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Climate and water: What tree rings tell us in Colorado, presented by Jeff Lukas, Western Water Assessment.
 

Climate and water: What tree rings tell us in Colorado, presented by Jeff Lukas, Western Water Assessment.

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Through the methods of dendrochronology, or tree-ring science, we can extract and interpret the “long view” of climate and water resources from trees. Presented to the Colorado Biology Teachers' ...

Through the methods of dendrochronology, or tree-ring science, we can extract and interpret the “long view” of climate and water resources from trees. Presented to the Colorado Biology Teachers' Association Symposium on Climate Change. The session also touched on tree biology, climate, and hydrology.

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  • Across the western US, with the exception of the wet coastal regions and the higher mountains, climates are generally arid or semi-arid, and so moisture is the main factor limiting tree growth. Thus, a dry year leads to a narrow growth ring, and a wet year leads to a wide growth ring. Shown here is an image of annual tree rings from 1973-1984 from a Douglas-fir tree growing at about 8000 ’ in south-central Colorado. The range in ring widths reflects the annual moisture variability, with 1977 being an exceptionally dry year locally and across the region, and 1983 being an exceptionally wet year. While it varies somewhat by tree species and the region of the West (the northern and northwestern parts of the western US are somewhat different), tree growth usually reflects the antecedent moisture from the fall, winter and spring prior to the summer growing season. Growth also responds to summer precipitation, but it is a less efficient source of moisture for the trees. In other words, the annual ring-width is typically an integrative measure of moisture over a roughly year-long period, corresponding closely to the October-September water year.
  • In most of Colorado, the annual precipitation is largely influenced by winter snows. Even in areas with a good summer rainfall season, this precipitation is less influential to streamflow since it is more subject to evaporation, while winter snows are stored in the seasonal snowpack and released over the snowmelt season.
  • There are three types of ring anomalies that would make a simple ring-count depart from the true annual sequence of growth: Micro rings : In very unfavorable (dry) years, a tree may grow a very thin layer of new wood, and the resulting ring may be so small and difficult to see that it will be missed during a ring count. Missing or absent rings : In the worst drought years, like 2002, no or virtually no growth will occur, and no annual ring will be seen in a core or cross-section. On the driest and most stressful sites, over 10% of the annual rings in any one tree may be absent, though a more typical figure is 1% to 4%. We will know these rings are missing because they will be present in trees that are growing in more favorable locations at that site. False rings : If there is a very dry period in the middle of the summer growing season (as is typical in Arizona and New Mexico, prior to the monsoon), the tree may begin to shut down its growth processes and form the dark band of latewood which indicates the end of the growing season. Then when the rains return in late summer, the tree will resume “normal” growth, and then finally put on a true latewood band as fall approaches. So the tree forms what at first glance appears to be two annual rings. By crossdating--comparing and matching the ring-width patterns among multiple trees, and multiple sites—we can identify these ring anomalies and ensure that each ring is assigned to the exact calendar year. This is critical if the tree-ring widths are to be calibrated with annual records of streamflow or climate. For an online exercise to practice cross dating, see: http://www.ltrr.arizona.edu/skeletonplot/introcrossdate.htm
  • There are three types of ring anomalies that would make a simple ring-count depart from the true annual sequence of growth: Micro rings : In very unfavorable (dry) years, a tree may grow a very thin layer of new wood, and the resulting ring may be so small and difficult to see that it will be missed during a ring count. Missing or absent rings : In the worst drought years, like 2002, no or virtually no growth will occur, and no annual ring will be seen in a core or cross-section. On the driest and most stressful sites, over 10% of the annual rings in any one tree may be absent, though a more typical figure is 1% to 4%. We will know these rings are missing because they will be present in trees that are growing in more favorable locations at that site. False rings : If there is a very dry period in the middle of the summer growing season (as is typical in Arizona and New Mexico, prior to the monsoon), the tree may begin to shut down its growth processes and form the dark band of latewood which indicates the end of the growing season. Then when the rains return in late summer, the tree will resume “normal” growth, and then finally put on a true latewood band as fall approaches. So the tree forms what at first glance appears to be two annual rings. By crossdating--comparing and matching the ring-width patterns among multiple trees, and multiple sites—we can identify these ring anomalies and ensure that each ring is assigned to the exact calendar year. This is critical if the tree-ring widths are to be calibrated with annual records of streamflow or climate. For an online exercise to practice cross dating, see: http://www.ltrr.arizona.edu/skeletonplot/introcrossdate.htm
  • As suggested by the examples of 1977 and 1983 in the previous slide, individual trees in the West are often remarkably accurate recorders of moisture variability. Here, we show a record of annual (August-July) precipitation across western Colorado in blue, plotted against the measured ring widths for a single pinyon pine tree sampled in western Colorado, in green. The correlation between the two time series over a 70-year period is almost 0.8. This robust moisture signal as recorded in individual trees is the basis for reliable reconstructions of streamflow and climate. We use sampling and data processing techniques designed to enhance the signal.
