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How diverse are extreme sites? How diverse can the species be, and their
adaptations and life strategies? How specialized do the species have to be,
which ecological amplitude can / must species on extreme sites possess? The
term extreme site is used without a clear definition in literature. This
paper gives a definition and aims at answering the questions mentioned above
with the help of examples, which are focussed on the ecology of trees.
A definition of the term ,extreme site' can be found in relation to Larcher'
s biological concept of stress: extreme sites are thus sites where one or
more factors of the local setting of environmental factors are under- or
overrepresented to an extent that the organisms inhabiting it can live there
only with the formation of special adaptations.
Such sites generally may be found in polar or alpine deserts, dry deserts or
inselbergs which normally are not colonized by trees. Ecosystems with
extreme factors where trees dominate, e.g. gallery forests in arid zones or
mangroves along tropical coasts, normally show a low diversity of species
and growth strategies.
Amazonian floodplains also show all characteristics of extreme sites. A very
long uninterruted period of flooding with a high amplitude, rapid changes of
water level, anoxic conditions in the rhizosphere, high sedimentation in
Várzea, lack of nutrients in sediment-poor Igapó, high mechanical stress and
even drought lead to difficult conditions for growth of all organisms living
in this ecosystem. Nevertheless, Amazonian floodplain forests have a very
high diversity of species and life strategies, also within the
phanerophytes. This diversity may result from the fact that this extreme
site has only a reduced number of factors representing stress for the trees,
and the regular disturbances may even represent a driving force for
resistance and adaptive evolution. The high complexity of the system and the
short but regular occurrence of factors enhancing tree growth allow the
formation of highly diverse survival strategies. The species are seldom
highly specialized - most have high ecological amplitudes. In these
floodplains, specialization is low enough to allow the trees to react to
changing environmental conditions, and is high enough to allow a high
diversity along the hydric and edaphic gradient.
