308 A NATIONAL PLAN FOR AMERICAN FORESTRY 



important ways in which they bring about their total effect on run-off 

 are explained in the following paragraphs. 



INTERCEPTION OF PRECIPITATION 



Anyone who has taken refuge under a tree during a summer shower 

 knows that the crown of both evergreen and broadleaf trees intercepts 

 and holds a certain amount of the rain, which is later evaporated, but 

 that if the rain is prolonged until the leaves and branches are thor- 

 oughly wet, the remainder of the fall reaching the tree drips and is not 

 caught but only delayed in reaching the ground. 



The Forest Service has recorded rainfall at paired stations inside 

 and outside of timber stands in several forest types. Records of 3 to 5 

 summers show that a good pulpwood stand of spruce, fir, and some 

 paper birch in Maine intercepted 26 percent of the rainfall; another 

 Maine stand of pure spruce-fir, 37 percent; a dense saw-timber stand 

 of white pine and helmock in Massachusetts, 24 percent; and a heavy 

 virgin white pine and hemlock stand in Idaho, 21 percent. Briefer 

 studies record that open second-growth forests of oak and hard pine 

 in southern New Jersey intercepted 13 percent of the summer's 

 rainfall; and jack pine and hardwood-hemlock stands in Wisconsin, 

 22 and 19 percent, respectively, of the spring and fall precipitation. 

 The Wisconsin hardwoods when in leaf intercepted 25 percent, as 

 against 16 percent after the leaves fell. 



Interception of snow by the crowns of ponderosa pines at about 

 4,500 feet elevation, in Idaho, was studied by the Forest Service 

 during 1931-32. In a good stand of virgin timber with an under- 

 story of young trees, C. A. Connaughton found that up to the time 

 of maximum storage 27 percent of the winter 's snow had been inter- 

 cepted; in similar mature timber without an understory it was 22 per- 

 cent; and in a somewhat open stand of ponderosa and Ipdgepole pine, 

 20 to 30 feet tall, 8 percent. Studies by Church,^ Jaenicke and Foer- 

 ster, 6 and Griffin, 7 however, indicate that snow interception is con- 

 siderably less in evergreen forest types elsewhere in the West. 



RETARDATION OF SNOW MELT 



Although MacKinney 8 found that light snows melted more rapidly 

 on litter than on mineral soil under a pine plantation in Connecticut, 

 in regions of heavy snow a forest cover retards melt in the spring, 

 thereby materially lessening destructive surface run-off and promoting 

 percolation of the melted snow into the ground. This is due in part 

 to shading of the ground, but mostly to reduction in wind movement; 

 Connaughton found the wind movement during the period of rapid 

 snow melt in Idaho to be more than nine times as great in the open as 

 in the heavy stand of mature ponderosa pine with an almost contin- 

 uous understory of advance reproduction. Even in the open ponder- 

 osa pine forest in which Jaenicke and Foerster worked the wind 

 movement was less than half that in the open. 



4 Church, J. E. Jr. "The conservation of snow. Its dependence on forests and mountains." Scientific 

 American Supplement, Sept. 7, 1912. 



6 Jaenicke, A. J., and Foerster, M. H. " The influence of western yellow pine forest on the accumulation 

 and melting of snow." Mo. Weather Rev., Mar., 1915. 



7 Griffin, A. A. "Influence of forests upon the melting of snow in the Cascade Range." Mo. Weather 

 Rev., July, 1918. 



8 MacKinney, A. L. "Effects of forest litter on soil temperature and soil freezing in autumn and 

 winter." Ecology, July, 1929. 