  • We ’ve criss-crossed the state, Gravitate towards dry, often rocky sites where soil moisture storage is minimal and trees may be moisture-stressed Look for old trees --age inferred from bark, crown, size, and sometimes previous collections Collect two cores on opposite sides of tree to average out within-tree variability Collect from many trees; replication = robust capture of shared signal
  • SO I ’ll try to give you an idea what these remnants look like…on most, the outer sapwood had eroded away, along with branches The GMR site was paydirt, with some very old remnants, including this one, probably dead 700-800 years
  • Shown here is a subset of the chronologies mapped in the previous slide, plus others collected since 2003, covering Colorado and parts of the surrounding states. These chronologies were all developed by Connie Woodhouse, Jeff Lukas, and colleagues at the University of Colorado - INSTAAR Dendrochronology Lab between 1999 and 2008. Many of these chronologies contribute to the latest streamflow reconstructions for the Colorado River basin, which are described later. Note that all of the chronologies are from Douglas-fir, pinyon pine, or ponderosa pine.
  • The dataset used by Cook et al. (2004) in the reconstruction of a gridded network of past drought (PSDI) includes 835 moisture-sensitive tree-ring chronologies collected across North America since 1979. Of these, 603 chronologies are from west of the 100 th meridian. This dataset represents nearly all of the moisture-sensitive chronologies useful for reconstructing climate and streamflow, with the exception of the most recently collected ones (collected since ~2003). You can see from the map that the density of chronologies varies across the West, with the highest density in the mountain regions of Colorado, Arizona, New Mexico, and California, and fewer chronologies in the intermontane basins, the northern Rockies, and the Pacific Northwest. These differences mainly reflect the relative abundance of long-lived moisture-sensitive trees, but other factors (the location of tree-ring research labs, and basins where interest in tree-ring data is highest) also play a part. Reference: Cook, E.R., C.A. Woodhouse, C.M. Eakin, D.M. Meko, and D.W. Stahle. 2004. Long-term aridity changes in the western United States. Science, 306, 1015-1018.
  • We ’ve now generated flow reconstructions for about 25 gages in several basins, and associated data products Our first and main partners have been Denver Water and NCWCD, who both have gages in South Platte and upper Colorado basins. Also have done several reconstructions for Bureau of rec Aspinall Unit on the Gunnison. Connie had a relationship with Hydrosphere Resource Consultants predating the project, they consult for Boulder. Also have done work with several other Front Range towns and cities, and the consultants for Coors Brewing. So a broad spectrumof entities, and likewise a big spread in the degree of interaction with the partners. I ’ll be focusing on our partnership with Denver Water, which has been very iterative
  • We ’ve now generated flow reconstructions for about 25 gages in several basins, and associated data products Our first and main partners have been Denver Water and NCWCD, who both have gages in South Platte and upper Colorado basins. Also have done several reconstructions for Bureau of rec Aspinall Unit on the Gunnison. Connie had a relationship with Hydrosphere Resource Consultants predating the project, they consult for Boulder. Also have done work with several other Front Range towns and cities, and the consultants for Coors Brewing. So a broad spectrumof entities, and likewise a big spread in the degree of interaction with the partners. I ’ll be focusing on our partnership with Denver Water, which has been very iterative
  • What Denver Water found, when they ran the model input for 1634-2002 derived from the reconstructions, was that there were two droughts prior to the observed period (in the 1680s and 1840s) which depleted reservoir contents more than the existing design drought. But even in the 1840 ’s drought their reservoirs were not quite depleted to their strategic water reserve, which indicated that under the assumed demand (345 KAF) and applying progressive drought restrictions on use, Denver Water’s system was robust to the worst reconstructed drought.
