WATER RELATIONS OF FOREST TREES 373 



limited for the potential need of the tree. In trees younger than 20 years, 

 dieback rather than cracking, occurs. 



It is obvious from the forgoing discussion that occasional drought 

 affects trees differently at various stages of their development, and that we 

 are dependent on natural occurrence for our data; exact duplication w^ith 

 mature material is nearly impossible without root disturbance. An excep- 

 tion to this might be found in the work of the sahnity laboratory at 

 Riverside, California, where, due to minimal rainfall, the water conditions 

 of any tree under investigation can be closely regulated. 



Many aspects of water relations of trees have been discussed by Kramer 

 (1952). He has stated that, in winter, except during warm periods, transpira- 

 tion of conifers is almost as low as that of bare branches of deciduous trees, 

 probably because of the cold soil hindering water absorption. The question 

 whether soil moisture is equally available between field capacity and wilting 

 point has been approached by Stanhill (1957) who, in his comprehensive 

 review, concluded that, in 80% of the experiments scrutinised, growth was 

 affected by differences in the amounts of available water. 



It is generally agreed that transpiration is affected by factors such as 

 Hght intensity, humidity, temperature and species selection as long as soil 

 moisture is above the wilting point (Pisek, 1950 ; Veihmeyer and Hendrick- 

 son, 1955 ; Ladefoged, 1956). This is shown by an increase in sap flow from 

 a morning low to a midday height, with further checking by sudden 

 drops of temperature or showers (Huber and Schmidt, 1937; Schubert, 

 1941; Kuntz andRiker, 1955). The maximum transpiration rate in July 

 with a gradual subsequent decrease, would point both to the high tem- 

 perature and long days as influential. Of course, even water availabihty 

 might be affected by the physical characteristics of the tree studied, i.e., 

 by the type and length of its root systems (Ladefoged, 1956). Kramer 

 (1952) quotes Wiggans who recorded apple trees in Nebraska drawing on 

 water to a depth of ten metres. An extreme of cHmatic effects on evapo- 

 transpiration is suggested by Wilm (1957), who beheves the dwarf 'rain 

 forest' of the Puerto Rican mountains to be caused by suppressed tran- 

 spiration rates in the saturated atmosphere. 



The relation of forests to run-off and water storage in the soil was 

 extensively reviewed by Zon (1927). Stalfelt (1944) studied water con- 

 sumption of spruce and found that interception of precipitation by spruce 

 crowns amounted to over 50%. A much smaller, yet significant rainfall 

 interception was found in forest stands at Chalk River (Fraser, 1956). 

 Stalfelt (1944) also reported on considerable cuticular absorption, par- 

 ticularly where water in the soil was deficient. It is significant that younger 



