S U C C E S S I V E A P P R O X I M A T I O N I N DESCRIPTIVE ECOLOGY
63
arguments developed above, however, the criteria used will be different
in small and in large regions. Much of the confusion which has arisen in
the classification of vegetation can be attributed t o attempts t o apply
the same criteria t o define all units.
Isoecia and isocoenoses should be distinguished by their physiognomy
and ecology, the remainder of the units by their floristic composition.
The floristic criteria available are the species occurring in a community
and their dominance, constancy and fidelity. Braun-Blanquet (1951)
and Poore (1955~) have given objections to the sole use of dominance,
Gams (1941) and Poore (1955~) to fidelity.
Constancy appears the most unexceptionable criterion, because a
definition based on constancy limits the range of the “nodum” t o that in
which the ecological amplitudes of those species named as constants
overlap. I would suggest as the definition of a “union” or “association”
the following: “A nodum defined in terms of its constant species” arid
any stand would be attributed t o that unit in which more than 80%
of the species named as constant t o the unit were to be found.
In the syiiusial classification this definition would require 110 qualification : in the phytocoenosal classification the condition of constancy
should refer only to the dominant synusia.
F. M U L T I D I M E N S I O N A L V A R I A T I O N
Many authors have concluded that variation in vegetation is multidimensional and that the most satisfactory way of representing vegetational variation in a region is by diagrams illustrating lines of variation (cf. Poore, 1955c; Gams, lY4J. ; Agnew in Greig-Smith, 1957). Units
can alternatively be separated by keys; and it should be possible to
construct two parallel keys in any region, one of vegetation and one
of habitat which coincide with one another. Curtis in The Vegetation
of Wisconsin has constructed keys ; but habitat and vegetation are
used as diagnostic characters in the same key.
Although the relationships of vegetation are undoubtedly multidimensional and it may be very useful t o arrange synusiae and phytocoenoses in diagrams t o illustrate relationships of various kinds, some
sort of hierarchical arrangment is necessary to make the available data
manageable and comprehensible. Several arrangements of this kind
are possible, some of which are more generally useful than others.
The system of Braun-Blanquet is unsatisfactory at the higher levels
because it limits the hierarchy to one Vegetationskreis, and therefore
widely separates communities of equivalent ecology. Van Steenis (1950)
also limits his scheme to one vegetation area but makes his subordinate
classification according to a hierarchy of habitat factors. He is classifying vegetation by a criterion that is not vegetational.
63
arguments developed above, however, the criteria used will be different
in small and in large regions. Much of the confusion which has arisen in
the classification of vegetation can be attributed t o attempts t o apply
the same criteria t o define all units.
Isoecia and isocoenoses should be distinguished by their physiognomy
and ecology, the remainder of the units by their floristic composition.
The floristic criteria available are the species occurring in a community
and their dominance, constancy and fidelity. Braun-Blanquet (1951)
and Poore (1955~) have given objections to the sole use of dominance,
Gams (1941) and Poore (1955~) to fidelity.
Constancy appears the most unexceptionable criterion, because a
definition based on constancy limits the range of the “nodum” t o that in
which the ecological amplitudes of those species named as constants
overlap. I would suggest as the definition of a “union” or “association”
the following: “A nodum defined in terms of its constant species” arid
any stand would be attributed t o that unit in which more than 80%
of the species named as constant t o the unit were to be found.
In the syiiusial classification this definition would require 110 qualification : in the phytocoenosal classification the condition of constancy
should refer only to the dominant synusia.
F. M U L T I D I M E N S I O N A L V A R I A T I O N
Many authors have concluded that variation in vegetation is multidimensional and that the most satisfactory way of representing vegetational variation in a region is by diagrams illustrating lines of variation (cf. Poore, 1955c; Gams, lY4J. ; Agnew in Greig-Smith, 1957). Units
can alternatively be separated by keys; and it should be possible to
construct two parallel keys in any region, one of vegetation and one
of habitat which coincide with one another. Curtis in The Vegetation
of Wisconsin has constructed keys ; but habitat and vegetation are
used as diagnostic characters in the same key.
Although the relationships of vegetation are undoubtedly multidimensional and it may be very useful t o arrange synusiae and phytocoenoses in diagrams t o illustrate relationships of various kinds, some
sort of hierarchical arrangment is necessary to make the available data
manageable and comprehensible. Several arrangements of this kind
are possible, some of which are more generally useful than others.
The system of Braun-Blanquet is unsatisfactory at the higher levels
because it limits the hierarchy to one Vegetationskreis, and therefore
widely separates communities of equivalent ecology. Van Steenis (1950)
also limits his scheme to one vegetation area but makes his subordinate
classification according to a hierarchy of habitat factors. He is classifying vegetation by a criterion that is not vegetational.