  • Very different picture for precipitation—which is driven by processes that are more spatially complex and chaotic and much harder to model than the relatively simple radiation physics which drives temperature For the Colorado River headwaters, not much consensus about future trend in annual precipitation – some models point up, others point down If we look at the model output seasonally, there is some consensus for more winter precip, less summer precip– but there ’s very large uncertainty in future behavior of SW monsoon To our south (inset map), it is likely that the Lower Basin (AZ) will get less precipitation as storm tracks shift north All models project continued high year-to-year variability
  • Output from 3 different GCMs was used for this study, and an ensemble of flow sequences were generated from each set of GCM output. Here is shown a true “worst-case scenario”, in which a “dry” GCM projection was combined with the reconstructed flows, to simulate the paleo period (1566-2002) under conditions of climate change. The height of the blue bars shows the extent of modeled water shortage in each year. The years from 1566-1700 contain many shortages, but very few during the most recent century (period of record). This shows how looking just at the period of record wouldn’t indicate the vulnerability of the city’s water system nearly as well; the combination of “paleo” and “future” is a harsher test of the system.
  • So to sum up, our ever-expanding chronology network allows us to develop many reconstructions, and makes a nice testing ground for new techniques And it perhaps goes without saying that the range of events in the full reconstructions exceeds that in the gaged and instrumental records Most notably, we repeatedly see severe droughts in the 1580s, mid 1680s, and late 1840s 2002
  • Although trees usually grow one ring per year, a tree may not grow a ring or a complete ring in years when climate is very stressful (i.e., an extreme drought year), particularly if a tree is growing on marginal site. These rings are called missing, absent, or locally absent ring and are detected through pattern matching with trees growing at more favorable sites.
  • Although trees usually grow one ring per year, a tree may not grow a ring or a complete ring in years when climate is very stressful (i.e., an extreme drought year), particularly if a tree is growing on marginal site. These rings are called missing, absent, or locally absent ring and are detected through pattern matching with trees growing at more favorable sites.
  • Old tree characteristics include spike top or flat top, heavy lower limbs, gnarled and crinkled limbs in crown (especially pinyon), large trunk diameter sometimes (as long as trees are not growing in moist sites, in which case we call them “ fat and happy ” , as they are not particularly sensitive to climate variations).
  • A ponderosa pine tree growing on a moist site with a high water table and thick soils (shown at left) will not be very responsive to the year-to-year variations in moisture availability, since moisture will be truly limiting to growth in only the driest years, and will tend to have “ complacent ” ring-width series. The moisture signal captured by the tree rings is relatively weak. A ponderosa pine growing on a dry, or stressful, site with thin soils, rapid runoff, and greater surface heating (i.e., south-facing slope), as shown at left, will have its growth more strongly limited by the moisture conditions that occur each year, and so it will have “ sensitive ” ring-width series, with a strong moisture signal.
  • Once all of the samples are crossdated, every ring is measured on a computer-assisted measuring system, in which a knob is turned to slowly move the sample across the microscope ’ s field of view. When a ring boundary is aligned with the microscope ’ s crosshairs, the push of a button records the position of the core (and thus the width of the ring) to the nearest 0.001 mm (1 micron). For comparison, typical rings from moisture-sensitive trees are between 0.1 and 1.5 mm wide (100 to 1500 microns), with the smallest “ micro ” rings being about 0.02 mm (20 microns) wide.

