DENDROCHRONOLGY

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DENDROCHRONOLGY
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General Principles

• In most trees, new water and food-conducting
  cells are added to the outer perimeter of the
  trunk at the start of each growing season
  following an inactive period in winter.

• Early in the growth season there is more demand
  for water so the cells tend to be larger in
  spring than late summer.

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General Principles

• As the cells get smaller, their walls get larger
  and form a distinct line between annual growth of
  wood.

• These lines (tree rings) allow the age of the
  tree to be established.

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General Principles

• Tree-ring width is seldom uniform. It is
  affected by a range of environmental factors and
  there are variations between species.

• The most important factor for tree ring growth
  is climate. The more favourable the conditions,
  the better the growth rate and the wider the tree
  rings.

• Determining climatic variations using tree ring
  growth is called dendroclimatology.

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Dendrochronological Procedures - Measurement

• Dead trees can be cut so that a complete cross
  section can be examined.

• Living trees can be sampled using a metal corer
  that extracts small diameter cylinders of wood
  from the tree trunk.

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Crossdating

• This is the technique of matching rings within
  trees from certain geographical areas.

• Distinctive rings, or groups of rings form
  markers which can be used to match trees with
  overlapping age ranges.

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Standardisation

• Trees grow faster when they are younger so there
  is usually a reduction in ring width with age.

• Each tree ring series is therefore standardised
  by changing the measured ring width values to
  ring width indices.

• This makes it hard to distinguish whether change
  is due to age or environment.

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Complacent Rings

• A complacent tree ring series is one which shows
  little variation.

• They are not very useful for crossdating because
  there are no distinctive markers.

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Sensitive Rings

• A sensitive series of rings shows a clear
  response to stress.

• Stress depends upon factors such as the slope of
  the ground surface, water-retentive capacity of
  the soil and the relative amount of shade and
  exposure.

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Missing Rings

• Trees are deliberately selected from stressed
  situations.

• There is always the possibility that during
  years of extreme climatic conditions a tree may
  fail to manufacture new cells.

• This is referred to as a missing ring.

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Bristlecone Pine

• Some of the oldest trees in the world grow in
  the American southwest mountains.

• The bristlecone pine can live in excess of 4000
  years. They are characteristically twisted and
  found in dry, rocky sites.

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Bristlecone Pine

• Bristlecone pines have a limited growing season
  of only one or two months per year.

• This produces narrow rings which are very
  sensitive to climatic variations.

• By crossdating between living and dead wood, and
  then between sub-fossil samples, a continuous
  master chronology has been developed which now
  extends back 8681 years.

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Dendroclimatology

• Dendroclimatolgy is the study of past climatic
  conditions using tree rings.

• It allows palaeoclimatic information to be
  precisely dated.

• When rings are very close together it indicates
  that there was not much water available for the
  tree, i.e. drought.

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Dendroclimatology

• Dendroclimatology enables deductions to be made
  about

1. Variations in summer temperature
2. Precipitation regimes
3. Linkages between glacier behaviour and climate
4. Relationships between climatic change and late
   Holocene volcanism.

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Autocorrelation

• The relationship between tree rings and climatic
  conditions can be affected by a lag time.

• Trees can store food reserves and water for a
  number of years and so the stored material could
  be used during adverse years.

• The width of a tree ring for a particular year
  is determined by environmental factors from both
  that year and previous years.

• A series of annual ring widths is autocorrelated
  using sophisticated statistical techniques.

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Bibliography

• Reconstructing Quaternary Environments by
  J.J.Lowe and M.J.C.Walker
• Geography An Integrated Approach by David
  Waugh
• Advanced Geography by David Redfern and Malcom
  Skinner
• www.sonic.net/bristlecone/dendro.html
• Presentation by Tom Cleverley