Crossdatingin Dendrochronology A. E. Douglass Laboratoryo! Tree-RingResearch, University o! •4rizona

This paper definesand illustratescrossdating, an initial processin dendrochronologyor tree-ring work by which accuratering chronologiesmay be built for dating purposes,for climatic information, or for certain ecologicalproblems. Here are briefly explainedits operationby an efficientmethod, its principles of interpretation and application, its character as differentiated from correlation, its proceduresfor reaching assurancein results, its significanceas a guide to special sites where certain climatic effects on tree rings can be distinguished, and finally references are given to some of its published discussions.The purpose of this paper is to call the attention of ecologists and others to this fruitful processthat carries convictionby tests on well-located trees but whose reality in certain well-assuredregions cannot be judged by misinterpretation of material or un- technical treatment of specimens. HEarti,c, lecalled "The Dendrochronology wcre the only onesthat had what we consider sensitivity,even though to a very small degree; berEnigma 1940 JOURNALbyA. W. OF Sampson FORESTRYintheis basedDecem- on and from that peculiarityhe becameuncertain of a seriousmisunderstanding of the principleon ring identity obtainedby countingin from the which dendrochronologyor tree-ring work is outside and omitted them. Those Davis did use founded. Our criterion of climatic character in had practicallyno sensitivityand, therefore,had the ring recordsin our trees is not sensitivity no patternsto crossdate,and thusno evidentcli- (the amountof fluctuationin thicknessfrom ring matic value so that negativeresults were to be to ring) as he statesand criticises,but crossdat- expected. ing qualitywhich is of an altogetherdifferent na- In localitiesnew to us our standardprocedure ture, Crossdatingis the recognitionof the same is to securea seriesof ring sequencesfrom incre- ring pattern in differenttrees, so that the actual mentcores or stump-cutstaken out in the fieldby growthdate of any onering of the patternis the someonewho understandsthe effectsof topogra- samein the differenttrees and onemay carry a phy, in order to see what climatic effectsthe chronologyacross from tree to tree. Sensitivity trees of the region are showing normally is a seconklaryfeature which is importantonly whengrowing in an environmentcapable of pro- in the presenceof crossdating.If thereis no sen- ducingclimatic effects in the rings. In the South- sitivitythere can be no patternsand thereforeno westwe seeksteep slopes and shallowsoils and a crossdatingby patterns,and no assurancethat rainfall about as small as the trees can stand. the specimencame from a sitein whichthe trees These are of course not the characters a lumber- canshow a climaticvariation year by year. manprefers; in fact he avoidsthem. In the study Inattentionto the initial crossdatingby pat- of large numbersof suchspecimens, we had be- ternswas the error in the studyof 21 increment comefamiliar with the normal Flagstaffpatterns coresby W. E. Davis• someyears ago, cores that and couldrecognize at sightthat eventhe better camefrom the SouthwesternForest and Range specimensdiscarded by Daviswere still not good ExperimentStation at Flagstaff,Arizona, and enoughto be usedin his test. Theseerrors would which ! later examinedvery carefullytogether have been avoidedby his adherenceto the re- with his reportmade available through that sta- quirementof initial crossdatingin real ring pat- tion. He discardedabout half of thosespecimens terns. becausehe was not sure of the identityof the It is surprisingto mostpeople to seecrossdat- rings. While this was apparentlya worthy rea- ing in the form in which we can show it. Our son,the fact wasthat the specimenshe discarded success is believed to be due to favorable sites in a dry region. Advantageousconditions reveal •Co-authorwith A. W. Sampsonof "Experimentin themselvesin certain superblocations in Mesa Correlation of Tree Growth Rings and Precipitation Cycles,"published in Trans. Amer. Geoph.Union, 493- Verde and near Durango,Colorado and many 496, 1936. This paper is referred to in "The Dendroch- otherplaces. Figure ! showsa very distinctive tonology Enigma." Mr. Edmund Schulman of this lab- topographicenvironment in the Mesa Verde Na- oratory in March 1939 examined substantially all of the collections and analysis on which that article was tional Park, one of the best sitesseen in forty based and found tundamental errors that showed the years of field work. The availablemoisture for necessityof a completerevision of that paper. In the text above I am giving my personal experiencewith an the fir treeson the slopeis reshic?edto the pre- earlier Davis test. cipitation that falls on or near the trees them-

