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Vegetation

2007 Schools Wikipedia Selection. Related subjects: Plants

   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.
   Aerial view of mixed aspen-spruce forest in Alaska
   Enlarge
   Aerial view of mixed aspen-spruce forest in Alaska

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 (including algal communities) 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 dominant species. 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 only into the lower (most specific) two levels. In
   Europe, classification often relies much more heavily, sometimes
   entirely, on floristic (species) composition alone, without explicit
   reference to climate, phenology or growth forms. It often empahsizes
   indicator or diagnostic species which separate one type from another.

   In the USA's 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.
   A temperate deciduous hardwood forest in the dormant season
   Enlarge
   A temperate deciduous hardwood forest in the dormant season

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 tempeate deciduous forests. Native grasslands in
   South Dakota, Arizona, and Indiana are visibly different from each
   other, low elevation chaparral differs from that at high elevations,
   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 large vertical distance.

   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.
   A freshwater wetland
   Enlarge
   A freshwater wetland

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 broadly 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.
   A coastal dune grassland on the Pacific Coast, USA
   Enlarge
   A coastal dune grassland on the Pacific Coast, USA

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 or horticultural 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 homogeneous 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, especially physiognomic ones, 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, and their
   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 during the
   warm season, when it is most useful, and consequently the type of
   vegetation to be expected. For example, two locations may have the same
   average annual precipitation and temperature, but if the relative
   timing of the precipitation and seasonal warmth are very different, so
   will their vegetation structure and growth and development processes
   be.

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 easily 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.

History

Pre-1900

   Vegetation science has its origins in the work of botanists and/or
   naturalists of the 18th century, or earlier in some cases. Many of
   these were world travelers on exploratory voyages in the Age of
   Exploration, and their work was a synthetic combination of botany and
   geography that today we would call plant biogeography (or
   phytogeography). Little was known about worldwide floristic or
   vegetation patterns at the time, and almost nothing about what
   determined them, so much of the work involved collecting, categorizing,
   and naming plant specimens. Little or no theoretical work occurred
   until the 19th century. The most productive of the early naturalists
   was Alexander von Humboldt, who collected 60,000 plant specimens on a
   five year voyage to South and Cental America from 1799 to 1804.
   Humboldt was one of the first to document the correspondence between
   climate and vegetation patterns, in his massive, life-long work "
   Voyage to the Equinoctial Regions of the New Continent", which he wrote
   with Aimé Bonpland, the botanist who accompanied him. Humboldt also
   described vegetation in physiogonmic terms rather than just
   taxonomically. His work presaged intensive work on
   environment-vegetation relationships that continues to this day
   (Barbour et al, 1987)

   The beginnings of vegetation study as we know it today began in Europe
   and Russia in the late 19th century, particularly under Jozef Paczoski,
   a Pole, and Leonid Ramensky, a Russian. Together they were much ahead
   of their time, introducing or elaborating on almost all topics germane
   to the field today, well before they were so in the west. These topics
   included plant community analysis, or phytosociology, gradient
   analysis, succession, and topics in plant ecophysiology and functional
   ecology. Due to language and/or political reasons, much of their work
   was unknown to much of the world, especially the English-speaking
   world, until well into the 20th century.

Post-1900

   In the United States, Henry Cowles and Frederic Clements developed
   ideas of plant succession in the early 1900s. Clements is famous for
   his now discredited view of the plant community as a " superorganism".
   He argued that, just as all organ systems in an individual must work
   together for the body to function well, and which develop in concert
   with each other as the individual matures, so the individual species in
   a plant community also develop and cooperate in a very tightly
   coordinated and synergistic way, pushing the plant community towards a
   defined and predictable end state. Although Clements did a great deal
   of work on North American vegetation, his devotion to the superorganism
   theory has hurt his reputation, as much work since then by numerous
   researchers has shown the idea to lack empirical support.

   In contrast to Clements, several ecologists have since demonstrated the
   validity of the individualistic hypothesis, which asserts that plant
   communities are simply the sum of a suite of species reacting
   individually to the environment, and co-occurring in time and space.
   Ramensky initiated this idea in Russia, and in 1926, Henry Gleason
   (Gleason, 1926) developed it in a paper in the United States. Gleason's
   ideas were categorically rejected for many years, so powerful was the
   influence of Clementsian ideas. However, in the 1950s and 60s, a series
   of well-designed studies by Robert Whittaker provided strong evidence
   for Gleason's arguments, and against those of Clements. Whittaker,
   considered one of the brightest and most productive of American plant
   ecologists, was a developer and proponent of gradient analysis, in
   which the abundances of individual species are measured against
   quantifiable environmental variables or their well-correlated
   surrogates. In studies in three very different montane ecosystems,
   Whittaker demonstrated strongly that species respond primarily to the
   environment, and not necessarily in any coordination with other,
   co-occurring species. Other work, particularly in paleobotany, has lent
   support to this view at larger temporal and spatial scales.

More Recent Concepts, Theories, and Approaches

   Since the 1960s, much research into vegetation has revolved around
   topics in functional ecology. In a functional framework, taxonomic
   botany is relatively less important; investigations centre around
   morphological, anatomical and physiological classifications of species,
   with the aim of predicting how particular groups thereof will respond
   to various environmental variables. The underlying basis for this
   approach is the observation that, due to convergent evolution and
   (conversely) adaptive radiation, there is often not a strong
   relationship between phylogenetic relatedness and environmental
   adaptations, especially at higher levels of the phylogenetic taxonomy,
   and at large spatial scales. Functional classifications arguably began
   in the 1930s with Raunkiaer's division of plants into groups based on
   the location of their apical meristems (buds) relative to the ground
   surface. This presaged later classifications such as Macarthur and
   Wilson's (1967) r vs K selected species (applied to all organisms, not
   just plants), and the "C-S-R" strategy proposed by Grime (1987) in
   which species are placed into one of three groups according to their
   ability to tolerate stress and the predictability of their
   environmental conditions.

   Functional classifications are crucial in modeling
   vegetation-environment interactions, which has been a leading topic in
   vegeation ecology for the last 30 or more years. Currently, there is a
   strong drive to model local, regional and global vegetation changes in
   response to global climate change, particularly changes in temperature,
   precipitation and disturbance regimes. Functional classifications such
   as the examples above, which attempt to categorize all plant species
   into a very small number of groups, are unlikely to be effective for
   the wide variety of different modeling purposes that exist or will
   exist. It is generally recognized that simple, all-purpose
   classifications will likely have to be replaced with more detailed and
   function-specific classifications for the modeling purpose at hand.
   This will require much better understanding of the physiology, anatomy,
   and developmental biology than currently exists, for a great number of
   species, even if only the dominant species in most vegetation types are
   considered.

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