26 FORESTRY 



cannot follow it. Deep accumulations of silt, washed down 

 from slopes into gullies or on flat benches, and supplied with 

 underground seepage from above, make ideal conditions of 

 soil moisture, since the storage room for water is large, the 

 supply is continuous and the drainage prevents too great 

 a surplus. Aside from the depth and consistency of the 

 soil, and the amount of rainfall it receives, the water con- 

 tent is determined by the topography and the drainage. If 

 the water has no chance to run off or seep away under- 

 ground, swamps are formed, in which the water stands con- 

 stantly at or near the same level, filling all the pores and 

 drowning the soil by exclusion of air. In other places un- 

 derground water comes to the surface, making marshy 

 spots, even on hillsides. The water in a swamp may not 

 necessarily overflow the surface at all times, but the dis- 

 tance down to the average water table will have a profound 

 influence on the character of the swamp vegetation. 



The Effect of Soil Moisture on Competition of Species. 

 — The difference in the requirements of tree species for 

 moisture is very marked. Some distinctly prefer swampy 

 soils, and have adapted themselves to them. The Bald 

 Cypress of the south has pushed this adaptation so far that 

 it develops special growths on its roots in the form of up- 

 right conical excrescences known as knees, through which 

 the roots receive the necessary air. The white cedar and 

 tamarack of the Lake States are swamp trees. But most of 

 these species are found to grow much more rapidly when 

 they obtain a foothold on soil which is elevated above the 

 level of the swamp. They have undoubtedly been forced 

 to occupy these wet soils by their inability to cope with 

 species better able to fight for room on well-drained soils. 

 Having become inured to an excess of moisture they would 

 probably suffer severely from drought if they should acci- 

 dentally get started on soil not supplied with constant un- 

 derground moisture. 



