CLIMAX COMMUNITIES : SHIFTS WITH TIME 



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FlG. 163. An example of a common form of pollen diagram, which also 

 illustrates what we know of the vegetational history of the northeastern 

 United States although derived from one place (Upper Linsley Pond, North 

 Branford, Conn.) Zone A indicates a spruce-fir forest; the high values for 

 pine are attributed to over-representation resulting from its light weight and 

 its abundant production. In the northeast, a secondary maximum for spruce 

 (A-2) is not uncommon in this period and is thought to represent a local 

 readvance of retreating ice. Zone B, a warm dry period, shows a pine maxi- 

 mum and the beginning of warmth-loving, deciduous trees. Then followed 

 deciduous dominance over a long period, in which hemlock-oak were first 

 important (C-l), then oak-hickory (C-2), and, with cooler moister condi- 

 tions (C-3), an increase of chestnut, followed by a reappearance of spruce in 

 some localities.— Fr om Deevey* 9 



quently, if samples of lake deposits are taken from the bottom up- 

 ward to the present surface of the sediment, the pollen content of 

 successive strata should indicate the nature of the forest, as to 

 genera and their relative abundance, throughout the period of ac- 

 cumulation. 



Sediments on lake bottoms as well as peat deposits have been 