825 826 JOURNAL OF FORESTRY

Fig. 1. Mesa herde site in Fewkes' Canyon. highly favorable to crossdating;the mesa lop at the right slopes generally away from this canyon. CROSSDATING IN DENDROCHRONOLOGY 827

selves,chiefly in the winter rainy season;the "Hopi" signatureas it is commonin the beams steepslope reduces local conservation of moisture of the Hopi villages.One can start here with the to a minimum; an elevation of 6,700 feet above two large rings in the left centerof each,1530- the sea and a mean annualrainfall of perhaps 1531,and especially the smallring of 1532; then twenty incheskeep thesetrees under a stressof list the smallrings both backwardand forward moisturedeficiency. Our incrementborings in and observethe agreementsbetween these trees. five of thesetrees, well distributedboth as to lo- That is the sort of crossdatingwe havefound in cationand age of tree, showa very high quality thousands of cases. of crossdatingin the ring patternsand a sensi- tivity that we havecalled class AA. But most people are familiar with plotted Peoplewho live in moistclimates have great curvesand so Figures 4 and 5 have been ar- difficukyin judgingthese topographic characters rangedto giveplots of measuresof the thickness as such sites are rare and experienceis often of thesesame rings shownin Figures2 and 3. neededin evaluatingthem. The chief requisites The similaritybetween these curves is evident.In hereare: first,the near impossibilityof the trees a regionwhere moisture is the criticalfactor, the getting"imported" water that has fallen on, or years of minimum growth seemsto be more in- been stored in, other localities and then moved dividualizedand to afford more ready pointsof to the area whose trees are under test; second, a correspondencebetween different trees. To these shallowsoil or one with small water storageca- examplesof crossdatingin plotsis addedFigure pacity; and third, a low mean precipitationso 6 showingthe agreementbetween two groupsof that the treesrarely or neverget all the moisture treesabout 70 milesapart. The uppershows the they can use. averagegrowth in seventrees in the Chuska Mountains between Fort Defiance and C,hinle. ILLUSTRATIONS OF CROSSDATING The lower curve is from a group of three logs Our crossdatingis done directly in the ring that camefrom near the storeat Pinyon in the patternson the wood (often aided by what we middle of the Black Mesa, about 70 miles west of call "skeleton"plots), hencein spiteof their un- the othergroup. familiarity direct photographsof ring sequences Finally Figure 7 givesa generalcurve from 55 constituteour first exhibit. Figure 2 represents treesin the northernparts of Arizonacompared patternsin four differenttrees of a sequenceof to the winter precipitationrecords from three yearsin the middle 1200's. We have calledthis widelyseparated points in the samearea that are patternthe "Gap" signaturebecause these rings highlyrepresentative of the area. The winterrec- were involved in the successful1929 dating ord is heremodified by introducinga lag of 2•/• acrossthe gap betweenthe earliesthistoric dates yearsin the smoothedvalues, which means that andthe mostrecent prehistoric ones. In examin- annualchanges are retainedbut that a conserva- ing this illustrationone shouldstart with the tion is introduced in the smoothed values. This threelarge rings near the left end of eachphoto, is a slight variant of the author'susage in test- the rings dated 1248, 1249, and 1250, and note ing Prescotttrees, published in 1919.a in eachtree the smallrings at 1251, 1254, 1258, 1263, 1270, and 1276 (this last in two only). The CROSSDATINGMEMORY illustrationwould be more appealingif rings and The most efficient and at the same time the dateswere not markedand onehad to "dig" out mostconvincing method of crossdatingis by that this similarityfor himself. Thus he wouldtake memorywhich develops on examiningscores of the smallring 1251 and then make a list of the specimensof approximatelythe same age in datesof succeedingvery smallrings for eachtree whichsimilar patternsare identifiedin the great andfind the agreementbetween the trees. majority of cases.Usually the processbegins in Figure 3 representssequences from four trees the selection of some feature such as a little com- that grew in the 1500's.2 We havecalled this the binationof ringsthat is foundto be commonto aFigures2 and 3 are photographsof specimenscol- two or three specimens;then some number, a lected under the auspicesof the National Geographic real dateor a hypotheticaldate is appliedto one Society. My sincere thanks go to that society, to the Carnegie Institution of Washington, to the University of Arizona and other institutions and personswithout SSeealso F. P. Keen, "Climatic Cycles in Eastern Ore- whose aid the developmentof the subject would have gon as indicated by Tree Rings," in Mort. Wea. Rev. been impossible. 65, 175, May 1937. 828 JOURNAL OF FORESTRY

•E TATAI•IN

BE TA TAKIN

Fig. 2. Crossdaling in the "Cap" signalnre.

