70 MAN ON THE LANDSCAPE 



willows confined to wet places. Few herbs can survive the dense and 

 year round shade of coniferous forests, and most coniferous seedlings 

 cannot survive without shade. Corn cannot survive the hot, dry 

 winds which are taken in stride by the cactus. 



The decision between survival and death may hinge on very slight 

 change in a habitat factor, when it nears the limit of tolerance. Being 

 shaded one hour more per day than a fortunate neighbor may mean 

 that one red oak sapling will die in its youth and its neighbor live. 

 On the other hand young beeches must have deep shade, and an hour 

 too many of direct sunlight per day will kill the exposed seedling. 



The evidence indicates that differences in plant growth and ability 

 to survive in a habitat are rarely due to the influence of only one 

 ecological factor, but rather to a complex influence. This can be 

 readily understood. Shade, for instance, reduces temperature of 

 air, leaf, and soil ; it reduces, also, evaporation from soil and transpi- 

 ration from leaf; in effect, shade is equivalent to greater rainfall, in 

 that soil moisture is increased or conserved. 



Leaves in shade tend to grow larger because of a need to intercept 

 more light for growth and survival. Shade leaves on a tree may have 

 a very different size and shape from those in the sun drenched top. 

 Plants whose leaves are valuable, such as tobacco, lettuce, kale, chard, 

 etc., may, in some respects, yield better crops if a certain amount of 

 shade is provided. On the other hand, reducing the sunlight may 

 reduce the nutritional quality of some plants; tomatoes, for example, 

 produce vitamin C in proportion to sunlight. Forest nurseries use 

 slatted frames placed on supports to provide the partial shade neces- 

 sary for the production of healthy seedling trees. Gradually, the 

 light is increased by periodic removal of the shades until the two to 

 four year old transplants are adjusted to full sunlight and ready for 

 planting on even bare eroded hillsides and gullies. 



Micro -climates. There is confusion in store for the student who 

 goes out to observe communities of plants. He has been told, let us 

 say, that a certain hilly, humid area comprising several counties, is, 

 or originally was, an oak-hickory forest. He has learned at second- 

 hand that oaks and hickories require a well drained soil and 30 to 50 

 inches of rain per year. He knows that wet soils in that general 

 climate will probably have an Elm-Willow-Sycamore association on 

 them, that somewhat less wet soils should have Beech-Maple, and that 

 the driest, best drained areas will be dominated by Red Oak-Black 

 Oak. So he goes out, and along the streams, sure enough, he finds 

 elms, willows and sycamores. But up on a dry, bare hillside there are 

 also sycamore seedlings, saplings, or trees. The student scratches his 

 head and wonders. Later he learns that the ecologists have about de- 

 cided that plenty of light is more essential to the sycamore than lots 

 of water. It needs both, but light seems to be the critical factor. 



The student moves on to a gentle slope, and there are the maples, 

 or the beeches, or both. Here and there he sees oaks, or hickories, or 

 both. Up on the hillsides are White oaks, and on the wind dried, sun 

 dried hilltops are Red and Black oaks. Then he discovers, well up on 



