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Zusammenfassung:
How diverse are extreme sites? How diverse can the species, and their
adaptations and life history stratgies be on extreme sites? How specialized
do species need to be, which ecological amplitude can / must the species
possess on extreme sites? The term ,extreme site' is frequently used in
current publications, but its delimitation and definition are not clear. In
this paper, the above mentioned question shall be analysed with examples
which are focussed on the ecology of trees. A definition of the term
,extreme site' is found basing on the biological stress concept: resulting
from cumbering climatic conditions, reduced nutrient availability,
biological competition and damage, plants are subjected to a variety of
strains for which the term stress is used. Extreme sites are therefore
environments in which one or more factors are over- or underrepresented in a
manner that the organisms can live there only if they possess special
adaptations. These environments normally are represented by polar deserts,
arid deserts, alpine environments or inselbergs which are not colonized by
angiosperm trees. Ecosystems with extreme conditions where non-coniferous
trees dominate, for example gallery forests in deserts and savannas, or
mangrove forests along tropical coasts, normally represent forests with low
diversity of species and a low variety of growth patterns.
Amazonian floodplain forests show all the characteristics typical for a
stressful extreme site: uninterrupted flood duration with high amplitudes,
rapid changes of water levels, anoxic conditions in the rhizosphere, high
sedimentation in várzea, nutrient scarcity in sediment-poor igapó, high
mechanical stress, sometimes even drought contribute to make growth and
establishment difficult for most organisms.
Despite these strongly restricting environmental conditions, Amazonian
floodplain forests are characterized by an extremely high diversity of
species and life history stategies, even within phanerophytes. This
diversity results from the fact that the stressors in this extreme site
possess only a low level of restriction for tree life, and disturbances may
even represent an enhancing factor for resistance and adaptive evolution.
The high complexity of the system and the short but regular presence of
factors favourable for tree growth allowed the evolution of highly diverse
survival strategies. The species living there partly are highly specialized,
but most have rather high ecological amplitudes. One may say, the ecological
specialization of the trees is low enough to be able to react flexibly to
changing environmental conditions and guarantee survival. On the other hand
specialization is high enough to allow a high diversity and establish
different species along hydric and edaphic gradients.
