Geoscience Reference
In-Depth Information
ring series, or 'chronology,' is equally essential as a standard for dating local wood
of unknown dates and as an estimate of ring growth as influenced by climate. In both
cases, sites, species, and trees are sought in which the ratio of the climate (common)
signal to non-climatic (primarily individual) noise is as great as possible.
It would make little sense to drill an ice core in a situation where stratigraphy
is likely to be distorted—or even inverted—to sample lake sediments in locations
where the record is prone to earthquake-induced slumps or extensive bioturbation,
or indeed to use meteorological station data without the application of appropriate
quality control standards and procedures for homogenization. In the same spirit,
strategies have been developed for selecting the sites, species, variables, and indi-
vidual trees most likely to show strong, consistent ring variability common to all
trees, and hence a clear climate signal in their rings, and for rejecting those where
there are a priori reasons to expect strong non-climatic influences that cannot be dis-
entangled. The aim in this case is not to take a representative sample of the trees of
the forest, but rather to maximize climate signal and minimize non-climatic noise.
A quite different sampling protocol would be used if the aim were to take a
representative sample of the forest. An optimal record of summer temperature is
likely to come from regions with cool, moist summers where drought influence on
wood growth is minimal. Within such regions, sites at upper elevations close to the
upper or poleward tree limit are likely to reflect regional temperatures, rather than
the peculiar regime of a particular valley. In cases where the uppermost or highest-
latitude trees have a stunted or dwarf growth form, they would not be sampled, as
their wood anatomy is strongly influenced by mechanical rather than climatic forces.
Individual trees with a lean, lightning damage, or other abnormal morphology would
also be avoided. A site from which a chronology is to be developed should be as
uniform as possible with respect to the small-scale conditions affecting tree growth,
such as aspect, slope, and substrate. Tree species produce ring series with charac-
teristic properties, and this too would be taken into account in designing a sampling
program.
Differing properties of tree rings reflect different climate signals, according to
the growth situation. Maximum latewood density of conifer tree rings in regions
with cool, moist summers is usually a better proxy for summer temperature than is
ring width from the same trees, for example. In regions of Mediterranean climate,
ring width may be the best available proxy for growing season soil moisture, or
for winter half-year precipitation, which is possibly the main determinant of early
summer growth. These considerations concerning the location and design of tree-
ring sampling are analogous to those that apply in choosing the best place to drill a
lake or a glacier.
This approach to sample design is almost universally applied 'a priori' to the
sampling of tree rings. Once the samples and data exist, the strength of the common
signal within the tree-ring dataset from an individual site is analyzed by using
information on the number of samples and the correlations between them (within
the tree-ring data, not with a climate variable), and a decision is made whether
or not to use the existing data for dendroclimatology, to reject it on the basis of
a weak common signal, or to seek more samples to strengthen the signal (Wigley
