ENVIRONMENT 111 



and sunlight are abundant and vegetation is densely crowded and 

 competition is intense, as in wet-climate forests, many individuals 

 must perish, and pests are largely the agent. Climatic conditions 

 influence these pests and we find therefore climatic variations in the 

 trees injured by them, but such effects are apt to be more hidden and 

 less clear and direct than in the dry Southwest, where the trees are 

 isolated and rainfall is the controlling factor. Pests, of course, attack 

 the trees in different ways, but when the growth is seriously interfered 

 with the rings show diminished size and may disappear, and abnormal 

 growths may enter. 



ENVIRONMENT INDICATORS 



The preceding pages of this chapter have dealt with the effects in 

 tree-rings of various exterior forces; the present paragraphs are 

 intended as a brief introduction to the general reversal of this process, 

 namely, estimation of exterior conditions by internal evidence in the 

 trees. So far as rainfall is concerned this is not new, for most of the 

 work done by the writer has had that purpose as its central theme. 

 But in approaching the study of prehistoric and geologic material, 

 the general consideration of all information contained in the rings 

 becomes more and more important. So long as one can apply the 

 principles of cross-identification, it is easy to isolate the climatic 

 effects, for climatic effects prevail over large areas for a short time, 

 while topographic influences modify the growth-rates in small areas 

 more or less permanently. Thus, as the use of groups of trees becomes 

 less and less possible in studying climates more and more remote, the 

 separation of climatic from topographic features requires notice to be 

 taken of all indicators of environment found in the trees. Without 

 any pretension to completeness, the following classification paves the 

 way to a future study of this interesting subject. 



EVIDENCE IN INDIVIDUAL RINGS 



This varies in different species, but in the yellow pine a widely 

 double ring means a double rainy season, especially if habitually 

 recurring. Narrow and indistinct doubles and multiples probably 

 mean the same, but in the extreme, multiple rings may refer merely 

 to individual storms. 



Average ring-size — This reflects water-supply, which consists (1) 

 of rainfall modified by continent, mountain ranges, latitude, and 

 altitude; (2) of ground- water, or secondary rainfall, modified by 

 drainage contours and kind of soil. 



EVIDENCE IN SINGLE TREES 



Ring-type — Ring-types are: (1) complacent, meaning reasonably 

 sure water each year; (2) complacent surges, meaning some slow 



