Paleo Slide Set: Tree Rings: Ancient Chronicles of Environmental Change
Plot of several individual ring-widths series illustrating the need for standardization.

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 standardization 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.

Photo Credits:
Graphic by Connie Woodhouse, based on Fritts 1976 fig. 1.9
NOAA Paleoclimatology Program