Precipitation for Five Representa- Introduction

Home , Kaibab Plateau, Pinaleno Mountains

Centuries-Long Tree Indices of Precipitation in the Southwest (I) EDMUND SCHULMAN Laboratory of Tree^Ring Research, University of Arizona, Tucson CONTENTS ations in winter (October-June) Summary- precipitation for five representa- Introduction. Climate and trees tive areas of the Southwest: Principles and technique Tucson, Chiricahua Mountains, Tree-ring indices: Mesa Verde area Gila River headwaters, Kaibab Kaibab Plateau area Gila River drainage area Plateau, and Mesa Verde. Tucson area (7) The precipitation spectrum of Chiricahua Mountains area Winter precipitation and tree growth : wet and dry years is apparently Sources of disagreements The recorded precipitation interval nearly identical at nearby, high- Improper selection and obscure chronologies and low-level stations. A comparative study in coastal California Tree growth and summer rainfall (8) In the Southwest, trees under Tree growth and temperature Tree growth and river run-off the climatic stress of precipita- Areal variations in chronologies tion deficit (soil moisture) may Areal variations in dry years Dendrochronologic areas disagree in ring chronology with Time fluctuations in the chronologies: others only a few yards distant Average duration of maxima or minima Frequency of extreme years but not under climatic stress, yet Cycle studies may closely parallel trees on SUMMARY difficult sites 400 miles away. (1) Tree-ring series, if they are (9) A 55-year "controlled experi- proven to be climatic records, ment": an example from Mon- have much potential value be- terey, California, which shows cause of great length, particular the effects on two old pines of ir- mountainous site, and centuries- rigation, 1880-1935, in destroy- long homogeneity. ing their crossdatability and (2) Significant dendrochronological value as precipitation records. work must recognize the import- (10) Summer precipitation in the ance of selection of specimens Southwest, roughly 50% of the and crossdating of records. annual total, has little influence, (3) Douglas fir, distributed in patch- in general, on variations in ring es over one-half million square thickness in Douglas fir and pon- miles of the Rocky Mountains, derosa pine. has been found to be one of the (11) No general effects of annual best recorders of precipitation; temperature variations are Douglas fir and ponderosa pine found in the ring records. form the basis of the present (12) Tree growth in the Gila River study. headwaters area provides a cen- (4) Synchronous fluctuation in an- turies-long and closely approxi- nual growth is illustrated in mate index of run-off. many Southwestern selected (13) Comparisons of chronologies trees. across 400 miles north-south and (5) Curves of mean annual growth 250 miles east-west emphasize are given for five areas from that even in regions of generally southern Arizona to Mesa Verde, similar climate there is a chang- Colorado; these vary in length ing areal domain under the in- from 300 to 560 years. fluence of drought in different Curves of growth are shown to years, a persistent feature of the represent to a marked degree three centuries of data. the recorded year-to-year fluetu- (14) A method is suggested for quan-

