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要旨:
The Amazonian flora has the highest tree diversity on earth: in 1000 km2 of
Terra Firme-forest near Manaus (Brazil) 1300 tree species were identified.
The world record was found in the Peruvian lowland forest near Iquitos with
300 tree species on one hectare. The search for explanations of this high
Amazonian diversity has been a focus of discussions for several decades
already. The refuge theory says that dry periods in the pleistocene caused
the formation of forest refugia within drier savannahs. The high local
endemism and radiation resulting from this led to a high speciation. On the
other hand, the theory of recent speciation says that in situ speciation is
responsible for the evolution of species basing on environmental gradients
and habitat differentiation. The current patterns thus have evolved since -
and not during the pleistocene. The forests often are not uniform, but build
mosaics and clines which allow speciation. Additionally, there is a
divergence of the peripheric, partially isolated forests due to the large
distances in Amazonia. The postulation of an equatorial climate which has
been stable for a long period and led to the gradual accumulation of species
points in the same direction. Today this diversity is endangered by logging
in many regions. In Brazil, which has the largest area of total Amazonia,
only180 Mio. ha of the original 286 Mio. ha remain to date. Intensive
research is being performed to analyse the effects of forest fragmentation.
In long-term studies it became apparent that fragmentation leads to a high
decrease of forest biomass by an increased tree mortality. The regrowing
vines and small trees or shrubs cannot compensate this loss. The whole
species composition changes, and diversity decreases rapidly.
