Principles of Dendrochronology
1. Uniformitarianism Principle
•
James Hutton, British geologist (published 1785–1788)
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“The present is the key to the past.”
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Corollary to this principle: “The past is the key to the future.”
•
Illustrates the “trajectory of science,” past, present, and future:
A. Study processes as they occur at present
B. Improved understanding comes from the past
C. Extrapolate/predict the future = applied!
1. Uniformitarianism Principle
Examples:
A. Reconstructions of past climate = Dendroclimatology
Future?
Past
Assumes that climatic processes operating today were
operating similarly in the past.
Present
1. Uniformitarianism Principle
Examples:
B. Reconstructions of past fire = Dendropyrochronology
Assumes that
wildfires operate
today as an
ecosystem
process just as
they did in the
past.
Future?
Past
Present
1. Uniformitarianism Principle
In recent years, this principle has come under scrutiny because
research is showing issues in three main areas:
1. The “Divergence” Issue: trees in the higher latitudes are no
longer tracking temperature as they faithfully did prior to 1960.
Basically, since then, temperature has been steadily rising but tree
growth has been declining! What gives?
2. Temporal Instability in Climate Response: dendrochronologists
are finding that the response to climate in many tree species at
many sites has actually been variable over time, this placing doubt
in our reconstructions of climate.
3. No-Analog Conditions in the 20th and 21st centuries: well, guess
what? The “present” we live in is unique (why?), so is it fair to say
now that the present is the key to the past? Our forests have
changed and therefore so have the processes (like wildfire) that
operate in them.
2. Principle of Limiting Factors
•
Basic principle in biology
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Adaptation to dendrochronology:
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“Tree growth can proceed only as fast as allowed by the
primary environmental and physiological mechanisms
that restrict growth.”
•
Sometimes, more than one mechanism operates to
restrict growth.
•
But whichever mechanisms limit growth, they must vary
year to year for tree-ring dating to work.
•
In other words, we MUST have variable tree growth!
•
And we specifically key in on those narrower rings that
indicate limiting conditions.
Wide ring, thick
latewood
Narrow ring, thin
latewood
2. Principle of Limiting Factors
The “low precip – high temp” model:
The “high precip – low temp” model:
2. Principle of Limiting Factors
2. Principle of Limiting Factors
3. Ecological Amplitude
•
Species have well defined ranges based on environmental
controls. (BTW, this is related to what former paradigm in
geography?)
•
A tree species will be more responsive and sensitive to
changes in environmental conditions in the outer limits of its
range.
•
Latitudinally and longitudinally:
Stressful locations, increased sensitivity
W
N
Optimal growth conditions, reduced sensitivity
S
E
Stressful locations, increased sensitivity
3. Ecological Amplitude
Range map of ponderosa pine.
Note: different spatial scales will
help us isolate several locations
where ponderosa pines may be
especially responsive.
3. Ecological Amplitude
Range map of sugar maple.
NOTE: Recent research in the
eastern U.S. is showing that
this principle is less important.
Environmental conditions at
range margins appear to be not
as stressful as conditions that
exist in the western U.S.
3. Ecological Amplitude
•
A tree species will be more responsive and sensitive to
changes in environmental conditions in the outer limits of its
range.
•
Also, elevationally!
Good
Not so good
Good
4. Principle of Site Selection
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Within any given area chosen for study, specific site
characteristics should be sought that will enhance a tree’s
responsiveness to environmental factors.
•
Notice how this is related to the principle of limiting factors.
We should select sites where factors are more limiting.
•
Notice also that recognizing the growth forms of trees will
provide clues where such sensitive sites exist.
4. Principle of Site Selection
Notice vastly
different
growth forms
of these trees
Slope:
better
Valley
bottom:
not
good
Thick soil: not good
Thin soil
with rocky
substrate:
better
Notice vastly
different ring
patterns
What to look for in trees that indicate longevity:
1. Dead spike top or broken
top
2. Heavy, drooping lower
limbs
3. Short stature, inverted
carrots
4. Erratic growth forms
5. Stripbark
6. Sparse foliage in crown
7. Exposed roots
8. Isolated individuals
El Malpais National Monument, NM
El Malpais National Monument, NM
Five feet tall,
broken top,
inside ring =
AD 1406
Alta Peak, Sierra
Nevada, CA
San Mateo Mountains, NM
Magdalena Mountains, NM
El Malpais National
Monument, NM
5. Aggregate Tree Growth
•
Tree growth can be “decomposed” into five basic parts:
R = ring width, t = the current year, and δ = presence (1) or
absence (0) indicator
1. A = age-related trend
2. C = climate
3. D1 = exogenous (external) disturbance processes (examples?)
4. D2 = endogenous (internal) disturbance processes (examples?)
5. E = random error
5. Aggregate Tree Growth
•
Tree growth can be “decomposed” into five basic parts:
Only ONE can be the desired signal. All OTHERS constitute
noise. We wish to maximize the signal to noise (S/N) ratio
(concept borrowed from engineering).
For example, if climate is our desired signal, we must (1)
mathematically minimize the effects of other parts, and (2)
sample to ensure other noise minimizes any affects on tree
growth in our study area.
5. Aggregate Tree Growth
Ideal age trend
Ideal age trend
Disturbance!
Ice storm
5. Aggregate Tree Growth
Release
from
logging
Death of
nearby tree
Release after
wildfire
6. Principle of Replication
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The environmental signal being investigated can be
maximized (and the amount of noise minimized) by sampling
more than one stem radius per tree and more than one tree
per site.
•
Obtaining more than one increment core per tree reduces the
amount of "intra-tree variability" = the amount of undesirable
environmental signal peculiar to only that tree.
•
Obtaining numerous trees from one site (and perhaps several
sites in a region) ensures that the amount of "noise" is
minimized.
6. Principle of Replication
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Follows the basic statistical rule: INCREASE YOUR SAMPLE
DEPTH !!! MORE IS BETTER !!!
7. Principle of Crossdating
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Matching patterns in ring widths or other ring characteristics
(such as ring density patterns) among several tree-ring series
allows the identification of the exact year in which each tree
ring was formed.
•
Both a principle and a technique. Without either,
dendrochronology is unscientific ring-counting.
•
The Principle of Crossdating concerns why trees have the
same ring patterns.
•
The Technique of Crossdating concerns how we can use this
property to (1) ensure we have precisely assigned the correct
calendar year to each tree ring, and (2) at the same time,
account for those problem rings, such as false or locally
absent rings.
•
We’ll cover the Technique of Crossdating later.
• Why does crossdating
work?
• Because trees within
a region will be
responding similarly to
the overall climate
regime in which they
grow.
• Different rates of
growth may occur due
to local microenvironmental effects,
but this does not
matter!
Look at these cores (taken from six different trees) from Mt. Graham in
southeastern Arizona, and pick out the wide and narrow rings they have in
common.
Again, these three cores (taken from three trees growing in El Malpais
National Monument in New Mexico) have ring patterns in common. For
example…
1793 = wide ring
with thick latewood
1816 Year Without a
Summer wide ring
1806 = narrow ring
(absent on bottom)
1847 = very
narrow ring
1840 = wide ring
Notice that crossdating uses both wide rings and narrow rings, although the
narrow rings are (for some reason) easier to visually key in on. These narrow
rings will be used later when we learn the technique of crossdating.