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Figure 7.5. Tree ring scars from landslide impacts (after Lang et al. , 1999 ).
event can still be made. If the outermost ring in the sample is not preserved, the
youngest remaining tree ring can give an estimate of the maximum age of the
movement. A minimum age for the event can be determined by establishing
theage of the oldest ring found in a tree established after the landslide event
(Lang et al ., 1999). In this fashion the age of the event can at least be bracketed.
Tree-ring data are collected by taking cores through the tree trunk. The loca-
tion of trees needs to be plotted on a topographic map, and the direction and
degree of any stem tilting and the direction of the core sampling should be
noted. Core samples are usually subjected to standard analysis procedures such
as sample surface preparation, skeleton plots, cross dating and ring-width mea-
surements with a micrometer to construct growth curves. The latter can also be
cross checked statistically using computer software (Fantucci and Sorriso-Valvo,
1999). Tree analysis or dendrochronology is most suitable when applied to the
analysis of the frequency and dynamics of shallow landslide events of a sudden
and episodic nature, such as debris flows and rock falls, or rotational slides and
mudslides that are reactivated within intervals of several years. Data recorded
from continual or seasonal mass movements provide less accurate estimates of
thetime of the event (Lang et al ., 1999)
Cores can also be subjected to a method called visual growth analysis. This
procedure is used to identify sudden increases or decreases of growth in the
sample cores. A sudden decrease of ring growth is referred to as 'suppression',
and a sudden increase of ring growth is known as a 'release' (Figs. 7. 6 and 7. 7 ).
Growth anomalies found during this process are used to create a graph which
is used to calculate an anomaly index (It) for each of the event-response cases
(Fig. 7. 8 ). The anomaly index considers the intensity of suppression as expressed
byacoefficient of intensity suppression, the number of observed rings affected
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