WATER RELATIONS OF FOREST TREES 371 



and growth regardless of species concerned. The site under scrutiny had a 

 niinimuni run-off and the rainfall data used were taken 20 kilometres from 

 the experimental area. 



Though we usually think of moisture in terms of availability , it should 

 be remembered that an excess can be just as detrimental to most species. 

 Soil aeration is intimately connected with soil moisture since surplus of the 

 latter partly excludes the former. As previously indicated, the amount of 

 oxygen to roots can become of paramount importance to healthy root 

 survival. However, Kramer (1950) considers that soil aeration is seldom a 

 serious problem in forests because natural selection tends to ehminate the 

 trees not well adapted to poorly aerated (usually very wet) habitats. Such 

 is the case in the great Clay Belt of northern Ontario (Fraser, 1954), where 

 black spruce predominates in areas of high water table and represents an 

 edaphic cUmax (Weaver and Clements, 1938). 



Cannon and Free (1917) showed that roots of mesquite cease to grow 

 when oxygen is lacking or where there is more than 25% carbon dioxide. 

 On the other hand, willow is very resistant to lack of oxygen in the soil 

 atmosphere. In Java, the oxygen requirements of root systems in several 

 hundreds of species of trees, shrubs, and herbs, were investigated by 

 Verhoef(i943). 



The gley horizon underlying the constantly or temporarily submerged 

 soils in the forest of Chaux, at less than 3 5 cm, can be penetrated only by 

 alders, willows and American red oak [Quercus borealis v. maxima) 

 (Lachausee, 1950). This oak has a powerful tap root which pierces the gley 

 horizon which the native Q. robin camiot do. The latter dies back at 80 to 

 100 years. 



White (Picea glauca (Moench.) Voss.) and black spruce, red (Pinus 

 resinosa Ait.), white and jack pine, as well as balsam fir {Abies balsamea (L.) 

 Mill.) in naturally flooded areas, were both measured and classified 

 according to health (Ahlgren and Hansen, 1957). Black spruce and balsam 

 fir showed the greatest endurance and abiHty for recovery, although all 

 trees were affected to a various degree. This varied not only with the 

 species, but age of the tree as well as the duration of flooding. The 30- to 

 6o-cm class was most seriously affected and damage appeared to decrease 

 progressively with size within the individual species. Some of the trees 

 lost all their needles during flooding, but disease did not become evident 

 in any of them until two years later. After 60 days of flooding, all sizes of 

 balsam fir showed 80 to 100% mortahty, and 4-5 m high white spruce 

 showed 100% mortahty; even 29 days of flooding affected the 30- to 60-cm 

 high white spruce. Of the hardwoods observed, paper birch was affected 



