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 de- rived 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.