Jeff Lukas

CIRES Western Water Assessment (WWA)University of Colorado Boulder

Colorado Biology Teachers Association – Spring SymposiumApril 20, 2013 – Boulder, CO

The Long View: What tree rings tell us about climate and water in Colorado

http://wwa.colorado.edu

Outline

1977 1983• How tree rings record climate

information

• How we extract and interpret that information

• What the tree rings tell us about past climate and water in Colorado

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750 1000 1250 1500 1750 2000Water Year

Ann

ual F

low

, MA

F

How can we get more context for “unprecedented” droughts and other events?

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1915 1930 1945 1960 1975 1990 2005

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a l F

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(1

000

acr

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South Platte R., Colorado - annual streamflows

Paleoclimatology: analysis and reconstruction of pre-instrumental climate, using environmental proxies

Lake sediments

Packrat middens

(vegetation) Tree rings(Dendrochronology)

Pollen

Ice coresCorals

Speleothems

Ocean sediments

Key attributes of tree rings as a paleo-proxy for climate and hydrology

• Annual resolution

• Absolute dating to calendar year

• Long, continuous records (200 to 10,000 yrs)

• Widespread distribution

• Straightforward translation into climate variables

Tree rings = a much longer view of past hydroclimatic variability, at annual resolution

Gaged record

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al fl

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Tree-ring reconstruction

Climate is typically the main factor limiting tree growth

• At the highest elevations and latitudes: energy availability (warmth)

• At lower elevations and mid-latitudes: moisture availability

Moisture availability varies greatly from year to year

81012141618202224

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annu

al p

reci

pita

tion,

in.

Annual precipitation, western Colorado 1977

1983

1977 1983

Douglas-fir, south-central CO

So, for most trees across Colorado:Dry conditions = Narrow ring Wet conditions = Wide ring

• This moisture signal integrates both precipitation and evapotranspiration, over a ~one-year period preceding and including the growing season

This moisture signal in trees can serve as proxy for multiple moisture-related variables

Annual or seasonal precipitation

Spring snow-water equivalent (SWE)

Drought indices (PDSI, SPI)

Annual (water-year) streamflow

Same climate influences the growth of all trees at a site = cross-dating

1900 1910 1920 1930

Two Douglas-fir trees near Eldorado Springs, CO

1900 1910 1920 1930

Two Douglas-fir trees near Eldorado Springs, CO

1925

1925

Same climate influences the growth of all trees at a site = cross-dating

Annual growth (ring-width) is not the only tree-ring indicator of climate

• Stable isotopes of carbon (12C, 13C) reflect carbon assimilation and thus moisture status

• Stable isotope of oxygen (18O) reflects temperature of the source water taken up by the tree

• Density of latewood reflects summer warmth in energy-limited trees

• Dry sites up to 9000’ (2750m)• Stands of old-appearing ponderosa pine, pinyon

pine, Douglas-fir• Collect cores from 15-30 trees (same species)

and sample dead wood if present• Cross-date and measure the rings, compile into

a site chronology 300-2000 years long

Collecting moisture-sensitive site chronologies in Colorado

Green Mountain Reservoir (GMR) Douglas-fir chronology (588-2005) (N of Silverthorne)

Living trees back to 1300s AD

Dead wood back to 500s AD

Pump House (PUM) pinyon pine: 1175–2005 (SW of Kremmling)

Eagle (EGL) Douglas-fir: 203–2005 (just outside Eagle)

Which Douglas-fir tree will tell us more about past climate?

Compilation of the site chronology enhances the common moisture signal

Ring

-wid

th in

dex

Van Bibber site, near Golden, Colorado (ponderosa pine)

Robust averaging

Tree-ring chronologies developed at CU from 2000 to 2009 (INSTAAR Dendrochronology Lab)

Over 1800 moisture-sensitive tree-ring chronologies across North America as of 2009 – 100+ in Colorado

Figure: Cook et al. (2009), J. Quaternary Science

Generating tree-ring reconstructions

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1015202530

1900 1915 1930 1945 1960 1975 1990 2005

annu

al fl

ow, M

AF

05

1015202530

1500 1600 1700 1800 1900 2000

Annu

al F

low

(MAF

)

Observed hydroclimate recordSubset of tree-ring chronologies

Best-fit statistical relationship between the tree rings and observations during overlap period

Tree-ring reconstruction of hydroclimate

Streamflow records reconstructed using tree-ring chronologies developed at CU

Streamflow records reconstructed using tree-ring chronologies developed at CU

South Platte at South Platte

(1634-2002)

Colorado River at Lees Ferry, AZ

(762-2005)

Colorado River at Kremmling

(1440-2002)

South Platte at South Platte, CO

Calibration of reconstruction model, 1916-2002

Calibration: R2 = 0.76

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ual F

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AF)

ObservedReconstructed

Full reconstruction of South Platte annual streamflow, 1634-2002

• 2002 is lowest reconstructed flow in entire record, but 1685 and 1851 are very close, within the uncertainty of the reconstruction

