640 FUNDAMENTALS OF FRUIT PRODUCTION 



way. Southern are generally warmer and earlier than northern slopes 

 because they receive the more direct rays of the sun. Shreve,^^ who 

 has studied the effect of varying physical environment on vegetation 

 in mountain regions, summarizes some of the more important influences 

 as follows: "Two slopes of the same inclination, which lie in opposed 

 positions so that one faces north and the other south, will present to 

 plants two environments differing in almost every essential physical 

 feature. The temperature of the air on two such slopes might be identical 

 as determined by the thermometer of a carefully established meteoro- 

 logical station, but they are distinctly different as they affect vegetation, 

 for the plants receive very different amounts of heat through diurnal 

 terrestrial radiation. This circumstance is of small importance to full- 

 grown trees and large plants, but is of great importance to young plants 

 and seedlings. The soil temperatures of opposed slopes are also widely 

 unlike, even in the presence of the undisturbed cover of natural vegeta- 

 tion. The two opposed slopes would in all likelihood receive the same 

 rainfall, although this is not necessarily the case. An equal amount of 

 rain might effect an equal elevation of the soil moisture on the two slopes, 

 and to the same depth, but the soil evaporation of the south slope would 

 greatly exceed that of the north slope, and a lower moisture would soon 

 prevail in the soil of the former. Greater or less differences may thus 

 be shown to obtain between the opposed slopes with respect to the most 

 vital features of plant environment. " 



Influence on Soil Temperatures and on the Plant. — Table 10 affords a 

 quantitative expression of the influence of slope on mean soil tempera- 

 ture. Even more significant are the differences in the temperatures of 

 the plants themselves on different slopes. Table 11 shows the mean 

 temperatures one inch beneath the surface of the bark on the north and 

 south sides of tree trunks at the summit of a hill and on its north and 

 south slopes during the winter months in Wisconsin. As would be 



Table 10. — Mean Soil Temperatures (Centigrade) at a Depth op 80 Centi- 

 meters FOR 3 Years on Different Slopes of an Isolated Conical 

 Sandhill at Innsbruck, Tyrol 

 {After Kerner and Oliver^'') 

 N. N.E. E. S. E. S. S.W. W. N.W. 



15.3° 17.0° 18.7° 20.0° 19.3° 18.3° 18.5° 15.0° 



expected, the trees on the south slope show higher midday and afternoon 

 temperatures than those on the northern slope. They also show rather 

 surprisingly lower early morning temperatures. This means that they 

 are exposed to greater extremes and more rapid temperature changes. 

 The relation of such conditions to certain forms of winter injury is pointed 

 out in . the section on Temperature Relations. The tendency of the 

 north side of the trunk on the north slope to be colder than the south side 

 in the early morning while on the south slope the reverse condition holds 