Climate and water: What tree rings tell us in Colorado, presented by Jeff Lukas, Western Water Assessment. Climate and water: What tree rings tell us in Colorado, presented by Jeff Lukas, Western Water Assessment. Presentation Transcript

  • Jeff LukasCIRES Western Water Assessment (WWA)University of Colorado BoulderColorado Biology Teachers Association – Spring SymposiumApril 20, 2013 – Boulder, COThe Long View: What tree rings tell usabout climate and water in Coloradohttp://wwa.colorado.edu
  • Outline1977 1983• How tree rings record climateinformation• How we extract and interpretthat information• What the tree rings tell usabout past climate and water inColorado101112131415161718750 1000 1250 1500 1750 2000Water YearAnnualFlow,MAF
  • How can we get more context for“unprecedented” droughts and other events?200201002003004005006007008001915 1930 1945 1960 1975 1990 2005AnnualFlow(1000acre-feet)South Platte R., Colorado - annual streamflows
  • Paleoclimatology: analysis and reconstruction of pre-instrumental climate, using environmental proxiesLake sedimentsPackratmiddens(vegetation) Tree rings(Dendrochronology)PollenIce coresCoralsSpeleothemsOcean sediments
  • Key attributes of tree rings as a paleo-proxy forclimate and hydrology• Annual resolution• Absolute dating to calendar year• Long, continuous records(200 to 10,000 yrs)• Widespread distribution• Straightforward translation intoclimate variables
  • Tree rings = a much longer view of past hydroclimaticvariability, at annual resolutionGaged record051015202530750 1000 1250 1500 1750 2000Annualflow,MAFTree-ringreconstruction
  • Climate is typically the main factor limiting treegrowth• At the highest elevations andlatitudes: energy availability(warmth)• At lower elevations and mid-latitudes: moisture availability
  • Moisture availability varies greatly from year to year810121416182022241900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010Annualprecipitation,in.Annual precipitation, western Colorado 19771983
  • 1977 1983Douglas-fir, south-central COSo, for most trees across Colorado:Dry conditions = Narrow ringWet conditions = Wide ring• This moisture signal integrates both precipitationand evapotranspiration, over a ~one-year periodpreceding and including the growing season
  • This moisture signal in trees can serve as proxy formultiple moisture-related variablesAnnual or seasonal precipitationSpring snow-water equivalent (SWE)Drought indices (PDSI, SPI)Annual (water-year) streamflow
  • Same climate influences the growth of all treesat a site = cross-dating1900 1910 1920 1930Two Douglas-fir trees near Eldorado Springs, CO
  • 1900 1910 1920 1930Two Douglas-fir trees near Eldorado Springs, CO19251925Same climate influences the growth of all treesat a site = cross-dating
  • Annual growth (ring-width) is not the only tree-ringindicator of climate• Stable isotopes of carbon (12C, 13C) reflect carbonassimilation and thus moisture status• Stable isotope of oxygen (18O) reflects temperature of thesource 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, pinyonpine, Douglas-fir• Collect cores from 15-30 trees (same species)and sample dead wood if present• Cross-date and measure the rings, compile intoa site chronology 300-2000 years longCollecting moisture-sensitive site chronologies in Colorado
  • Green Mountain Reservoir (GMR) Douglas-fir chronology (588-2005)(N of Silverthorne)Living trees backto 1300s ADDead wood back to500s 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 pastclimate?
  • Compilation of the site chronology enhances the commonmoisture signalRing-widthindexVan Bibber site, nearGolden, Colorado(ponderosa pine)Robust averaging
  • Tree-ring chronologies developedat CU from 2000 to 2009(INSTAAR Dendrochronology Lab)
  • Over 1800 moisture-sensitive tree-ring chronologies acrossNorth America as of 2009 – 100+ in ColoradoFigure: Cook et al. (2009), J. Quaternary Science
  • Generating tree-ring reconstructions0510152025301900 1915 1930 1945 1960 1975 1990 2005annualflow,MAF0510152025301500 1600 1700 1800 1900 2000AnnualFlow(MAF)Observed hydroclimate recordSubset of tree-ring chronologiesBest-fit statistical relationshipbetween the tree rings andobservations during overlapperiodTree-ringreconstruction ofhydroclimate
  • Streamflow recordsreconstructed usingtree-ringchronologiesdeveloped at CU
  • Streamflow recordsreconstructed usingtree-ringchronologiesdeveloped at CUSouth Platteat SouthPlatte(1634-2002)ColoradoRiver at LeesFerry, AZ(762-2005)ColoradoRiver atKremmling(1440-2002)
  • South Platte at South Platte, COCalibration of reconstruction model, 1916-2002Calibration: R2= 0.7601002003004005006007008001915 1930 1945 1960 1975 1990 2005AnnualFlow(KAF)ObservedReconstructed
