THE FOREST AND THE PRAIRIE. 253 



nials to pastures. The grass type has an advantage over some other 

 forms of herbaceous plants in another respect. The growing region of a 

 grass leaf is at the base, often below the surface of the ground. This 

 part is less likely to be injured by fires or by grazing animals. As soon 

 as the danger is passed, growth is resumed if the other conditions are 

 favorable. 



Young trees are subject to destruction by grazing animals. Especi- 

 ally is this true early in the spring, in the late fall, and in winter, for then 

 there is often little to eat. Trees with needle-like leaves are protected 

 more or less in that the needles, acting as prickles, are disagreeable. 

 Yet even the shoots of conifers are often eaten. Young trees of bull 

 pine and Douglas spruce have been found thus injured by cattle. Herds 

 of cattle and sheep kill many trees in the young stages by trampling 

 them. The older trees are less likely to be injured by grazing animals 

 than the younger. The most important growing parts of trees, unlike 

 that of the grass leaf is at the tip of the young shoot, and as this is the 

 most tender as well as the most exposed portion it is likely to be eaten, by 

 which the tree may be damaged or killed. 



From the above it will be seen that the young tree is more subject 

 to drought and is more likely to be destroyed by fire and by grazing ani- 

 mals than -the perennial grasses and other herbaceous forms that grow 

 in the prairie. In the drier regions, then, the prairie forms can exist 

 where trees cannot, and even in regions where the moisture is sufficient 

 to make tree growth possible, the occurrence of fires and the presence of 

 grazing animals may favor the prairie rather than the forest. Of course, 

 the forests that border on the prairie are open and prairie plants can 

 exist there, but where the forest forms a heavy canopy, nearly all grasses 

 are excluded, for as a rule, they require more light than they can get in 

 the deep shade of the trees. 



It should be emphasized that the seedling stage is the most critical 

 period of a tree's life. Probably many trees could exist in drier regions 

 than those in which they are found, if they could survive the juvenile 

 stage. Especially would this be true of those trees that have deep and 

 widespreading root systems, for they might later be able to get a suffi- 

 cient supply of water from the soil to supply the excessive loss by trans- 

 piration. A number of successive favorable seasons no doubt permits 

 some trees to be established in places where otherwise it would be im- 

 possible for them to get a start. 



Another factor that is against tree growth is the prevalence of wind. 

 Of course, occasional violent storms may destroy forests. But since on 

 a windy day there is more water given off than on a still day, even moder- 

 ate winds may be prohibitive of tree growth whereas the occasional storm 

 only partially destroys it. Because this factor is not so obvious, it is 

 often overlooked. Even if the rainfall is sufficient to support tree life, 

 in regions of excessive wind trees are often absent. A good sample of 

 this is found in contrasting the two sides of the Rocky mountains. At 

 the east base of the mountains where the Great Northern Railroad 

 crosses, there is a rainfall of twenty inches or more, with little or no tree 

 growth. The west base in the Flathead valley with a rainfall of ap- 



