribution.
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Forest |
Closed treen stand, one or more stories, 7-40 m crown |
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Woodland |
Intermittent stand of trees with canopy cover >20%, mostly herbaceous understory with shrubs less than 10% of canopy cover |
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Bushland |
Assemblage of shrubs and trees, dominated by shrubs with trees conspicuous, canopy cover >20% and primarily below 10m in height |
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Shrubland |
Stand of shrubs less than 6m in height with canopy cover >20%, understory often thin with trees less than 10% of canopy cover |
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Wooded Grassland |
Grassland with conspicuous scattered or grouped trees, woody canopy <20% |
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Bush Grassland |
Grassland with scattered or grouped trees and shrubs, each conspicuous, woody canopy <20% |
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Shrub Grassland |
Grassland with conspicuous scattered or grouped shrubs, woody canopy <20% |
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Dwarf Shrub Grassland |
Grassland often sparse, but set with stands of dwarf shrubs <70cm in height, occasional widely scattered larger shrubs or stunted trees |
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Grassland |
Grasses and other herbaceous cover with woody canopy <2% |
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Permanent Swamp |
Reeds, sedges, rushes, trees, and shrubs in standing water |
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Barren Land |
Land naturally devoid of vascular plants |
The reflectance, transmittance and scattering of radiation in a canopy is affected by vegetation structure and optical properties (spectral reflectance and transmittance) of leaves or needles. These key factors interact to determine the spectral characteristics (including reflectance) of the landscape. Variations in vegetation structure can be observed by remote sensing instruments.
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