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adjacent rock detritus and soils are moved, fresh substrate is colonised by new
lichens. These periodic landslide events are usually recorded in the complex age
differences of several colonies of lichen. The technique is limited where younger
deposits cover older blocks, as the older surfaces cannot be sampled. Hence, bias
against older deposits can occur and this can create difficulties when aiming to
derive a magnitude--frequency analysis of events at a particular location.
Site selection, therefore, is important in the quest to obtain valid results,
particularly where analysis of closely spaced events is desirable. Bull and Bran-
don (1998)note that several factors are important when choosing sampling
sites. These include the diversity and frequency of geomorphic processes, lichen
species and abundance, quality of thalli, substrate smoothness, the size of rock
fall blocks and the ability to recognise old, stabilised block fields where lichen
communities are not related to the times of substrate exposure because the first
generation of lichens have died and been replaced. The latter generally provide
minimal chronological information. First generation lichen communities are
usually recognised by nearly circular, isolated lichens whereas lichen colonies
that were established after the initial event are recognised by large thalli with
highly irregular margins or a mosaic of thalli that have grown together. Slower
growing lichens are usually preferable for dating prehistoric events.
There is no one specific method within the field of lichenometry. Traditional
methods usually estimate the age of a geomorphic event by the size of the
single largest lichen in a deposit or the mean size of the five largest lichens.
The largest lichen, i.e. in terms of lichen radius or longest axis, is deemed to
be the largest lichen found within a particular unit of time (e.g. 1 h). Bull
and Brandon (1998)developed an alternative method based on populations of
large lichens on several blocks within a deposit. The technique is known as the
FALL or fixed-area largest-lichen method. The approach is based upon the style
of sampling strategy employed and requires sufficient cobble and boulder size
blocks in a sampling area to allow analysis of a number of individual lichen
colonies. The FALL method measures the longest axis of the largest lichen in
each of 100 or more sample sites of about the same size, typically 1 m 2 . This
sampling strategy allows the averaging of the effects of locally variable colonisa-
tion times and growth rates, taxonomic misinterpretation and inherited lichens
that have merged to form composite thalli. Ideally, the sampling strategy should
aim to minimise inherent variability in measurements by considering the sam-
pling area and the density of lichen thalli in each area. Thus, sampling will
ideally only include the largest isolated lichen in a number of sampling areas
where the conditions for colonisation and growth are near identical but oth-
erwise independent of each other. This is an ideal sampling strategy; however,
actual variables may differ between sites depending on differing conditions. FALL
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