CLIMAX COMMUNITIES : SHIFTS WITH TIME 



307 



adequate sample in the count of grains is not agreed upon by all 

 investigators but fewer than 150-250 grains are rarely counted. 



When the proportions for genera are known for each stratum, 

 they are represented in a standardized form, known as a pollen 

 diagram, in which pollen spectra— the relative importance of each 

 genus in a stratum— are plotted on horizontal lines, one spectrum 

 above another to show the progressive changes for genera, which, 

 are shown on vertical lines. A pollen diagram is no more than a 

 means of visualizing the pollen spectrum of a section— a vertical 

 series of samples from the bottom to the surface of a deposit. 

 Changes in the spectra from the bottom upward are, of course, to 

 be correlated with time. 



The shortcomings and pitfalls of pollen analysis as a method of 

 determining past vegetation and climate are appreciated by all 

 who have used it. 48 ' 97 There are sources of error in methods, in 

 records, which may be incomplete, and in identifications which 

 may not always be correct, and interpretations may be based upon 

 inadequate data. Because of its simpler flora and greater amount 

 of study, the pollen spectrum for Europe is better established and 



Age in 

 years 



2O0O 



400O 



6000 



6000 



10,000 



12000 



14.000 



16.000 



16.000 



Period itr 

 Cooler 



Moijler 



Per/od n'i 

 Maximum 



Warmth 

 and 



Dryness 



Period li 



Increasing 



Warmth 



and 

 Dryness 



Period / 

 Cool-Moisf 



Pmus 

 monhcola 



Pjeudolsupa 

 la*, i folia 



Tsuga 

 hererophy/la 



% eo 



to 



60 



20 



20 



-hO 



oO 



eo 



4-0 



FlG. 165. A composite of ten pollen profiles from the Puget Sound region, 

 which is indicative of postglacial climate and vegetation in the northwest 

 although not typical of all areas as to species. The volcanic ash level, present 

 in all northwestern profiles, is considered to be of common age. Such com- 

 posite profiles, because they eliminate the sharp fluctuations from level to 

 level found in individual profiles, give a better picture of the trend of post- 

 glacial vegetation.— From Hansen. 



115 



