OQ 



FOREST INFLUENCES. 



The upper straight line represents preeipitatioii, Aviiile tlie u])per curve 

 represents evai)()ration in tlie fields, the hjwer that in the forest. L're- 

 Aious diagrams showed that the amount of evaporation increased rap- 

 idly up to May and June; this one shows that the percentage of rain 

 evaporated increases even more rapidly and reaches its maximum iu 

 April, a month earlier. From this point it descends more sh)wly to the 

 winter months, when the minimum i« reached. Almost the same course 

 is taken by the evaporation within woods. The hatched spaces below 

 represent the percentage of precipitation evaporated within woods and 

 without, the cross-hatched part that saved by the woods or evaporated 

 in open fields in excess of that evaporated in woods. The saving is 

 greatest at the time when the evaporation could dispose of the largest 

 eprcentage of precipitation, namely, in spring and summer. 



It is interesting to see whether the evaporation iphenomena ])resent 

 any difference with different kinds of trees or with trees of ditt'crcnt 

 uges. Figs. 41)/> and c exhibit the annual ([uantity of precipitation 

 and evaporation for stations Under and outside of deciduous and ever- 

 green trees respectively. It will be noticed that the saving of evapo.- 



EVERGREEN TREES. 



DECIDUOUS TREES 



YOUNG TREES. 



Fig. 53. — Pcrceutajic of cv;i point ion in -woods to tlKit in the opiai air 



ration under trees is more marked for deciduous trees. The percent- 

 age of evaxjoration in forests to that outside is 41 for deciduous trees and 

 45 for evergreens. Fig. 41)J gives the same data for young trees. It 

 will be noticed that the j)ercentages are here entirely changed. The 

 evaporation in fields approaches the quantity of precipitation (which is 

 here small), while the evajtoratiou under trees hugs closely that in the 

 open. The evaporation in the open is here 73 ]»er cent of the rainfall 

 Avliile that under trees is 58 per cent, which is 80 per cent of the former. 



Fig. 53 gives the percentages of evaporation in woods compared to 

 that outside for tlie various months. That ior dcciducms trees (the 

 dotted line) varies more during the months and sums up slightly less 

 for the year. The effect of the leafless condition in early spring is 

 faintly visible. 



The unbroken line represents the i)ercentages for young trees. It is 

 very uneven (on account of the series of (►bservations, only seven years), 

 but it run>; much higher aiul ocu'asionally the evaporation under the 

 trees is greater than Hint onrsidc. 



