<P> 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 incorporates the names of the dominant one to three (usually two) species of a 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", referring to Pacific madrone - tanoak forests which occur in California and Oregon, USA . 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 . </P> <P> Victoria in Australia classifies its vegetation by Ecological Vegetation Class . </P> <P> Like all the biological systems, plant communities are temporally and spatially dynamic; they change at all possible scales . Dynamism in vegetation is defined primarily as changes in species composition and / or vegetation structure . </P> <P> 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 wildfires, 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 composition very quickly and for long time periods, and they can do so over large areas . 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 seeds, spores, and living meristems representing the potential next generation, and because of fire's impact on fauna populations, soil characteristics and other ecosystem elements and processes (for further discussion of this topic see fire ecology). </P>

What kind of vegetation is found in north america