To fix the place of natural vegetation units in the scheme of life on earth we begin with the total system, or biosphere, which encompasses the entire near-surface zone of the earth favorable to life in one form or another. The biosphere may be subdivided into three environmental divisions, or cycles:
(l) salt water (oceans),
(2) fresh water (stream, lakes, ponds), and
(3) land (soil and the air in contact with it).
We are interested primarily in the land cycle, although the geographer's concern extends into the fresh water and salt-water cycles as well.
The land cycle is divided into ecological system of decreasing size and complexity, schematically. Four great biochores comprise the land cycle
(a) forest,
(b) savanna,
(c) grassland, and
(d) desert.
These four classes of vegetation are separated on the basis of the structure of the plant assemblages and represent the fundamental response of vegetation to global climate controls principally available moisture (precipitation and evaporation), but secondarily light, heat, and winds.
The four biochores comprise systems too vast in area and having too wide a variation within each biochores to use in the mapping of world vegetation. Therefore, within the biochores are formation classes- altogether from 15 to 20 or more in number, depending upon the judgment of the plant geographer whose classification system is used. Thus there are clearly several varieties of forests (e.g. rainforest, deciduous forest, needleleaf forest each responding to the climate regime to which it is subjected. The distinction among the several biochores and their included formation classes is made on the basis of the plant structure, rather than species. Thus classification is structural, rather than floristic.
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