Principles of DendrochronologyPrinciples of Dendrochronology

1. Uniformitarianism Principle

• James Hutton, British geologist (published 1785–1788)

• “The present is the key to the past.”

• 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!

Examples:

A. Reconstructions of past climate = Dendroclimatology

Assumes that climatic processes operating today were operating similarly in the past.

PresentPast

Future?

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.

PresentPast

Future?

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.

1. Uniformitarianism Principle

2. Principle of Limiting Factors

• Basic principle in biology

• Adaptation to dendrochronology:

• “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.

Narrow ring, thin Narrow ring, thin latewoodlatewood

Wide ring, thick Wide ring, thick latewoodlatewood

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.

NN

S S

E E W W Optimal growth conditions, reduced sensitivityOptimal growth conditions, reduced sensitivity

Stressful locations, increased sensitivityStressful locations, increased sensitivity

Stressful locations, increased sensitivityStressful locations, increased sensitivity

• Latitudinally and longitudinally:

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

Good

Not so good

4. Principle of Site Selection

• 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.

Valley bottom: not good

Slope: better

Notice vastly different ring patterns

Notice vastly different growth forms of these trees

Thick soil: not good

Thin soil with rocky substrate: better

4. Principle of Site Selection

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

Five feet tall, broken top, inside ring = AD 1406

El Malpais National Monument, NM

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

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

• Tree growth can be “decomposed” into five basic parts:

Ideal age trend

Disturbance!

5. Aggregate Tree GrowthIdeal age trend

Release from

logging

Death of nearby tree

Release after wildfire

5. Aggregate Tree GrowthIce storm

6. Principle of Replication

• 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.

• Follows the basic statistical rule: INCREASE YOUR SAMPLE DEPTH !!! MORE IS BETTER !!!

6. Principle of Replication

7. Principle of Crossdating

• 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 micro-environmental 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

1793 = wide ring with thick latewood

1806 = narrow ring (absent on bottom)1806 = narrow ring (absent on bottom)

1816 Year Without a Summer wide ring

1816 Year Without a Summer wide ring

1840 = wide ring1840 = wide ring

1847 = verynarrow ring1847 = verynarrow 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.