Vegetation
From Freepedia
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants, and is, by far, the most abundant biotic element of the biosphere. The term vegetation does not, by itself, imply anything regarding species composition, life forms, structure, spatial extent, "naturalness", or any other specific botanical or geographic characteristics. It is broader than the term flora which refers exclusively to species composition. Perhaps the closest synonym is plant community, but vegetation can, and often does, refer to a wider range of spatial scales. Primeval redwood forests, coastal mangrove stands, sphagnum bogs, desert soil crusts, roadside weed patches, wheat fields, cultivated gardens and lawns; all are encompassed by the term vegetation.
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Importance
Vegetation serves several critical functions in the biosphere, at all possible spatial scales. First, vegetation regulates the flow of numerous biogeochemical cycles (see biogeochemistry), most critically those of water, carbon, and nitrogen; it is also of great importance in local and global energy balances. Such cycles are important not only for global patterns of vegetation but also for those of climate. Second, vegetation strongly affects soil characteristics, including soil volume, chemistry and texture, which feed back to affect various vegetational characteristics, including productivity and structure. Third, vegetation serves as wildlife habitat and the energy source for the vast array of animal species on the planet. Vegetation is also critically important to the world economy, particularly in the use of fossil fuels as an energy source, but also in the global production of food, wood, fuel and other materials. Perhaps most importantly, and often overlooked, global vegetation has been the primary source of oxygen in the atmosphere, enabling the aerobic metabolism systems to evolve and persist. Lastly, vegetation is psychologically important to humans, who evolved in direct contact with, and dependence on, vegetation, for food, sheleter, and medicines.
Classification
Much of the work on vegetation classification comes from European and North American ecologists, and they have fundamentally different approaches. In North America, vegetation types are based on a combination of the following criteria: climate pattern, plant habit, phenology and/or growth form, and species composition. In the current US standard (adopted by the Federal Geographic Data Committee (FGDC), and originally developed by UNESCO and The Nature Conservancy), the classification is hierarchical and incorporates the non-floristic criteria into the upper (most general) five levels and limited floristic criteria into only the lower (most specific) two levels. In Europe, classification relies much more heavily, often entirely, on floristic (species) composition alone, without explicit reference to climate, phenology or growth forms.
In the FGDC standard, the hierarchy levels, from most general to most specific, are: system, class, subclass, group, formation, alliance, and association. The lowest level, or association, is thus the most precisely defined, and incoporates the names of the dominant one to three (usually two) species of the type. An example of a vegetation type defined at the level of class might be "Forest, canopy cover > 60%"; at the level of a formation as "Winter-rain, broad-leaved, evergreen, sclerophyllous, closed-canopy forest"; at the level of alliance as "Arbutus menziesii forest"; and at the level of association as "Arbutus menziesii-Lithocarpus densiflora forest". In practice, the levels of the alliance and/or association are the most often used, particularly in vegetation mapping, just as the latin binomial is most often used in discussing particular species in taxonomy and in general communication.
Vegetation Structure
A primary characteristic of vegetation is its three-dimensional structure, sometimes referred to as its physiognomy, or architecture. Most people have an understanding of this idea through their familiarity with terms like "jungle", "woods", "prairie" or "meadow"; these terms conjure up a mental image of what such vegetation looks like. So, meadows are grassy and open, tropical rainforests are dense, tall, and dark, savannahs have trees dotting a grass-covered landscape, etc.
Obviously, a forest has a very different structure than a desert or a backyard lawn. Vegetation ecologists discriminate structure at much more detailed levels than this, but the principle is the same. Thus, different types of forests can have very different structures; tropical rainforests are very different from boreal conifer forests, both of which differ from temperate deciduous forests. Native grasslands in South Dakota, Arizona, and Indiana are visibly different from each other, low elevation chaparral differs from that at high eleveations, etc.
Structure is determined by an interacting combination of environmental and historical factors, and species composition. It is characterized primarily by the horizontal and vertical distributions of plant biomass, particularly foliage biomass. Horizontal distributions refer to the pattern of spacing of plant stems on the ground. Plants can be very uniformly spaced, as in a tree plantation, or very non-uniformly spaced, as in many forests in rocky, mountainous terrain, where areas of high and low tree density alternate depending on the spatial pattern of soil and climatic variables. Three broad categories of spacing are recognized: uniform, random and clumped. These correspond directly to the expected variation in the distance between randomly chosen locations and the closest plant to such locations. Vertical distributions of biomass are determined by the inherent productivity of an area, the height potential of the dominant species, and the presence/absence of shade tolerant species in the flora. Communities with high productivities and in which at least one shade tolerant tree species is present, have high levels of biomass because of their high foliage densities throughout a relatively large vertical area.
Although this discussion centers on biomass, it is difficult to measure in practice. Ecologists thus often measure a surrogate, plant cover, which is defined as the percentage of the ground surface area that has plant biomass (especially foliage) vertically above it. If the vertical distribution of the foliage is broken into defined height layers, cover can be estimated for each layer, and the total cover value can therefore be over 100; otherwise the values range from zero to 100. The measure is designed to be a rough, but useful, approximation of biomass.
In some vegetation types, the underground distribution of biomass can also discriminate different types. Thus a sod-forming grassland has a more continuous and connected root system, while a bunchgrass community's is much less so, with more open spaces between plants (though often not as drastic as the openings or spacings in the above-ground part of the community, since root systems are generally less constrained in their horizontal growth patterns than are shoots). However, below-ground architecture is so much more time-consuming to measure, that vegetation structure is almost always described in relationship to the above-ground parts of the community.
