THE FOREST AND SPRINGS 369 



The action of the forest besides differs appreciably at different seasons of the year. 



In winter the trees are denuded, although at that period they are more than ever 

 favorable to the condensation of atmospheric vapors. Also once in every three times 

 almost the gauge under the trees is the fullest, and this one contains on an average 

 96.9 per cent of the water fallen during the months from November to April — almost all 

 of it. 



The condensation by the tree crests suffices therefore to compensate almost entirely 

 for the loss due to the adherence of a part of the water fallen upon the trees. 



During the summer, on the contrary, the branches covered with foliage intercept the 

 rain more efficaciously. If one compares the maximum rainfall occurring in the months 

 from May to October on the open ground, and under the trees, respectively, one finds 

 that they are equal to the numbers 100 and 88.8. 



If the proportions in which the forest soil is watered vary according to the season, 

 they ought therefore to be somewhat different according to the age and density of the 

 plantations. In fact, calculating by averages of three successive periods approximately 

 equal, and no longer by the total period during which observations have been made, it 

 has been ascertained that under the trees in summer 89.1 per cent, 88 per cent, and 

 89.4 per cent of the quantity of water precipitated by atmospheric water have been 

 collected. The portion retained by the crowns of trees increases from 1867 to 1888 at 

 the same time as does that under cover, owing to the growth of the tree under observa- 

 tion from 40 to 62 years old. It then diminishes as a result of the gradual lightening of 

 the foliage coinciding with the decline of the vegetation, which shows itself in the 

 hornbeam sprouts in the forest of Haye about every 60 to 70 years under ordinary 

 conditions. 



Let us quote a few figures taken from abroad relative to the proportion of rain inter- 

 cepted by the tree tops. 



The absolute quantity of water that adheres to the trees and returns to the atmosphere 

 by evaporation is regular for the same tree, whatever may be the duration of the rain- 

 fall, but evaporation is sufficient to absorb the rainfall entirely, especially in summer, if 

 the rain is of short duration, while it will only absorb a steadily decreasing quantity in 

 proportion to the increase in the duration of the rainfall. 



The loss of water owing to shelter is therefore very variable according to the local 

 distribution of rains. 



The same applies to different species of trees. By placing a great number of rain 

 gauges under the crown of the same tree at varying distances from the trunk, M. Boppe 

 has tried to obtain an average of the quantity of water which passes directly through 

 the tree tops. He has then measured separately the water running down the trunks. 

 Combining these results with those obtained by himself, M. Ney ^^ has arrived at the 

 conclusion that the loss due to adherence to the crowns is: 15 per cent of the annual rain- 

 fall for the beech tree; 20 per cent for Scotch pine; 33 per cent for spruce. 



If one considers the water rainfall separately, the only important one for the feeding 

 of springs, one ascertains according to the same author: 



That the crowns of beech trees retain 7 per cent, those of pines 15 per cent, and 

 those of lower spruce pines 20 per cent. This last figure should be still in relation to the 

 surplus of rain acquired by the presence of forests, according to French experiments. 



The figures given by M. Ney do not take into account the water supplied by the tree 

 crowns through the process of condensation, and moreover are only based on a small 

 numlier of measurements. 



It remains now to draw conclusions from all the preceding data, relative to the feeding 

 of springs.25 The question to be resolved is as follows: Is the ground under cover of 



24 Op. cit., p. 10. 25 Dq Bouville, op. cit. 