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC titatively delineating a homo- of culture developments, (3) series geneous dendrochronologic area, which are homogeneous throughout interpretable as a climatic unit. many centuries, in contrast to the (15) With the frequently violent fluc- striking heterogeneity of the longer tuations in successive years of the meteorological series. smoothed out, the average dura- Attention has been called in this tion of excess (wet) or deficient journal2 to the wealth of climatolog- (dry) growth in the Southwest ical information developed by Doug- is about 8 or 9 years, on the lass for the Southwest and elsewhere. basis of the last three centuries The present paper deals with a con- of data; intervals substantially siderable body of new data represent- more than twice this are rela- ing collections of tree-ring specimens tively rare. of especially high indicator value. The (16) Since 1640 in the southern South- emphasis here is on the factual ma- west about 4% of the years are terial. characterized by extreme winter Of the several hundred papers now droughts with growth less than published on tree-ring analysis3 a con- half of normal, and about 15% siderable number have as a principal of the years show growth less objective the development of one or than three-fourths of normal; more climatic series; many such series the significant percentages are of value have been established for a somewhat greater in the Mesa number of areas. In many regions Verde area. the climatic influences operating on (17) In some areas the absence and trees represent a complex which ap- in other areas the presence of pears to be difficult or even impossible the sun-spot cycle in tree growth to resolve into the separate climatic leads to a suggestion of a pos- components. Only in specially favored sible relations to climatic lati- regions, such as the Southwest, has it tude. been possible to find trees which give a long and clean-cut record of seasonal fluctuations in one element, such as INTRODUCTION precipitation. Within this generally HE OBVIOUS VALUE of a centuries- favored region there are local forested long meteorological series lies in areas which are particularly suitable T its providing a substantial basis for dendrochronologic study, and many for the understanding of climatic of these have been sampled in the last changes. It has been shown, espe- few years by the writer. Some pre- cially by Douglass,1 that if a certain liminary but highly significant results set of analytical and site requirements may now be outlined here. are fulfilled then growth curves derived from tree-ring series provide CLIMATE AND TREES this basis. Theoretically, tree-ring The climate of Arizona and neigh- records of climate can supplement the boring areas to the north and east is observed meteorological data by pro- generally characterized by a double viding (1) a length of record many rainy season, roughly November-April times that of the longest meteorolog- and July-August. The relative per ical series, (2) records in mountain- cent of the summer rains in a general ous areas and on sites where meteoro- JA. E. Douglass, Ecology, v. 1:24-32, 1920 ; logical data are rarely obtained; such ibid., Monthly Weather Rev., v. 37:225-237, 1909. sites are often more representative of 2Schulman, Bull. Am. Met. Soc., v. 19:204- general conditions than those in areas 215, 1938. 3Ibid., Tree-Ring Bull., v. 6 :27-39, 1940.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC way increases southward and east- groups of cores collected by the writer ward in this area. Daytime tempera- from all parts of the West and at va- tures are often fairly high in mid- rious seasons of the year (mostly winter even in northern Arizona, al- spring and summer) shows that the though, with the frequent highly deposition of a new ring in general transparent sky, night-time ground begins in May or June and ends by radiation can occasionally lower tem- September or sooner. The end dates peratures to extreme values. During are to a certain extent a function of the summer, daytime temperatures are topography, elevation, species, climate, usually very high over most of the and the year-to-year fluctuations in state, especially in the valleys and on climatic elements such as precipitation the tablelands. Very low relative hu- and temperature. midity is the rule. Thus the evapora- tion rate is very excessive, and the PRINCIPLES AND TECHNIQUE more mesophytic plants such as the The prime objective in dendrochron- "timber trees" seek the relatively cool ologic studies of climate is the devel- moist slopes of the higher mountain opment for a given locality of as ranges. faithful an index of one or more cli- Much of the data herein is derived matic elements as it is possible to ob- from Douglas fir (Pseudotsuga taxi- tain. This aim at once leads to meth- folia Britt.). This species has recently ods of study strikingly different from been found to yield perhaps the best those current in the closely related precipitation records of any species in and fundamental botanical and eco- the Southwest. In the Rocky Moun- logical investigations, where the anal- tain region it seldom grows in exten- ysis of all factors usually operating sive stands but is able to maintain it- on trees is attempted.4 self in innumerable scattered patches Proper analysis of tree-ring records over an area of more than a half- involves the precise solution of all million square miles. In locations as problems of identification of non- far apart as southern Arizona and annual or "false" or "double" rings, central Colorado it is alike in seeking locally-absent rings, injury rings, and north slopes where soil moisture and many other anomalous characters of shade favor its existence. In southern growth. The theory of these cases has Arizona its optimum site is a north been treated extensively in various slope near 9000 feet elevation, al- publications,5 