98 



Transpiration. 

 Great nmounts of water are returned to the air through evaporation 

 from leaves aud stems of plants. Tliis is known as transpiration. Careful 

 experiments and estimates have shown that ])liints 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 

 .<^even hundred thou.sand leaves, will transpire one hundred and eighty 

 gallons of water per day. A'on Ilohnel estimates that a beech will trans- 

 jjii'o about two thousand two hundred and fifty gallons of water in one 

 summer. Schleider believed that a forest trauspii'ed three times as 

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

 to the territory covered by the forest. Schvibler 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 

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

 that a forest transpired .06 as much as evaporated from bare soil and 

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

 tliat 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 

 water from the soil as the average agricultural crop. Some investigators 

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

 open, and that this is caused by excessive transpiration. Othei's draw 

 opi)osite conclusions. On the whole, however, it may be said that the 

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

 form of vegetation. 



Forests and Kun-okf. 



It is generally believed that forests are great regulators of run-oft", 

 that is, that they increase seepage run-otT and decrease surface run-oft" 

 This is true to such an extent that tlie government has recently mad. 

 provision for buying certain timber lauds with the express puri>ose of 

 Iirotecting the headwaters of several navigable streams. 



Many factors enter into the question such as the sloi»e of the ground, 

 the underlying rock, the amount and leiii,'th nf time (•!" precipitation, etc. 

 The forest canopy intercepts the raindi-nps aiui extends the period of time 

 during which the rain readies the urimiid. 'I'his gives the soil more 

 time in wliich to ;il)sorb tjie preeipihilion ;ind thus lessens the surfOCf 



