(348) 



"as determined by various factors, which may be quite different 

 in different successional series, should be the criterion." If shade 

 is the critical factor, the climax might well be less moist than a 

 preceding stage. ^ 



The hemlock type well illustrates the above conception. 

 Hemlock can stand much more shade than the oaks; hence when 

 protected from fire, it is able to survive under their crowns and 

 eventually come up and crowd them out. Two hardwoods only, 

 beech and sugar maple, ^ are more tolerant of shade than hemlock, 

 but are less abundant around here than the oaks. It is reasona- 

 ble, therefore, to consider hemlock as the climax forest. 



Since hardwoods make up the climax forest further south, and 

 hemlock is the climax to the east and north of New York City, 

 it has been suggested that perhaps we have on the Botanical 

 Garden grounds two climax forests existing side by side. 



Whatever may be the cause, the two types have not mixed 

 appreciably in the long period that they have lived in contact 

 with each other, with every opportunity for the mutual inter- 

 change of seed and for either to invade the other. 



The climatic differences which the above records show between 

 the two types of forest do not seem sufficient to account for the 

 distinctness of the two, unless the hemlock grove is at the absolute 

 limit of the warmth which this type of forest will endure. If this 

 were the case, any increase of temperature would prevent the 

 establishment of hemlock. But the soil temperatures in the 

 hardwoods had a maximum of only 2° F. higher at 6 inches, and 

 only .6° higher at i8 inches, which seems so small as to require a 

 dividing line much sharper than appears reasonable. It will be 

 necessary to look elsewhere for the cause of the difference, and 

 soil conditions appear to offer the most promising line of attack, 



SUMMARY 



Representative examples of hemlock forests, and two examples 

 of hardwoods, were selected to cover roughly the north and south 

 range of the hemlock type. The southerly representative was 



* Nichols, G. E. "A Working Basis for the Ecological Classification of Plant 

 Communities." Ecology, 4: 11-23; I54~i79- 1923- The quotation is from a 

 letter written by Dr. Nichols about this particular study. 



' Burns, George P. "Minimum Light Requirements Referred to a Definite 

 Standard." Vermont Agr. Exp. Sta. Bull. 235, 1923. 



