Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 40

are particularly useful climatic proxies when they are near their climatic limits of
either rainfall or summer warmth as then it is possible to attribute variability in ring
thickness with one, or other, of these climatic parameters. Conversely, trees in the
tropics do not exhibit a pronounced annual cycle and so it is not possible to obtain
climatic information from their rings.
Where possible, rings not only serve as climatic indicators but are also used in
dating. Sections of tree trunks can be compared with others from either slightly
younger or older trees in the region, and then the overlap in each tree record can
be used to extend the overall ring record backwards or forwards in time, as Andrew
Douglass did. Indeed, starting from recent trees in the time of meteorological records it
is possible to make a calibration relating tree rings from appropriate samples directly
to either rainfall and/or summer warmth back to centuries before meteorological
records began. Starting from modern times and using overlapping sequences of ring
it is possible to work back in time not just centuries but even, in some cases, a few
thousand years. Such is the success of dendrochronology that it has not just been of
use to palaeoclimatologists but also to archaeologists wishing to date local wooden
artefacts. Palaeoclimatologists who use dendrochronology have created a number of
standard sequences against which new samples can be checked. Dendrochronology
has proved particularly useful in understanding the palaeoclimate of part of our current
interglacial, the Holocene (the time since the end of the last glacial to the present;
see Chapter 4), which itself has so far lasted more than 11 700 years (Briffa, 2000).
Further, more detailed information than just the nature of a particular season can
be obtained by a closer examination of the wood structure within individual rings.
Parameters such as maximum latewood density, minimum earlywood density and
width of early- and latewood growth can provide insight into weather fluctuations in
the principal growing season.
There are a number of tenets underpinning dendrochronology. The main ones are
described here.
The uniformitarian principle: variability evident in tree-ring growth can be linked
to past environmental variability and used to identify environmental change.
The limiting-factor principle: the fastest rate that plant growth (and other processes
such as seed production) can occur is equal to the greatest limiting factor.
The principle of ecological amplitude: tree species will be most sensitive to envir-
onmental flux at the latitudinal and altitudinal limits of their range.
The principle of aggregate tree growth: any individual tree-ring growth series can be
broken down into an aggregate of environmental factors, both natural and human,
that have affected the patterns of tree growth over time (and these might include
age-related growth trends, the climate that occurred over the course of the year and
various other factors that occur within and outside of the forest stand).
The principle of cross-dating: matching patterns in tree-ring width, density and
other characteristics across several tree-ring series allows for the identification of
the year in which the growth ring was formed.
The principle of site selection: sites can be identified based on the criteria that will
produce growth-ring series that are most sensitive to the examining factor.