• 100-150-year return interval implied for 2002-type flow years

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1630 1660 1690 1720 1750 1780 1810 1840 1870 1900 1930 1960 1990

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Full reconstruction of South Platte annual streamflow, 1634-2002, with 4-year running average

• 1953-1956 was 3rd lowest reconstructed 4-year flow since 1634 (lowest: 1844-48)

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1630 1660 1690 1720 1750 1780 1810 1840 1870 1900 1930 1960 1990

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(KA

F)

Colorado at Kremmling, CO

Calibration of reconstruction model, 1916-2002

Calibration: R2 = 0.70

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1915 1930 1945 1960 1975 1990 2005

Ann

ual F

low

(K

AF)

ObservedReconstructed

Full reconstruction of Colorado at Kremmling annual streamflow, 1440-2002

• 2002 is 9th-lowest reconstructed annual flow since 1440 • 30-50-year return interval implied for 2002-type flows (but not evenly

distributed)

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1440 1480 1520 1560 1600 1640 1680 1720 1760 1800 1840 1880 1920 1960 2000

Ann

ual F

low

(KA

F)

Full reconstruction of Colorado at Kremmling annual streamflow, 1440-2002, with 4-year running average

• 1953-1956 exceeded in severity by 13 other 4-year periods• 1844-1848 was lowest 4-year flow by a large margin

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1,200

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1440 1480 1520 1560 1600 1640 1680 1720 1760 1800 1840 1880 1920 1960 2000

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ual F

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Full reconstruction of Colorado at Kremmling annual streamflow, 1440-2002, and South Platte, 1634-2002 (4-year running averages)

• Streamflows (and trees) in the Colorado headwaters closely track those in the South Platte headwaters since they mainly experience the same weather events

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Tree-ring reconstructed annual flows, Colorado River at Lees Ferry 762-2005, with 20-year running average

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Tree-ring reconstructed annual flows, Colorado River at Lees Ferry 762-2005, with 20-year running average

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Mid-1100s megadrought

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1120 1130 1140 1150 1160 1170 1180

Annu

al fl

ow, M

AF 46 dry years in 57-year period

• Only the reconstructed 14th century averaged flow was higher than the average since 1900

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al F

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Colorado River at Lees Ferry, AZ – reconstructed annual flows, 762-2005: 100-year averages

Observed average, 1906-2012

Even if anthropogenic climate change were not occurring, we’d want to prepare for droughts worse than any modern (>1900) droughts

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Water Year

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ual F

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Denver Water: water supply yield analyses with South Platte and Colorado at Kremmling flow reconstructions, 1634-2002

Reservoir contents with 345 KAF demand and progressive drought restrictions

The ongoing and projected warming alone will reduce streamflows and make droughts worse

1950 2000 2050 2100

50°F

60°F

North-central Colorado mean annual temperature Ensemble of 16 GCMs, medium emissions scenario

Observed temp.

Source: Marty Hoerling, NOAA ESRL PSD

Future precipitation trend is unclear, but unlikely to compensate for the warming

1950 2000 2050 2100

24”

12”

Observed precip.

Source: Marty Hoerling, NOAA ESRL PSD

North-central Colorado mean annual precipitation Ensemble of 16 GCMs, medium emissions scenario

From Smith et al, 2009.

Worst case scenario: A “dry” GCM projection imposed on the tree-ring reconstruction (blue bar = modeled reduction in water delivery)

City of Boulder, CO - Integration of tree-ring reconstructed flows for Boulder Creek with future climate projections

The TreeFlow web resource

http://treeflow.info

• Access to flow reconstruction data

• Descriptions of applications

• Technical workshop presentations

• Resources and references

• Colorado River Streamflow: A Paleo Perspective

Recap of messages from the trees

• Many trees in Colorado record a strong moisture signal we can use to reconstruct past streamflow and droughts

• The full range of past natural variability in climate and water supply (e.g., severe droughts) is broader than the past 100 years would suggest

• The 20th century was generally wetter and less drought-prone than the previous 4+ centuries

• Even without considering climate change, we would want to prepare for conditions worse than 2002/2000s

• Please contact me (lukas@colorado.edu) if you have any further questions, or need assistance with teaching resources

Additional slides and graphics

Trees need to allocate their growth for multiple purposes

Fine roots and mycorrhizal fungi

Larger roots

Foliage

Height growth

Diameter growth

Reproductive structures (cones)

Branches

Production of resins

The formation of annual growth rings (conifer)

Cambium

Phloem

BarkPith

Annual ring (earlywood + latewood)

Resin duct

Tracheids (cells)

Two types of wood (xylem):

Sapwood: transports water & nutrients (sap) from roots to canopy

Heartwood: cells fill with gums, resins, tannins, no longer transports water, structural only

Heartwood

Sapwood

Generic old-tree characteristics

flat or spike top

heavy and gnarled limbs

bent/leaning trunk

thick bark

large size*

Stressful sites produce ring series with a stronger moisture signal

from Fritts 1976

Lab setup for measuring tree-ring samples

Measuring stage Measurement path is

perpendicular to the ring boundaries