266 FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



Irrigation is generally too expensive to be applied to wood crops, except perhaps in the arid 

 regions, where the benefit of the shelter belt may warrant the expense. 



Attention to favorable moisture conditions in the soil requires the selection of such kinds of 

 trees as shade well for a long time, to plant closely, to protect the woody undergrowth (but not 

 weeds), and to leave the litter on the ground as a mulch. 



Different species, to be sure, adapt themselves to different degrees of soil moisture, and the 

 crop should therefore be selected with reference to its adaptation to available moisture supplies. 



While, as stated, all trees thrive best with a moderate and even supply of moisture, some can 

 get along with very little, like the conifers, especially pines; others can exist even with an 

 excessive supply, as the bald cypress, honey locust, some oaks, etc. The climate, however, must 

 also be considered in this connection, for a tree species, although succeeding well enough on a dry 

 soil in an atmosphere which does not require much transpiration, may not do so in a drier climate 

 on the same soil. 



In the selection of different kinds of trees for different soils, the water conditions of the soil 

 should therefore determine the choice. 



LIGHT CONDITIONS. 



To insure the largest amount of growth, full enjoyment of sunlight is needed, But as light 

 is almost always accompanied by heat and relative dryness of air, which demands water from the 

 plant, and may increase transpiration from the leaves inordinately, making them pump too haid, 

 as it were, young seedlings of tree species whose foliage is not built for such strains require 

 X>artial shading for the first year or two. The conifers belong to this class. 



The great extent of our country, involving as it does wide ranges of climatic and soil condi- 

 tions, makes it impossible to give complete lists of trees adapted to various soil conditions in all 

 parts of the United States. The safest rule for the planter to follow is to be guided in his 

 selection of species by the character of the growth in similar sites near the land to be planted. 

 Speaking generally, the following lists may be useful: 



Trees that endure wet soils. — South of the Ohio Eiver and central Missouri: Bald cypress, 

 white cedar, red cedar, black gum, holly, water oak, red birch, cottonwood. North of the Ohio 

 and Missouri rivers: White cedar, arbor vitas, larch, black spruce, cottonwood, white willow, 

 sycamore. 



Dry soils. — South of the Ohio Eiver and central Missouri: Mesquite (Texas and southwest), 

 black oak, hackberry, shortleaf pine. North of Ohio and Missouri rivers: Bull pine, jack pine, 

 scrub pine, white oak, post oak, jack oak. 



The remaining species, north and south, require moist or fresh soils for their development, 

 conditions under which all species succeed best. 



In later life the light conditions exert a threefold influence on the development of the tree, 

 namely, with reference to soil conditions, with reference to form development, and with reference 

 to amount of growth. 



The art of the forester consists in regulating the light conditions so as to secure the full 

 benefit of the stimulating effect of light on growth without its deteriorating influences on the soil 

 and on form development. 



As we have seen, shade is desirable in order to preserve soil moisture. Now, while young 

 trees of all kinds, during the u brush" stage of development, have a rather dense foliage, as they 

 grow older they vary in habit, especially when growing in the forest. Some, like the beech, the 

 sugar maple, the hemlock, and the spruce, keep up a dense crown ; others, like the chestnut, the 

 oaks, the walnut, the tulip tree, and the white pine, thin out more and more, and when fully grown 

 have a much less dense foliage; finally, there are some which do not keep up a dense shade for any 

 length of time, like the black and honey locust, with their small, thin leaves; the catalpa, with its 

 large but few leaves at the end of the branchlets only, and the larch, with its short, scattered 

 bunches of needles. So we can establish a comparative scale of trees with reference to the amount 

 of shade which they can give continuously, as densely foliaged and thinly foliaged, in various 

 gradations. If we planted all beech or sugar maple, the desirable shading of the soil would 

 never be lacking, while if we planted all locust or catalpa the sun would soon reach the soil and dry 

 it out, or permit a growth of grass or weeds, which is worse, because these transpire still larger 



