Schulman • Tree-rings and History in the Western United States 



187 



impossible, C^"* dating greatly increases 

 the value of possible relative tree-ring 

 dates in some of these regions. 



DENDROCLIMATOLOGY 



Principles. Evaluation of all the in- 

 fluences responsible for the observed 

 absolute growth of trees is obviously a 

 far more complex matter than that of 

 setting up a centuries-long tree-ring 

 index of climate. Nevertheless, even 

 with the latter limited objective, the 

 numerous pitfalls in analysis and in- 

 terpretation of ring-growth permit no 

 simple generalizations of the results of 

 work in dendro-climatology. It will per- 

 haps be sufficient to sketch certain 

 broad outlines as follows: 



1. A most fundamental property of all 

 phases of ring-growth is variability; 

 no conclusions based on one species 

 or stand are necessarily generally ap- 

 plicable. 



2. Coniferous species are usually pref- 

 erable to hardwoods as sources of 

 climatic chronologies because of 

 longevity and ease of sampling and 

 analysis; however, their dominance 

 in the most "sensitive" belts thus far 

 studied may unduly determine this 

 view. 



3. As a single meteorological element 

 becomes severely limiting in tree 

 growth— e.g., temperature near the 

 arctic tree line or moisture near the 

 lower forest border on mountain 

 slopes of semi-arid regions— the door 

 seems to close to the entry of nu- 

 merous random factors which in 

 part control the fluctuations in radial 

 tree growth in mesophytic areas. 



4. A powerful tool for the solution of 

 uncertainties and ehmination of er- 

 rors in ring identity is provided by 

 the cross-dating technique already 

 emphasized, in which the sequences 



of widths are matched ring by ring; 

 only when a general tendency exists 

 for parallelism among the various 

 sequences, whether from a tree, lo- 

 cality or large but climatically homo- 

 geneous area, are absolute dating 

 and a significant index possible. 



S. The dendro-climatic history of a re- 

 gion, if derivable at all, is an approxi- 

 mation which it is usually possible 

 to replace with a better one. By re- 

 peated sampling of trees, selected 

 according to improved field criteria, 

 and by the construction of indices 

 having a wider statistical base, it 

 may be possible to go far towards 

 avoiding or cancelling out the in- 

 numerable biotic, climatic and other 

 factors which tend to distort the cli- 

 matic index in trees. 



OVER-AGE CONIFERS AND CHRONOLOGY 



The spectacular achievement in 

 archaeological application of a bo- 

 tanical variable was not the only un- 

 expected offshoot of dendrochronologic 

 research. An intensive field search by 

 the writer during the past 14 years for 

 suitable tree sources of climatic data 

 has brought to light a remarkable cate- 

 gory of long-lived growth-stunted trees 

 of high index value. These not only 

 provide a unique kind of tree-gage rec- 

 ord of past rainfall but exhibit very 

 suggesHve properties of growth under 

 extreme adversity. 



For many miles bordering the up- 

 per Colorado River, as in the area just 

 west of Eagle, Colorado, stunted Doug- 

 las-firs and pinyon pines (P. edulis 

 Engelm.) dot the steep slopes and are 

 readily accessible from the highway. 

 Standing dead poles are common. That 

 this site happens to be a gypsum forma- 

 rion may have no great significance, 

 since trees of comparable age, sensitiv- 

 ity and slow growth have been found 

 on sandstone or limestone slopes 