to which reference is though it has been found in moist can- made. In outline, there are three prin- yons at 6000 feet. In southern Colo- ciples which form the scientific basis rado the optimum site is considerably of dendrochronologic study: selection, lower, and in the mountainous areas crossdating, and sensitivity. The prin- fine stands are found at 7000 feet or ciple of selection states that, because less. Outposts of very slow, stunted of growth characteristics and. topo- growth may be found, especially in the graphic and biotic environment, only northern parts of the area, at much certain selected species and individu- lower elevations. Its habits in west- als will yield growth records which ern Wyoming, where it exists exten- are simple indices of climate. The sively in small stands, bear some sur- principle of crossdating involves the prising resemblances to the preceding; a study of growth in this rather dif- 4Failure to appreciate this fundamental fact and its implications has sometimes led to very- ferent climatic region is now under confused conclusions, both in actual projects in tree-ring analysis and also in occasional way. critiques. 5A. E. Douglass, Carnegie Inst. Washington Examination of over 150 selected Publ. 289, vols. I-III, 1919, 1928, 1936.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 1. The Southwestern region, showing the location of recent tree-ring collections. detailed control of the ring record of paper are based are in the form of one tree radius by the records in other radial cores obtained by the writer radii and in other trees of a homo- from selected living trees with the geneous group, and is fundamental to Swedish increment borer. These have precision analysis in dendrochronol- been mounted, surfaced, and the rings ogy. The principle of sensitivity per- dated according to the standard tech- mits an observational estimation of niques of the Tree-Ring Laboratory,® the quality of climatic record in a and the ring thicknesses measured to crossdatable ring series by the magni- the nearest 0.01 mm. The resulting tude of fluctuation in width of succes- series, each representing one tree, sive annual rings. have then been merged, either by Almost all of the specimens on 6Douglass, Tree-Ring Bull., v. 7 :2-8, 1940 ; v. which the climatological series in this 7:28-34, 1941.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC straight addition of the measures or of dendroclimatic studies are to be by addition of the standardized values found in Mesa Verde,7 near the south- (per cent departure from the mean western corner of Colorado. Mesa trend line), to form group mean Verde is a relatively large and slightly growth curves. Where useful some of tilted table-land, with cap rock of these curves have been simply sandstone and shale, rising only 1000 smoothed by the formula 6' = {a + feet to about 3000 feet out of the arid 2b + c)/4; this smoothing retains to mile-high semi-desert. The topography a maximum degree fidelity to the orig- favors a minimum of local variability inal data and at the same time re- in precipitation as related to oro- moves some of the more violent year- graphic influences. In July, 1941, to-year fluctuations. Finally, these cores were obtained from seven trees group curves are studied for their in- in three well separated shallow can- dicator-value as climatic records. yons at elevations of about 6800 feet. Statistical constants, such as corre- The agreement in fluctuations in these lation coefficients, are in general trees equals or excels the best cross- avoided in this paper. Although of dating in any of the thousand or more much value when properly applied, tree groups analyzed at the Tree-Ring their lack of significance as proof of Laboratory in the last 40 years. This physical relationship between short- agreement holds throughout the entire time series is well-known though often five centuries of data, the outer 100 overlooked. As mere indices showing years of which is plotted in fig. 2. Al- concurrence of fluctuations between though the five-year means computed curves they are often very poor and separately for each canyon show some- misleading substitutes for the original what less crossdating than do the an- data presented as curves. It is felt nual growth curves for the individual that a real view of the significance of trees, there is obviously a general con- tree-rings as climatic indicators is bet- currence in the longer time fluctua- ter gained by a common-sense consid- tions in chronology also. eration of many comparative curves The mean growth curve of the area, of growth vs. climatic element. MVR in fig. 3, when compared with the winter precipitation at Mesa Verde, TREE-RING INDICES Mancos, and Durango, all in Colo- The sites represented by the follow- rado, reveals perhaps the most faith- ing groups are selected areas in Ari- ful tree-ring record of winter precipi- zona, New Mexico, and southwestern tation yet obtained. In this figure are Colorado (map, fig. 1). A brief de- plotted also curves for two groups of scription of the specimen site, a dia- Douglas firs from the west slopes grammatic illustration of the relation above the Animas River, seven miles of the growth curves to winter precip- north of Durango and 50 miles north- itation, and the longest, most repre- west of Mesa Verde, obtained in July, sentative growth curve for each area 1941. For further comparison an old are given first. Discussion of the de- group is shown representing Johnson tails of climatic relationships is re- Canyon, about 10 miles east of Mesa served for the following section, since Verde Park headquarters; this in- it can best be treated for the groups cludes 11 trees (5 ponderosa pines, 3 in their entirety. Douglas firs, 3 pinyon pines) collected by A. E. Douglass in June, 1931. The Mesa Verde area. — Sites most close agreement in growth fluctuations nearly answering all the requirements in all groups is striking.