Fig. 3. Cr.ssdating in the "Hopi" signature. CROSNI)ATING IN DENDROCHRONOLOGY 829 of therings and a "skeleton"count is made,often meritsjoin until one beginsto seewhat tile pic- in memoryonly. giviugthe relatedchronologi- lure really is. cal positionof the xarious small rings. This is appliedto eachspecimen in turn. If one has a PRINCIPLES OF I'•TERPRET •,TION &\D flair for numbers•,such as many persons have in %.PPLIC&TION rememberingtelephoue uuinbers) one sooufinds Looking back after almostthirt) )ears of cou- thatone has iu hismind a pictureof ringpatlerns stahl use and testsof crossdatiugon sometwenty in whicheach ring is identifiedb• a numbcr.In thousandspecimens. after dating probablya mil- tile •vriter'sexperience that first picturecame in lion rings and measuringperhaps half of that 1911 after aboutt•enh Prescottspecimeus had number we find no evidence that leads us to heencarefull• examiued. In largerand larger douht the geaeral accuracxof the chronolog•in groupsof specimensthe originalsmall numhered this Puebloarea and in the big sequoiasand in patterngrows and grosssto greatersize aml in- othercarefulh testedareas. We havedeveloped terestjust like a picturepuzzle whose small frag- man) ver) stringentrules of techniquewhich, if ignored,cast doubt on the results. Certaiu prin- (.iplesof interpretationand applicationhave been developed.The principlesof interpretationare as f,)lhlws: 1. If man; independenttrees in a forestover a ,•ide area and for a hmg interval of time show similar ring variations in ideutical )ears, the cause of such variation is climatic because ctim- ate is the common continuous factor in their sur- Fig. 4. Crossdatingin growth curvesof four treea shown roundings.4 in Figure 2. 2. A forestborder dependent on generaltopo- graphic featuressuch as altitude or latitude or coastline hecomesall importantguide in the search for the climatic ele]nent influencingthe tree• becausethat element's importanceto the tree is emphasizedat the foresthotder. 4 3. Geueral areas like forest borders are com- monlyinvaded b• localitiescontr.lled by exterior conditionssuch as dryer upland ridges,shallow soils and so forth l in criticalh dry regionsl or Fig. 5. Cros,-dming in grox•d• curxes of four trees 1ocalhextended b; areasshowing forest interior shos•n in Figure 3. coaditionssuch as stremns.•noist valley bottoms and so forth {in dry regions); thesesurround- ings have strong influenceupon the ring record of the individual tree. The principlesof applicationare as follows: 1. Crossdatingprocedures have changedfun- damentallvthe method of gettiug information frmn tree rings by introducingas the unit of in- Fig. 6. Crossdatingacross 70 iniles. formation not the individual tree but a group of treeswhose ring patternscrossdatefi 2. Cro,,sdatlngshould not he acceptedin any ues•localit) until it is provedby testto be there. So alsoits at,sencein one locality carriesuo in- ferencewith respectto its geuuinenessin auother locality that differs ill climatic aud topographic featuresfi

Fig. 7.- Tree growth (solid line) of 55 trees and winter q.arnegle |nslitntmn of WashingIon.Year Book 1932, precipitmion (dotted line) with 2i•z years conservation page 217. in the srnoo•hed values. •Tree Ring Bu!lelin, 6. z6, April, 1940. 830 JOURNAL OF FORESTRY

