WATER RELATIONS OF FOREST TREES 367 



When considering the various sources of moisture to the tree, we might 

 think of them in terms of ground water, or as precipitation which in turn 

 becomes available to the plant through the roots, when it permeates the 

 soil. The various types of soil moisture have been outlined by Kramer 

 (1944). The alternative is moisture that becomes directly available from 

 the atmosphere in the form of dew. Krecmer (195 1) and Stone (1957a) 

 have recently reviewed the hterature on dew as an ecological factor. Stone 

 and several co-workers explored the survival value of dew, though their 

 experimental work has been restricted to seedling material of ponderosa 

 pine [Pinus ponderosa Dougl.), (Stone and Powells, 1955 ; Stone, 1957b) and 

 coulter pine [Pinus coulteri D. Don.), (Stone, Went and Young, 1950). They 

 considered that dew and fog prolonged survival of trees in soil at or near 

 the wilting point by causing a re-saturation of leaf tissue and concomitant 

 reduction of moisture removed from the root system. Waisel (1958) in a 

 study of dew absorption by evergreen trees and shrubs of arid zones stated 

 that there was a negative correlation between soil humidity and dew 

 absorption, but the amounts of dew taken up were completely inadequate 

 to balance the daily transpiration loss. 



The distribution and growth of tree species may be influenced by per- 

 sistent dew and fog in areas of relatively low annual precipitation. The 

 location o£ Sequoia sempervirens (Lamb.) Endl. along the foggy part of the 

 Cahfornia coast and of S. washingtoniana (Winsl.) Sudw. on the misty 

 slopes of the Sierras in inland southern California, is attributed to this 

 cHmatic factor (Harlow and Harrar, 1941). 



Although it is usual to think of fog in terms of moisture, Wilson (1948), 

 while studying the increased apparent photosynthesis on Camellia japonica 

 and Ligustrum lucidutn, came to the conclusion that it was the 20-25% 

 increase of carbon dioxide in the air on foggy days that was responsible for 

 favourable plant growth. 



Tree species may be found over a range of soil moisture sites which vary 

 with latitude and altitude. For example beech's [Fagiis grandifolia Ehrh.) 

 amplitude is fairly wide in southern Ontario, but not as wide as in the 

 southern United States where the population is also more diverse genetically 

 (Camp, 1950). Frequently a species is hniited to edaphic conditions by its 

 intolerance to shade when it fits into niches where competition is less 

 severe. Jack pine (Pinus banksiana Lamb.) is usually associated with dry soils 

 although it does occur on very wet situations with black spruce [Picea 

 mariana (Mill.) B.S.P.) as a codominant in areas south-east of Nighthawk 

 Lake in northern Ontario. Similarly the same black spruce, which is 

 usually found on wet soils, occurs on dry sites in north-western Ontario 