7Douglass, Tree-Ring Bull., v. 6:10-11, 1939. The complete mean growth curve

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 2. Above: crossdating (synchronous fluctuations in chronology) in annual ring thickness of seven Douglas firs, Mesa Verde, Colorado. Below: five-year means of complete growth records, Mesa Verde 500-year firs, averaged for each of three canyon sites and showing general agreement in long-period fluctuations. These comparison graphs are best viewed by sighting at a low angle, rotating the figure on its horizontal axis. Circled figures give the number of trees on which each group curve is based. for Mesa Verde is given in fig. 4. inches of radius. Since this curve represents straight Kaibab Plateau area.—In the ex- averages of the individual ring-widths tensive forest which covers the Kaibab with no statistical manipulations of Plateau for 40 or 50 miles north of the data, it presents a striking pic- the Grand Canyon, two groups of ture of fluctuations in growth over Douglas fir were collected in July, five centuries and the corresponding 1941. One group of two stunted firs changes in winter precipitation. The from a steep, rocky, foothill ridge age curve of growth in these trees has 5700 feet in elevation and about 10 not been removed in the averages, so miles west of Jacob Lake represents that the 1400's particularly are influ- an outpost of the forest encroaching enced by the relatively rapid "in- on the woodlands and semi-desert of fancy" growth. The annual growth, lower elevations. This group is given which is all laid down during the late in full in fig. 5; the measured ring- spring and summer, depends to a widths have been averaged without remarkably close degree on the pre- removing the age trend in mean ceding winter's precipitation in spite growth. For comparison a growth of the extremely small total growth curve, 1800-1940, is also plotted in this in any season; in the last 400 years figure and represents a straight aver- these trees each grew only about five age of four firs growing well within

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 3. Agreement in fluctuations of tree-growth and winter precipitation in southwestern Colorado. Growth variations at three stations show concurrence across 50 miles. Unweighted average growth in mm at Mesa Verde and near Durango (DUR) and standardized growth, i.e., average percentage de- partures from trends, at Johnson Canyon (LP J). The LP J group was collected by A. E. Douglass in 1931. the main forest on the plateau top at that in the dry low level group, it an elevation of about 8500 feet and closely supports the dry-site curve. some six miles south of Jacob Lake. The small number of specimens enter- Although the growth in the relatively ing into the low-level group especially moist site of the latter is somewhat would normally be insufficient for a less sensitive and its average rate of satisfactory elimination of effects volume accretion at least sixteen times local to each tree, but the striking

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 4. Mean tree growth at Mesa Verde, Colorado; unweighted average ring thickness in mm of seven Douglas firs. Note change of vertical scale for the upper two segments.

