Topics, Tools and Techniques in
Paleoclimate Research
Dendroclimatology
Speaker: Malcolm Hughes, LTRR
March 31st 2004
Summary prepared by Leslie McCluskey
The lecture by Malcolm Hughes was dedicated to how dendrochronology is
done and why it is valid. As in the assigned reading (Hughes 2002),
there was a concentration on the specific strengths and weaknesses of
dendrochronology/climatology. We started by learning about the biology
and physiology of tree growth and tree-ring formation. Then we had a
crash course in cross dating. Hughes spent a good deal of time going
over the justification for dendroclimatology and its applications.
Finally, he concluded by
discussing the Vaganov-Shashkin model of tree-ring growth as was
covered in Vaganov et al (1999).
Tree rings are able to be seen because of variation in cell size of
light-colored earlywood and darker latewood. During peak growing season,
a tree will produce wide cells of low density. These are the light
portions of the tree rings. When conditions for growth become more
limiting (according to available water, temperature, amount of
daylight, etc.), cells become smaller in cross-section and, therefore,
more dense. The close proximity of tree cell walls is what gives the
darker color.
These seasonal variations in tree ring growth are the basis of
cross-dating and dendroclimatology. All kinds of measurements can done
to help build chronologies and rebuild climate. Anything from ring
width to ring density to cell size to isotopic compositions have been
used for these purposes in the past. Cross dating is done by matching
patterns of thin and wide rings in different trees. This can be applied
to climate if patterns of growth are similar over a wide geographic
range. Having large-scale consistencies implies
that climate drives the similarities.
The inherent strengths of tree rings in climate reconstruction are that
1) their dating is both accurate and precise and 2) they exhibit linear
relationships with climate variables. The weaknesses of tree ring based
studies of climate include issues of number of years in a sample and
related loss of long-term variability by data detrending. Hughes,
however, believes that some records with especially long individual
members are able to show century-scale variability and perhaps beyond.
The conclusion of our discussion focused on the application of the
Vaganov-Shashkin model that imitates tree-ring formation through the
growth processes of cell division, expansion and maturation. Though
there was not enough time to get into deep detail Hughes showed us how
well it can be applied, even to trees that are not particularly limited
by their environment.
Back
to TTT04 syllabus.