Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 44

equilibrium or disequilibrium with the climate. The disequilibrium model is
based on the assumption that physical barriers and different spread rates cause
migrational lags. Tinner and Lotter (2001), in their study of rapid changes in vege-
tation composition at 8200 years BP based on pollen counts from sediment cores
from lakes in Switzerland and Germany, have attempted to address this issue by
comparing corresponding records from the Greenland ice cores. Both the Green-
land ice core and European lake chronologies indicate an abrupt climatic change
around 8200 years BP (towards a cooler climate). Tinner and Lotter's (2001)pollen
records indicate a substantial vegetation change for the same period, suggesting
that the vegetation regime at that time, and at their study sites, was in equi-
librium with climate change i.e. there was little or no lag time. Their pollen
analysis was based on the presence or absence of Corylus ,aspecies also found
in present vegetation regimes in Central Europe. The period around 8200 years
BP was also found to coincide with the appearance of previously absent or rare
taxa. Tinner and Lotter estimated the vegetational lag in response to climate
change in this case to be no more than a few decades.
Another issue arises from the Tinner and Lotter (2001)study. Their results
indicate that it would be easy to suggest that Corylus is affected (i.e. decreases
in abundance) by colder temperatures and that climatic change was the spe-
cific factor behind the vegetation changes that occurred simultaneously with
asignificantly cooler climate. This is unlikely, however, as Corylus is presently
found within the climatic range estimated for this cooler period around 8200
years BP. The decrease of Corylus is more likely to be the result of inter-specific
competition. Dry weather conditions and drought are usually accompanied by
areduction of the radial and height growth of vegetation along with foliage
loss. This causes tree crowns to thin resulting in more light reaching the under-
storey of the forest. Tinner and Lotter (2001)havehypothesised that drought
was common prior to 8200 years BP which favoured the light dependent Corylus .
When drought stress decreased as a result of cooler temperatures, taller growing
trees were favoured and out-competed Corylus ,because it is a shade-intolerant
species. These findings emphasise the complexity of using pollen records and
other biological data to estimate past climate change.
It is not always possible to rule out human interference in vegetation composi-
tion when pollen analyses are used to estimate prehistoric precipitation regimes.
If human interference has occurred, the pollen analysis will not describe a nat-
ural chronology. Although this may pose a problem in some studies, it has
been assumed that such interference would result in a continuously decreasing
trend in species composition rather than a cyclic trend. Cyclic trends that range
over longer time spans are therefore often assumed to be natural chronologies
(Bonnefile and Chalie, 2000).