Provided that one life-form predominates, communities, floras, or climatic zones can be classified to reflect ambient climatic conditions. In general, the deserts are dominated by theraphytic floras, the tropics are phanerophytic in composition, alpine and arctic climates are dominated by chamaephytic species, and most temperate regions are hemicryptophytic in composition. Chamaephytes show steady increase in number with altitude. Classifying climatic regions using subdivisions within each of the five life-form classes is also possible, for example, evergreen megaphanerophytes without bud scales are abundant in tropical rainforests, and deciduous nanophanerophytes with bud scales are typically found in deserts and shrub steppes.
However, no geographically broad environment is characterized by a single life-form, just as no single lifeform is associated exclusively with a single habitat. From its inception, Raunkiaer intended that his classification system would provide a basis for statistically comparing different floras according to their life-form 'spectra', that is, the percentage distributions of species sorted into the five major life-form classes. To provide a yardstick with which to make these comparisons, Raunkiaer selected 1000 plant species from across the globe to produce a random sample of species that, once sorted into the different life-form categories, produces what Raunkiaer called the 'normal spectrum'.
Based on this analytical approach, the normal (worldwide) spectrum consists of 46% phanerophytes, 9% chamaeophytes, 26% hemicryptophytes, 6% cryptophytes, and 13% therophytes. Within this spectrum, all gymnos-perm species and the majority of dicot species are phanerophytes; the majority of monocot species are hemicryptophytes. Based on extensive comparisons between this normal spectrum and the life-form spectra of numerous floras, Raunkiaer adduced four major phytoclimatic types: (1) the phanerophytic climate of the warm humid topics, (2) the hemicryptophytic climate of the mid-latitudes (including both needle-leaved and deciduous forests as well as the moister steppes), (3) the therophytic climate of tropical and subtropical deserts, and (4) the chamaephytic climate ofhigh latitudes and altitudes.
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