Vegetation Processes
Like all biological systems, plant communities are temporally and spatially dynamic; they change at all possible scales. Dynamism in vegetation is defined primarily as changes in either or both of species composition and vegetation structure.
Temporal Dynamics
Temporally, a large number of processes or events can cause change, but for sake of simplicity they can be categorized roughly as either abrupt or gradual. Abrupt changes are generally referred to as disturbances; these include things like fire, high winds, landslides, floods, avalanches and the like. Their causes are usually external (exogenous) to the community--they are natural processes occurring (mostly) independently of the natural processes of the community (such as germination, growth, death, etc.). Such events can change vegetation structure and species composition very quickly and for long time periods, and they can do so over large ares. Very few ecosystems are without some type of disturbance as a regular and recurring part of the long term system dynamic. Fire and wind disturbances are particularly common throughout many vegetation types worldwide. Fire is particularly potent because of its ability to destroy not only living plants, but also the spores and seeds representing the potential next generation, and because of fire's impact on faunal populations and soil characteristics.
Temporal change at a slower pace is ubiquitous; it comprises the field of ecological succession. Succession is the relatively gradual change in structure and composition that arises as the vegetation itself modifies various environmental variables, including light, water and nutrient levels over time. These modifications change the suite of species most adapted to grow, survive and reproduce in an area, causing floristic changes. These floristic changes contribute to structural changes that are already inherent in plant growth even in the absence of species changes (especially where plants have a large maximum size, i.e. trees), causing slow and usually predictable changes in the vegetation. Succession can be interrupted at any time by disturbance, setting the system either back to a previous state, or off on another trajectory altogether. Because of this, successional processes may or may not lead to some static, final state. Moreover, accurately predicting the characteristics of such a state, even if it does arise, is not always possible. In short, vegetative communities are subject to many and unpredictable variables that limit predictability.
Spatial Dynamics
As a general rule, the larger an area under consideration, the more likely the vegetation will be heterogeneous across it. Two main factors are at work. First, the temporal dynamics of disturbance and succession are increasingly unlikely to be in synchrony across any area as the size of that area increases. That is, different areas will be at different developmental stages due to different local histories, particularly their times since last major disturbance. This fact interacts with inherent environmental variability, which is also a function of area. Environmental variability constrains the suite of species that can occupy a given area, and the two factors together interact to create a mosaic of vegetation conditions across the landscape. Only in agricultural systems does vegetation ever approach perfect uniformity. In natural systems, there is always heterogeneity, although its scale and intensity will vary widely. A natural grassland may seem relatively homogenous when compared to the same area of partially burned forest, but highly diverse and heterogeneous when compared to the wheat field next to it.
Global Vegetation Patterns and Determinants
At regional and global scales there is predictability of certain vegetation characteristics, which are related to the predictability in certain environmental characteristics. Much of the variation in these global patterns is directly explainable by corresponding patterns of temperature and precipitation (sometimes referred to as the energy and moisture balances). These two factors are highly interactive in their effect on plant growth, so their seasonal relationship to each other throughout the year is critical. Such relationships are shown graphically in climate diagrams. By graphing the long term monthly averages of the two variables against each other, an idea is given as to whether or not precipitation occurs when it is useful, i.e. during the warm season, and consequently the type of vegetation to be expected.
Scientific Study
Vegetation scientists study the causes of the patterns and processes observed in vegetation at various scales of space and time. Of particular interest and importance are questions of the relative roles of climate, soil, topography, and history on vegetation characteristics, including both species composition and structure. Such questions are often large scale, and so cannot be addressed by experimentation in a meaningful way. Observational studies supplemented by knowledge of botany, paleobotany, ecology, soil science etc, are thus the rule in vegetation science.
See also
External Links
Classification
- Terrestrial Vegetation of the United States Volume I – The National Vegetation Classification System: Development, Status, and Applications (PDF)
- Federal Geographic Data Committee Vegetation Subcommittee
- Vegetation Classification Standard [FGDC-STD-005, June 1997] (PDF)
Mapping-related
- USGS - NPS Vegetation Mapping Program
- Checklist of Online Vegetation and Plant Distribution Maps
- Free Vegetation Distribution Site – "free access to almost the entire Spot Vegetation ten daily synthesis archive"
Climate Diagrams
References
- Archibold, O. W. Ecology of World Vegetation. New York: Springer Publishing, 1994.
- Barbour, M. G. and W. D. Billings (editors). North American Terrestrial Vegetation. Cambridge: Cambridge University Press, 1999.
- Breckle, S-W. Walter's Vegetation of the Earth. New York: Springer Publishing, 2002.
- Burrows, C. J. Processes of Vegetation Change. Oxford: Routledge Press, 1990.
- Feldmeyer-Christie, E., N. E. Zimmerman, and S. Ghosh. Modern Approaches In Vegetation Monitoring. Budapest: Akademiai Kiado, 2005.
- Kabat, P., et. al. (editors). Vegetation, Water, Humans and the Climate: A New Perspective on an Interactive System. Heidelberg: Springer-Verlag 2004.
- Mueller-Dombois, D., and H. Ellenberg. Aims and Methods of Vegetation Ecology. The Blackburn Press, 2003.
- Van Der Maarel, E. Vegetation Ecology. Oxford: Blackwell Publishers, 2004.
- Vankat, J. L. The Natural Vegetation of North America. Krieger Publishing Co., 1992.