  • Full reconstruction of South Platte annual streamflow,1634-2002• 2002 is lowest reconstructed flow in entire record, but 1685 and 1851 arevery close, within the uncertainty of the reconstruction• 100-150-year return interval implied for 2002-type flow years01002003004005006007008001630 1660 1690 1720 1750 1780 1810 1840 1870 1900 1930 1960 1990AnnualFlow(KAF)
  • Full reconstruction of South Platte annual streamflow,1634-2002, with 4-year running average• 1953-1956 was 3rdlowest reconstructed 4-year flow since 1634 (lowest:1844-48)1002003004005001630 1660 1690 1720 1750 1780 1810 1840 1870 1900 1930 1960 1990AnnualFlow(KAF)
  • Colorado at Kremmling, COCalibration of reconstruction model, 1916-2002Calibration: R2= 0.70050010001500200025001915 1930 1945 1960 1975 1990 2005AnnualFlow(KAF)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 evenlydistributed)04008001200160020001440 1480 1520 1560 1600 1640 1680 1720 1760 1800 1840 1880 1920 1960 2000AnnualFlow(KAF)
  • 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 margin6008001,0001,2001,4001,6001,8001440 1480 1520 1560 1600 1640 1680 1720 1760 1800 1840 1880 1920 1960 2000AnnualFlow(KAF)
  • 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 inthe South Platte headwaters since they mainly experience the same weatherevents6008001,0001,2001,4001,6001,8001440 1480 1520 1560 1600 1640 1680 1720 1760 1800 1840 1880 1920 1960 2000AnnualFlow(KAF)
  • Tree-ring reconstructed annual flows, Colorado River atLees Ferry 762-2005, with 20-year running average101112131415161718750 1000 1250 1500 1750 2000Water YearAnnualFlow,MAF
  • Tree-ring reconstructed annual flows, Colorado River atLees Ferry 762-2005, with 20-year running average101112131415161718750 1000 1250 1500 1750 2000Water YearAnnualFlow,MAFMid-1100smegadrought911131517191120 1130 1140 1150 1160 1170 1180Annualflow,MAF46 dry years in 57-year period
  • • Only the reconstructed 14thcentury averaged flow was higher than theaverage since 190013.514.014.515.015.5AnnualFlow,MAF800 1000 1200 1600 1800 20001400Colorado River at Lees Ferry, AZ – reconstructedannual flows, 762-2005: 100-year averagesObserved average, 1906-2012
  • Even if anthropogenic climate change were notoccurring, we’d want to prepare for droughts worsethan any modern (>1900) droughts101112131415161718750 1000 1250 1500 1750 2000Water YearAnnualFlow,MAF
  • Denver Water: water supply yield analyses with South Platteand Colorado at Kremmling flow reconstructions, 1634-2002Reservoir contents with 345 KAF demand and progressive drought restrictions
  • The ongoing and projected warming alone will reducestreamflows and make droughts worse1950 2000 2050 210050°F60°FNorth-central Colorado mean annual temperatureEnsemble of 16 GCMs, medium emissions scenarioObserved temp.Source: Marty Hoerling, NOAA ESRL PSD
  • Future precipitation trend is unclear, but unlikely tocompensate for the warming1950 2000 2050 210024”12”Observed precip.Source: Marty Hoerling, NOAA ESRL PSDNorth-central Colorado mean annual precipitationEnsemble of 16 GCMs, medium emissions scenario
  • From Smith et al, 2009.Worst case scenario: A “dry” GCM projection imposed on thetree-ring reconstruction (blue bar = modeled reduction in water delivery)City of Boulder, CO - Integration of tree-ring reconstructed flowsfor Boulder Creek with future climate projections
  • The TreeFlow web resourcehttp://treeflow.info• Access to flowreconstructiondata• Descriptions ofapplications• Technicalworkshoppresentations• Resources andreferences• Colorado RiverStreamflow: APaleo Perspective
  • Recap of messages from the trees• Many trees in Colorado record a strong moisture signal wecan use to reconstruct past streamflow and droughts• The full range of past natural variability in climate andwater supply (e.g., severe droughts) is broader than thepast 100 years would suggest• The 20thcentury was generally wetter and less drought-prone than the previous 4+ centuries• Even without considering climate change, we would wantto prepare for conditions worse than 2002/2000s
  • • Please contact me (lukas@colorado.edu) if you haveany further questions, or need assistance withteaching resources
  • Additional slides and graphics
  • Trees need to allocate theirgrowth for multiple purposesFine roots andmycorrhizalfungiLarger rootsFoliageHeight growthDiametergrowthReproductivestructures(cones)BranchesProduction ofresins
  • The formation of annual growth rings (conifer)CambiumPhloemBarkPithAnnual ring(earlywood +latewood)Resin ductTracheids(cells)
  • Two types of wood (xylem):Sapwood: transports water& nutrients (sap) from rootsto canopyHeartwood: cells fill withgums, resins, tannins, nolonger transports water,structural onlyHeartwoodSapwood
  • Generic old-treecharacteristicsflat or spike topheavy and gnarledlimbsbent/leaning trunkthick barklarge size*
  • Stressful sites produce ring series with a strongermoisture signalfrom Fritts 1976
  • Lab setup for measuring tree-ring samplesMeasuringstageMeasurement path isperpendicular to thering boundaries