374 E. E. GAERTNER 



trees consumed more water than older ones. This fact should be particularly 

 pertinent to species estabhshment. 



Adequate moisture and subsequent growth optimum might not be in 

 the best interest of the tree if longevity is so considered. According to 

 Schulman (1954) trees with great tolerance grown under adverse conditions 

 and with extremely small annual increment (4-6 mm radial mcrement per 

 century o£Fitzroya aipressoides (Molina) Johnston) hve the longest. 



The humidity aspects of the water cycle have been most thoroughly 

 investigated by meteorologists (Geiger, 1950). However, Thornthwaite 

 and Hare (1955) approached the problem from a chmatological point of 

 view and 'expressed the hope that the conceptual framework of forest 

 ecology will approach that of the chmatologist more closely in the future'. 

 Thornthwaite (1948) and Penman (1948, 1956) have independently de- 

 veloped methods of calculating potential evapotranspiration from meteoro- 

 logical records. Whereas Penman (1956) in his approximations utihses 

 diuration of bright sunshine, air temperature, air humidity, and wind speed, 

 Thornthwaite (1948) calculates the potential evapotranspiration using 

 nomograms derived from average temperatures and latitude. Fraser (i957» 

 1961) reported on fluctuations in soil moisture and potential evapotrans- 

 piration over a decade and indicated that although total annual rainfall at 

 Chalk River, Canada, always exceeded potential evapotranspiration, 

 periods of drought did occur in certan years when the evapotranspiration 

 in summer months exceeded current rainfall. Consequently there was a 

 gradual depletion of stored soil moisture with a resultant decrease in 

 growth (Fraser, 1958) of trees on sites that usually have adequate water. 

 Trees on wet sites with seepage water were not affected to the same extent 

 except where the root systems were shallow. Zahner (i955) hi a study of 

 soil water depletion by pine and hardwood stands duruig a dry season, 

 noted httle difference in the rate of loss of soil moisture between the two 

 stands in southern Arkansas. His work supports the contention that evapo- 

 transpiration within a given climatic area is independent of the type of 

 forest cover. 



Daily and seasonal moisture changes in the leaves of yellow birch were 

 elucidated by Fraser and Dirks (1959) in their study of the internal water 

 relations of that species. They noted that wood moisture decreased from 

 almost 100% ui May to about 60% in late June when the leaves were 

 fully unfolded. Relative turgidity of leaves was usually less durmg the 

 day and during periods of drought it decreased even at night. Fraser and 

 Gaertner (1959) outlined micro-environmental research in Canadian 

 forests and emphasised the importance of usmg physiological techniques to 



