368 E. E. GAERTNER 



as a codoniinant with jack pine (Fraser, 1954). In both cases it may be 

 partly intolerance to shade that forces each species to edaphic extremes. 



Soil moisture classification (Briggs, 1897J includes gravitational, capillary 

 and hygroscopic types as well as the vapour phase (Lebedeff, 1928), the 

 first two classes of which are available for tree growth. Condition of 

 aeration (telluric and stagnant water (Hills, 1953)) merits a special con- 

 sideration for it is important not only for growth but even for selection of 

 the tree species on different sites (Clements, 1 921). In north-eastern North 

 America yellow hivch.{Betula lutea Michx. f.) and black ash {Fraxinus nigra 

 Marsh.) are usually found on wet soils abundant with telluric (aerated) 

 water, but they are replaced by black spruce and tamarack (Larix laricina 

 (DuRoi) K. Koch.) if the soil water is stagnant with low oxygen content 

 (Fraser, 1954)- 



Earher workers have been interested in tolerance of trees both to light 

 and water. Fricke (1904) showed that competition for moisture by roots of 

 older trees could cause death of young growth under the shade of mother 

 trees. His experiments were in a 70- to loo-year-old Scot's pine [Piuus 

 sylvestris L.) stand on a dry sandy soil. Trenches 25 cm deep were cut 

 around groups of ten-year-old 40 cm pines under this stand to isolate them 

 from the mature trees. In the first year after trenching the young pines in 

 the isolated groups doubled their terminal growth of the previous year. 

 Similar changes were noted in the ecesis of a rich ground vegetation. 

 Fricke (1904) found the soil moisture conditions in the experimental 

 trenched plots two to three times greater than in untrenched areas. 

 Bjorkman (1944-45), Surmac (1958), and others considered root competi- 

 tion for moisture and nutrients an important factor in Hmiting growth of 

 tree seedlings in a closed stand. In other instances light was considered 

 the limiting factor, for Moore (1926) showed that the addition of moisture 

 to plots under the shade failed, in white pine [Pinus strohns L.), to counteract 

 the shade effect. The seed in the experimental flats in shade showed markedly 

 better germination than those in the open, because the soil surface was 

 prevented from drying. However, the growth of the survivors in the open 

 was many times that of the shaded seedlings in spite of ample moisture. 

 Trenched and untrenched plot experiments were also conducted by 

 Korstian and Coile (1938), who demonstrated that competition between 

 individuals of the forest vegetation for soil moisture is a highly significant 

 factor in growth, development, and reproduction of loblolly and short-leaf 

 pine in the Piedmont plateau. 



Burns (1923) pointed out the variability of annual ring width in New 

 England forest trees, and stated that there was no direct correlation between 



