308 THE STUDY OF PLANT COMMUNITIES * Chapter XI 



accepted than in North America. Most of our studies have been 

 made in areas where bogs are common and within a reasonable 

 range of accessibility of an individual or his students : the north- 

 west, the north-central states, and the northeast. There is still 

 much to be done within the glacial area to complete the picture. 



It is somewhat surprising that investigators are in as close agree- 

 ment as they are. Most generally accepted is a postglacial climatic 

 series beginning with increasing warmth, followed by a period of 

 maximum warmth and drought, followed by a period of decreas- 

 ing warmth, the present. 224 This is applicable to both Europe and 

 America. Some students would subdivide these three major pe- 

 riods, claiming that greater refinement is possible. Others contend 

 that their data contain no evidence of a warm dry period in North 

 America. 



More studies are certainly necessary in North America before 

 agreement can be reached as to all phases of the basic normal pollen 

 spectrum and its meaning in terms of climate. Several scattered 

 studies have been made of deposits beyond the limits of glaciation, 

 and these offer real possibilities. Likewise, there must be more ef- 

 forts to correlate all sources of contributing evidence, 89 ' 203 a truly 

 paleo-ecological approach : floristic, vegetational, zoological, geo- 

 logical, archeological, as well as evidence from pollen analysis. 



DENDROCHRONOLOGY 



Another bioclimatic approach to past history was originated by 

 an astronomer. Dr. A. E. Douglass, when he began studies of an- 

 nual growth rings of trees in an attempt to correlate their differ- 

 ences with climatic variations, presumably related to solar activity. 

 Cross-dating, or matching the growth patterns year by year, for 

 modern trees in Arizona was first accomplished in 1904, but its 

 significance was not fully appreciated until several years later. 91 

 Then a chronology was established from modern times back to 

 A.D. 1400 by matching ring records of modern trees to the ex- 

 terior ring records of earlier trees and so on with trees that grew 

 still earlier. When these records were matched with rings in beams 

 taken from ancient pueblos, the records became complete back 

 to A.D. 1299, then to A.D. 700 and, more recently, successively to 

 A.D. 643, A.D. 500, and finally to A.D. 11. Recent finds suggest 