FIG. 5. Mean tree growth on Kaibab Plateau, Arizona; unweighted average ring thickness in mm. KBL: two firs at 5700 feet elevation. KBH: four firs at 8500 feet elevation.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 6. Agreement in fluctuations of tree growth and winter precipitation, northern Arizona. Distances of the raingauge stations from the area of the specimens is indicated; linear interpolation can yield only an approximation to the precipitation curves at the collection site. sensitivity of these trees in part com- rainfall. pensates for this. Gila River drainage area.—During Meteorological data of any length June, 1940, a field survey was begun are not available in the immediate of growth records in the Gila River area of these collections. In fig. 6 the drainage. The most sensitive and winter precipitation at two Utah sta- crossdatable groups of specimens in tions to the north and west has been the collections, representing eight plotted, as well as that at two Arizona well-scattered sites, were merged, stations 40 and 120 miles to the south. after individual standardizing, into a Interpolations indicate a satisfactory mean areal curve for comparison agreement of growth with winter studies with the run-off of this stream.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 7. Agreement in fluctuations of areal tree growth and precipitation, Gila headwaters region, Arizona-New Mexico. The upper eight curves represent groups within a strip roughly 125 miles east-west. Mean tree growth for the entire area com- pares well with two raingauge stations near its eastern and western limits. All tree groups have been individually standardized. These groups are briefly described in SSE of the GCF group), and are the Table 1. longest meteorological records near the Gila headwaters area. Consider- The growth curves for every group able variation in chronology between listed in Table 1 have been plotted the tree groups is in evidence, trace- in Fig. 7 for the interval 1868-1940. able in part to the heterogeneity of In addition, the mean curve for the sites and to the dependence largely entire area has been given, to facil- on pine records; these local variations itate comparison with the winter pre- tend to cancel out on averaging. cipitation curves; these represent The mean areal curve representing Fort Apache, Arizona (about 30 mi. these eight groups, comprising 47 ENE of the HTP group) and Fort trees, is plotted as the middle curve Bayard, New Mexico (about 30 mi. in fig. 17.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 8. Agreement in fluctuations of tree growth and winter precipitation, southern Arizona. The Santa Rita Mountains (SRH, SRL) and the Santa Catalina Mountains (SCH, SCL), rising over a mile out of the surrounding semi-desert and separated by some 50 miles of valley, yield Douglas fir ring series of extraordinary similarity.

TABLE 1. SPECIMEN GROUPS IN THE GILA RIVER DRAINAGE AREA Elev. No. Group1 Location2 Ft. Site Soil Species3 trees HTP 130 mi. WbyN 6000 upland hilltop limestone PP;DF 7 PYO 100 mi. WSW 9000 ridge top granitic PP;DF 6 BRL 65 mi. WNW 7000 divide crest malpais; cinders PP 3 ROZ 65 mi. WbyN 8500 NW upper slope malpais PP 4 MGN 50 mi. NW 7500 ridge top malpais PP 5 MGL 35 mi. WNW 7000 stony foothill ridge granitic PP;DF 6 MGH 30 mi. NW 9200 mountain crest granitic PP;DF 5 GCF 0 6000 mesa top tuff PP;DF 10

^TP-Hilltop, Arizona; PYO-Pinaleno Mountains, Ariz.; ROZ-Rose Peak, Ariz.; BRL-Blue Rim, Ariz.; MGN-Reserve (near), New Mex.; MGL-Mogol- lon Mountains, New Mex.; MGH-Mogollon Mountains, New Mex.; GCF-Gila Cliff Dwellings, New Mex. 2Location with respect to Gila Cliff Dwellings Ruin, 32 miles north of Silver City, New Mexico. 3PP-ponderosa pine; DF-Douglas fir.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 9. Mean tree growth, Santa Rita Mountains; average of six trees, individually standardized. Elevation 8500 feet. Tucson area.—Two groups of Doug- growth in the widely-separated las fir cores were obtained during stands of forest which these groups 1940-41 from the Santa Catalina represent, it is obvious from fig. 8 Mountains about 15 miles north of that they are responding in essenti- Tucson. The SCH group (fig. 8), 6 ally similar fashion to fluctuations in trees at an elevation of about 8500 some element commonly affecting feet, came from a steep north slope them all. The lower set of two curves, of Mount Lemmon, the highest peak giving the winter precipitation (Octo- in the range; the SCL group came ber-June, upper, and November-April, from the head of Bear Canyon, at an lower) for the interval 1868-1940, elevation of about 6000 feet, five miles shows a significant relation to the east of the first group and on the tree growth curves. south side of the range. The SRH standardized growth Two groups of Douglas fir cores curve, most sensitive and longest were also obtained during 1940-41 in this area, is given in full in fig. 9. from the Santa Rita Mountains about Chiricahma Mountains area.—From 40 miles south of Tucson. The SRH the Chiricahua Mountains, another of group, 6 trees at about 8500 feet on a the island mountains of southern Ari- north slope near the top of the range zona about 100 miles east of the Santa corresponds closely in site to the SCH Ritas, an excellent set of borings in group 55 miles across the valley; the Douglas fir was obtained in March, SRL group, 5 trees at about 6000 feet 1941. The CHH group of fig. 10 came elevation in the north-draining Flor- from a north slope just above Rustler ida Canyon, corresponds roughly in Park, in the heart of the mountains site to the SCL group. at an elevation of 8500 feet. The In spite of the entirely independent CHM group, from a dry site at Onion

