TREE RINGS AND HISTORY — SCHULMAN 467 



variability ; no conclusions based on one species or stand are necessarily 

 generally applicable. 



2. Coniferous species are usually preferable 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 m 

 tree growth — e. g., temperature near the Arctic tree line or moisture 

 near the lower forest border on mountain slopes of semiarid regions — 

 the door seems to close to the entry of numerous 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 elimination 

 of errors in ring identity is provided by the cross-dating teclinique 

 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, locality, or large 

 but climatically homogeneous area, are absolute dating and a signifi- 

 cant index possible. 



6. The dendroclimatic history of a region, if derivable at all, is 

 an approximation which it is usually possible to replace with a better 

 one. By repeated 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 toward avoiding or can- 

 celing out the innumerable biotic, climatic, and other factors which 

 tend to distort the climatic index in trees. 



Over-age conifers and chronology. — The spectacular achievement 

 in archeological application of a botanical variable was not the only 

 unexpected 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 category 

 of long-lived gTowth-stunted trees of high index value. These not 

 only provide a unique kind of tree-gage record of past rainfall but 

 exhibit very suggestive properties of growth under extreme adversity. 



Rain-recording trees of great longevity are of specially high value 

 in dendroclimatic studies, not only because they provide long histories 

 but also because they make possible a greatly improved estimate, 

 as compared with young trees, of the absolute values of past rainfall. 



* Sampling is done principally with the Swedish increment borer. This permits 

 a widespread survey without damage to the trees. The cores, about 4 mm. in 

 diameter and up to 15 inches long, may be quickly surfaced with a razor blade 

 by use of a sliding cut with the grain and at a low angle to it. On the resultant 

 undamaged cellular surfaces, rings 2 to 3 cells thick, that is, less than 0.10 mm. 

 wide, may be readily seen with a low-power hand lens and proper lighting, and 

 several cores may be examined simultaneously. 