CROSSDATINGAND CORRELATION shallowsoils, steep slopes, high ridges,and the Crossdatingis not to be confusedwith correla- dryer east or "shadow"sides of the mountains tion. It is not a correlation coefficient between (away from the moistwesterly winds) are favor- the recordsof differenttrees; it is not identity able to crossdating. In conclusionwe arequite aware that these em- of thicknessof a givenring in manytrees, exact phasizedclimatic effects in tree rings are not a thicknessof a ring is not highlyimportant. In general characterof all trees about the world; ourSouthwestern work crossdating is finding in many different trees the same relative chrono- on the contrarythey are the resultof specialized logicalplacement of yearsof deficientgrowth. conditionsbut when finally found in relative The writer feels that no correlation formula purity they offerrare opportunitiesfor important shouldbe used to decidewhether sequences from studiesin climaticsand prehistoryas well as in dynamicecology. two differenttrees are showingthe samedates. In doubtfulcrossdating cases there are several SOME PUBLISHED DISCUSSION OF CROSSDATING proceduresthat carryone toward final assurance. First,review the rings very carefully under ab- 1. Douglass,A. E. 1909. Weather cyclesin solutelyrigorous conditions ofmicroscopic power the growth of big trees. Mon. Wea. Rev.: and of standardsurface and illuminationand see 37, 226. if anychange or uncertaintyis indicated in them. 2. 1914. Method of estimating Second,withhold any decisionin the mattertill rainfallbythe growth oftrees. Bull. Amer. thelength of thesequences susceptible of com- Geog.Soc. 46: 323-325. parisonis sufficientlyextended to givea definite 3. 1914. Method of estimating decision.This usually means waiting for new rainfall by growthof trees. Chap.XI in specimens.On the average, datable Arizona pine "The climatic factor" by EllsworthHunt- treesneed crossdating patterns about fifty years ington,Carnegie Inst. Wash.Pub. 192, 104- in length; Mesa Verde firs often work well in 109. patternsof twenty-fiveyears; giant sequoias com- 4. . 1919. Climaticcycles and tree monlyneed one hundred years and in northerly growth.Carnegie Inst. Wash. Pub. 289 (Vol. grovesthey requireseveral hundred. Third, I): 15-20. strengthenthe localchronology by crossdating5. 1920. Evidences of climatic betweenall availablespecimens. Fourth, se- effectsin the annualrings of trees. Ecology cure datablering recordsfarther and farther 1: 24-27. awayfor properchecking purposes. 6. 1922. Someaspects of theuse SIGNIFICANCEOF CROSSDATING of the annualrings of treesin climaticstudy. Sci. Mo. 15: 5-22. (Also Smithsonian Ann. Crossdatingis not a magicformula which only Reportfor 1922,225-229). theelect can use; on the contrary it is a valuable 7. 1928. Climaticcycles and tree guideto thosetopographic laboratories where growth.Carnegie Inst. Wash. Pub. 289 (Vol. nature has isolatedcertain climatic elementsin II): 31-33. theireffects on growth rings. It pointsthe way 8. . 1931. Tree ringsand their re- by whichwe ourselvescan find theselaboratories lation to solar variations and chronology. and study the effectsunder measurablecondi- ResearchCorp. Award Paper, Smithsonian tions.This process was first used by thewriter Ann. Reportfor 1931,306-307. fordating purposes in 1904 but it hastaken. years 9. 1935. Dating PuebloBonito of unendingtests in thefield on living trees and and other ruins of the Southwest. Nat. a minuteacquaintance with theirindividual sites, Geog. Soc. Tech. Papers; Pueblo Bonito to reachthe moreevident phases of the rela- Series No. 1: 7-8. tion of ring characterto thesurroundings of the 10. 1936. Climaticcycles and tree tree in whichit grew. In a generalway it is growth.Carnegie Inst. Wash. Pub. 289 (Vol. realizedthat higherelevations (with more rain- III): 7-14. fall), runningstreams and deepmoist soils de- 11. 1937. Tree-ringwork. Tree- creasecrossdating, while lower elevations(near ring Bull.4 (No. 1): 3. the forest's lower border where the rainfall is 12. 1937. Tree rings and chro- less), sandstoneand limestone bed rock under nology.Univ. Arizona Bull. 8: No. 4. CROSSDATING IN DENDROCHRONOLOGY 831

13. 1940. Editorial: A bibliog- 16. 1938. Tree-ring dating: Fac- raphy. Tree-ringBull. 6: 26-27. tors pertainingto accuracy.Tree-Ring Bull. 14. Giddings,J. L. 1938. Recenttree-ring work 4 (No. 4): 8. in Alaska. Tree-RingBull. 5: 16. 15. Glock,W. S. 1937. Principlesand methods 17. Schulman,E. 1938. Classificationof false of tree-ringan/dysis. Carnegie Inst. Wash. annual rings in Monterey pine. Tree-Ring Pub. 486: 9-12, 16-20, 6, 64. Bull. 4 (No. 3): 4-7.

Philadelphia Industrial Union Counci.l Adopts Resolution Supporting Fed- eral Regulation of Private Forest Lands Whereas,For more than 50 years forestersand farsightedpeople have fought againstthe cut-outand get-outpolicy pursued by many lumbermenon privatelands. This policyof selfishdevastation has turnedmillions of acresof productiveland into barrenwaste. Insteadof a nationaland communityasset, it has becomea fire and flood menaceand an economicburden to stateand nation. Prosperouscommunities dependenton the timber for livelihoodhave becomeghost towns. Workers and local businessmenand their families hive beenthrown into dire povertyand the few re- maininglocal taxpayers have had to meetan ever-increasingand unbearabletax load, and Whereas,Under propermanagement these wasted lands would have continuedto producetimber and providework in forest,mill and construction. Tourists,sports- men and vacationists would secure recreation and contribute income to local resi- dents. Water would be conservedfor fishing,bathing, industry and home. Rapid runoff and erosionwith resultantflood damage would be reducedand both the local worker,the landowner,the communityand the nation at large wouldhave the bene- fits and securityresulting from a stableand assuredincome, and Whereas,Because of thesefacts and becauseorganized labor is vitally inter- estedin all measureswhich promotestable and fruitful use of and conservationof our naturalresources whether these be privatelyor publiclyowned, therefore be it Resolved,That the PhiladelphiaIndustrial Union Councilgo on recordas support- ing the federal regulationof private forestlands and endorsingH. R. 3849, knownas the Pierce Bill, and be it further Resolved,That Localsaffiliated with this Councilbe urged to take similar action, and be it further Resolved,That copies of this resolutionshall be sent to both Pennsylvania United StatesSenators and to all Congressmenfrom this area, and to Congressman Pierceand to the Speakerof the Houseof Representatives.