In describing the four great biospheres, emphasis has been placed on the vast range of climates spanned by each. Essentially, the biospheres are determined by the degree to which moisture is available to plants in a scale ranging from abundant (forest biochore) to almost none (desert biochore). But within each biochore conditions of temperature are vastly different from low to high latitudes and from low to high altitudes. Consequently there is need to subdivide each biochore into a number of formation classes.
The formation classes. The description of world vegetation in terms of formation classes was first developed fully by Professor A.F.W. Schimper whose monumental work in two volumes entitled Plant Geography on a Physiological Basis was first published in German in 1903. This work was subsequently revised by Professor F.C. von Faber and published in 1935. The classification system described below follow in many respects the recent work of Professor Pierre Dansereau and is based essentially on Schimper's and Rubel's principles.
The eighteen formation classes listed in this chapter are all included within Rubel's ten map units so that in certain eases two to four formation classes are combined into a single map unit. The map is thus simplified in appearance and brings out broad world patterns that are obviously related to the climate regimes.
The table lists eighteen formation classes within the four biospheres.
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