WATER RELATIONS OF FOREST TREES 



E. E. Gaertner 



Professional Agrologist, Petawawa Forest, 

 Chalk River, Ontario, Canada 



Seventy years have passed since R. Hartig (1891) in his Anatomic und 

 Physiologic der Pjianzcn considered the factors influencing the amount of 

 growth in a tree. Under 'soil' he recognised depth, moisture, and mineral 

 content ; under 'cHmate', temperature and the humidity of the air ; next was 

 the area for crown and root development; the occurrence of seed years, 

 and finally the inherited characteristics of the individual trees. Today we 

 can still, to a degree, follow this outline to plot the relationship between a 

 tree and its environment. In the present contribution we shall concentrate 

 on the water relations of forest trees. 



It is well recognised that water is an essential component of all living 

 matter, yet its overall effect on the growth of forest trees is still not com- 

 pletely comprehended. It can be said that trees, through their nature, are 

 subjected to a greater variety of water stresses than any other plants, often 

 more devastating because, due to their size, several of these stresses can be 

 active simultaneously. While their roots are active in soil where the drying 

 cycle acts slowly, the shoot tip, often fifty metres or more in the air is 

 subjected to a rapidly changing environment. 



Russell (i960) in the study of flow phenomena in plants has aptly pointed 

 out the difficulties involved in this study. The technique of accurate mea- 

 surements of the flux of material past a given point or of the space distribu- 

 tion of potentials in the flow system still remains to be developed, even 

 for the flow of water through a single plant part such as a root, stem or a 

 petiole. 



Each species requires its own specific water supply for most favourable 

 conditions of growth, and the quantity of water in the soil has a greater 

 influence than any other factor on the distribution of plant species (Bow- 

 man, 191 1). This statement suggests in part, the vast variety of conditions 

 affecting the relationship between the water content of the soil and the 

 individual tree. It has been of concern not only to the forester, but also to 

 the horticulturist and to those concerned with watershed management and 

 irrigation. 



