98 



Tkanspiration. 

 Great amounts of water are returned to the air through evaporation 

 from leaves and stems of plants. This is known as transpiration. Careful 

 exjierimeuts and estimates have shown that plants differ widely as to the 

 amounts transpired and that conditions such as wind, the amount of 

 humidity, sunlight, etc., affect this to a great extent. An oak tree, with 

 seven hundred thousand leaves, will transpire one hundred and eighty 

 gallons of water per day. Von Hohnel estimates that a beech will trans- 

 pire about two thousand two hundred and fifty gallons of water in one 

 summer. Schleider believed that a forest transpired three times as 

 iiuich water as would be evaporated from a water surface equal in extent 

 to the territory covered by the forest. Schiibler considered it only one- 

 fourth as much, and Pfeff, who studied only one oak, found it to 

 vary from 0.87 to 1.50. Hartig believed the transpiration from a forest 

 loss than the evaporation from bare soil of equal extent. Schiibler found 

 that a forest transpired .OG as much as evaporated from bare soil and 

 from sod three to five times as much. Investigations by WoUny show 

 that agricultural crops and forms of low vegetation, such as weeds, 

 transpire greater amounts than do forests. Risler, after a long series of 

 experiments, concludes that forests take up less than one-half as much 

 \\ater from the soil as the average agricultural crop. Some investigators 

 claim that the ground water levd of a forest is lower than that in the 

 ()l>en, and that this is caused by excessive transpiration. Others draw 

 opiK)site conclusions. On the wlmle, Jiowever, it may he said that the 

 forest, at least, transpires no more water than docs any other ordinary 

 form of vegetation. 



Forests anu Run-ofp\ 



It is generally lielieved that forests are great regulators of run-off. 

 that is, that they Increase seepage run-off and decrease surface run-off. 

 This is true to such an extent that the government has recently made 

 provision for buying certain timber lands with the express pur^wse of 

 protecting the headwaters of several navigable streams. 



Many factors enter into the question such as the slope of the ground, 

 the underlying rock, the amoimt and length of time of precipitation, etc. 

 The forest canopy intercepts the raindrops and extends the period of time 

 during which the rain reaches the ground. This gives the soil more 

 time in which to absorb the precipitation and thus lessens the surface 