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC FIG. 10. Agreement in fluctuations of tree growth and winter precipitation, southeastern Arizona. Growth curves (standardized) for the Chiricahua Mountains crossdate well, and agree with weather stations in the area.

FIG. 11 Mean tree growth, Chiricahua Mountains, Arizona; five Douglas firs individually standardized.

Unauthenticated | Downloaded 09/24/21 04:37 AM UTC Saddle some two miles north and 7500 the general agreement of tree growth feet elevation, showed the expected with precipitation of the area is very slower growth and greater sensitivity close. Records from the two sites as compared with the higher group. were merged to form the standardized The map of sites, fig. 1, shows the group curve of fig. 11. rather wide dispersion of raingauge stations with which these groups are (To be concluded in the following compared in fig. 10; in spite of this issue)

Climatic Types of California, According to the Decimal Scheme of World Climates* W. GORCZYNSKI, D.SC. As may be seen from the accom- II. ARID GROUP panying chart (fig. 1), we distinguish Limits: Per cent of aridity at least 20 five large climatic groups divided into or more; average temperature of ten types. The limits of each type in the coldest month above 23° F. this classification are given below. The (-5° C.) State of California together with the Type 3. Desert (per cent of aridity more than 40). peninsula of Baja California has an Type U. Steppe (per cent of aridity at area slightly exceeding 200,000 square least 20 and no more than 40). miles. This is roughly four-tenths of III. MODERATE GROUP one per cent of the total area of all Limits: Average temperature of the continents of the entire globe (57,- coldest month between 72° F (22° 000,000 square miles). The area of C.) and 23° F. (-5° C.) ; per cent of all California, from Oregon border to aridity less than 20. Cape San Lucas which is the southern- Type 5. Average temperature of the coldest month at least 46° F. (8° C.) most end of the peninsula near the and no more than 72° F. (22° C.); tropic of Cancer, is 2 per cent of that during winter months less than one- of the North American Continent; it half (five-tenths) of the sky ob- is 7 per cent of the area of the United scured as an average for the season. Type 6. Same limits for temperature States. The following table presents as for type 5, but no restrictions for percentages of areas having each cli- cloud amount. matic type, according to the decimal Type 7. All areas with cooler winters, scheme of world climates. within the limits of the moderate group. IV. EXTREME GROUP Limits: Average temperature of the FIGURE 1. DECIMAL SCHEME OF WORLD coldest month below 23° F. (-5° C.) CLIMATES and of the warmest month above 50° F. (10° C). No restrictions con- I. TROPICAL GROUP cerning per cent of aridity. Type 8.. Precipitation, mostly in Limits: Average temperature above winter. 72° F. (22° C.) during the cold- Type 9. Precipitation, mostly in est month. summer. Type 1. Wet (no dry month on the average). V. SNOWY GROUP Type 2. Wet, with dry periods. Polar regions and higher mountains in all latitudes. •Condensed summary of a lecture presented Type 10. Highest mean monthly tem- to San Diego branch of the American Mete- orological Society. (Meeting of November 19, perature not exceeding 50° F. (10° 1941, held in the premises of U. S. Weather C.) Bureau, San Diego Airport). The same lec- A subdivision of Group V is made if ture was also delivered on December 31, 1941, at the Dallas Meeting, American Association the average temperature is below for the Advancement of Science. freezing point in all months.

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