G4
M. E. D. POORE
Clements and Tansley classify according t o the climax or climax
mosaic, and thereby combine in one hierarchy communities which are
poles apart ecologically, and separate those which are ecologically
similar.
The physiognomic-ecological classification seems to be the most
natural for our purpose, as the physiognomy and structure of the
phytocoenose seem t o reflect rather faithfully the sum total of
the ecological factors of the habitat. I would suggest therefore that the
major units of a vegetational classification should be physiognomic,
while the lower units may be arranged in floristic series. It is perhaps
notable that this is the treatment which Curtis has followed in his
Vegetation of Wisconsin. I would like also t o suggest some tentative additional reasons why physiognomic units may be the most
satisfactory.
It is very striking that similar structural patterns occur under
similar environments in different parts of the world, even when there are
great geographical discontinuities between them and the floras are very
different. When the same structural pattern extends over large areas of
country it is noteworthy that there is a gradual change in the floristic
composition of the community without a change in structure. Lippmaa
( 1939) has worked this out in detail for the Galeobdolon-Asperula-Asarurn
union of the Baltic deciduous forest region; Poore and McVean (1957)
describe such series across Scotland from the oceanic west t o the more
continental east - series which continue into Scandinavia. The place
occupied by one species in the structure of one community is exactly
filled by an equivalent species of the same life form in another. The
two species may be of the same genus, but may be of related genera if
the distance between the communities is great.
This suggests that there are only a limited number of structural
patterns which occur in vegetation, and that these have considerable
inherent stability. It is certainly common experience that ecotones
between communities that are structurally different are much more
abrupt than between structurally identical communities. Also, although
mosaics may occur composed of communities which have a different
physiognomy, true intermediates rarely occur. Those stages of a succession, too, which involve a total change of structure, are usually
relatively rapid.
VI. DISCUSSION
The structure of vegetation is very complex and its variation in
space and time is determined by a multiplicity of factors or factor complexes whose effects and interactions are often little understood. Hence
experiment, the normal tool of the scientist, is of limited usefulness.
M. E. D. POORE
Clements and Tansley classify according t o the climax or climax
mosaic, and thereby combine in one hierarchy communities which are
poles apart ecologically, and separate those which are ecologically
similar.
The physiognomic-ecological classification seems to be the most
natural for our purpose, as the physiognomy and structure of the
phytocoenose seem t o reflect rather faithfully the sum total of
the ecological factors of the habitat. I would suggest therefore that the
major units of a vegetational classification should be physiognomic,
while the lower units may be arranged in floristic series. It is perhaps
notable that this is the treatment which Curtis has followed in his
Vegetation of Wisconsin. I would like also t o suggest some tentative additional reasons why physiognomic units may be the most
satisfactory.
It is very striking that similar structural patterns occur under
similar environments in different parts of the world, even when there are
great geographical discontinuities between them and the floras are very
different. When the same structural pattern extends over large areas of
country it is noteworthy that there is a gradual change in the floristic
composition of the community without a change in structure. Lippmaa
( 1939) has worked this out in detail for the Galeobdolon-Asperula-Asarurn
union of the Baltic deciduous forest region; Poore and McVean (1957)
describe such series across Scotland from the oceanic west t o the more
continental east - series which continue into Scandinavia. The place
occupied by one species in the structure of one community is exactly
filled by an equivalent species of the same life form in another. The
two species may be of the same genus, but may be of related genera if
the distance between the communities is great.
This suggests that there are only a limited number of structural
patterns which occur in vegetation, and that these have considerable
inherent stability. It is certainly common experience that ecotones
between communities that are structurally different are much more
abrupt than between structurally identical communities. Also, although
mosaics may occur composed of communities which have a different
physiognomy, true intermediates rarely occur. Those stages of a succession, too, which involve a total change of structure, are usually
relatively rapid.
VI. DISCUSSION
The structure of vegetation is very complex and its variation in
space and time is determined by a multiplicity of factors or factor complexes whose effects and interactions are often little understood. Hence
experiment, the normal tool of the scientist, is of limited usefulness.