| The raw ring-width
measurements from each core collected at one ecologically homogeneous sampling
site are combined into a site chronology, which reflects the common tree
growth variations at the site over time. However, the ring measurement series
for each core must first be standardized before they can be combined into
a site chronology. Standardization is necessary for several reasons. First,
individual trees grow at different rates, and so faster growing individuals
with higher absolute ring widths would dominate a simple average of ring
widths. Second, most individuals put on smaller rings later in life, and
thus a simple average of growth rings would tend to retain the long-term
ring-width decline due to increasing trunk circumference of the trees, instead
of reflecting a desired signal such as climatic variations. Finally, the
process is used to remove variations in growth due to factors unrelated
to the desired signal. For example, low frequency variations in growth due
to disturbances such as fire or insect infestations can be removed when
assessing climatic variations.
The standardization process involves fitting a smooth curve to the ring-width
series and then dividing each ring width value by the corresponding curve
value to produce a series of ring-width indices with a stationary mean.
This process allows samples with large differences in growth rates to
be combined, and can be used to remove any undesired growth trends present.
For example, a typical sample might display exponentially declining growth
with age, the classic biological growth curve. Standardizing this sample
using a negative exponential function results in data values that represent
the departure from the expected biological growth trend for each year.
Indices from numerous trees can then be combined into a site chronology.
The chronology is a time series of indices that represent the departure
of growth for a given year from the series mean. Higher or lower values
for a given year represent proportionally higher or lower tree growth
for that year.
Graphic by Connie Woodhouse, based on Fritts 1976 fig. 1.9
NOAA Paleoclimatology Program
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