Man has been able to produce trees where Nature did not see fit to grow 

 them. By using proper cultural methods — such as tillage of the soil before 

 planting to kill grass and weeds and to store moisture, planting only the 

 best seedlings of species adapted to the climate, cultivation for a few years 

 to prevent loss of water from the soil, and protection from livestock and 

 jack rabbits — the Prairie States forestry project in 1939 had about 90,000,- 

 000 trees growing in field windbreaks on farms in the prairie-Plains region 

 (fig. 3). In the few seasons since they had been planted many of these trees 

 had made faster growth than similar plantings in forest regions. 



Local climatic effects of forests. — Limited studies made tend to show mea- 

 surable differences in certain climatic factors, such as wind velocity and 

 temperature, traceable to the presence of forests. Most scientists are of 

 the opinion that such influence is limited to the forest areas or to very short 

 distances beyond the forest borders. The horizontal extent of windbreak 

 influence leeward is held to be about 20 times the height of the trees. Such 

 influence cannot be expected to change the climatic factors over large and 

 remote areas. 



Rainfall and windbreaks . — Forests are a result rather than a cause of rain- 

 fall. Although trees may be grown in much of the Great Plains region if 

 given careful attention, the possibility of affecting rainfall by tree planting 

 is remote. These facts have been recognized in the Prairie States forestry 

 project. The primary purpose in the tree-planting work of this under- 

 taking has been to provide the prairie-Plains region with some of the bene- 

 ficial influences of forests, not to change climate. 



Windbreaks should be grown for what it has been proved they can do; 

 namely, check wind velocity within the limited area of their influence; 

 protect growing crops from hot winds; prevent soil from blowing; protect 

 buildings, livestock, and man from strong winds, both hot and cold; 

 attract birds; supply posts, poles, and fuel (fig. 4). The windbreak also 



Figure 3. — Field windbreak on the Great Plains. How can man insure the success of tree 

 plantings in areas where favorable conditions for their growth are lacking? 



