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short lived before they are gradually replaced by more mature forest species. The
natural evolution of these units organise the species composition of the forests,
and each of the individual units, around light availability. Pollen can record
changes in the composition of forests which result from extreme events that
result in episodic increases in light levels at the forest floor. A sudden increase
in the abundance of pioneer species in a pollen record for example suggests
that the forest light conditions changed rapidly which may be suggestive of
thedeath of many mature trees during an extreme event. If the region is not
prone to strong winds from tropical cyclones or tornadoes, then changes in
precipitation may be inferred as a possible cause. Pollen records, therefore, can
be used to identify these events and the severity of the event may be registered
by the extentofthechange in the record over time and area.
Van't Veer et al .(2000)usedpollenanalysistoidentify the Younger Dryas in
South America. The Younger Dryas was a sharp and rapid return to cooler and
drier conditions around 12 000--11 000 years BP as climate was slowly warming or
ameliorating after the Last Glacial Maximum. Pollen samples were taken from
sediment cores extracted from the shores of several lakes. Counts of various
pollen species showed a decrease in arboreal pollen around the time of the
Younger Dryas suggesting a decrease in effective precipitation and temperature
similar to that found in pollen species dated to the Last Glacial Maximum.
The palynological interpretations were confirmed by sedimentological evidence
which suggested a coincident lowering of lake levels and development of local
marsh vegetation due to decreased precipitation levels.
Pollen analysis of Hyrax dung has also been used to reconstruct past rain-
fall regimes (Hubbard and Sampson, 1993). In parts of Europe, the amount and
reliability of rainfall affects the relative predominance of grasses to other plants
(perennial grasses versus Karoo shrublets). The content of airborne pollen reflects
the composition of the vegetative ground cover and the pollen content of Hyrax
dung reflects the pollen content in the air. Increased values of composite and
decreased values for grasses indicates a reduction of rainfall and drier than nor-
mal conditions. Pollen records, however, are sensitive to local variations in pre-
cipitation, so it is important to know the seasonal mix of pellets in a fossil dung
sample for this method to provide accurate estimates of past rainfall. The sample
also needs to contain an even mix of seasonal markers before accurate estimates
can be made. The study of modern pollen in Hyrax dung suggests that fossil
dung could be used to provide a useful indicator of past precipitation regimes.
Problems relating to the use of fossil pollen concern the rate at which veg-
etation responds to climate change. Estimates have ranged from decades to
millennia. The main point of argument has been whether vegetation is in
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