106 



FOREST INFLUENCES. 



RAINFAT.Ij in, above, ANT) NEAR FORESTS. 



The first result from the parallel system of forest observations with 

 re.i^ard to rainfall is the determination of the quantity of rain under 

 trees as compared Avith that outside. Aside from any possible differ- 

 ence of rainfall over forests and ontside, the tree and its folia,i>e serve 

 as a cover for the space underneath. The result is that the rain 

 gauges under the trees fail to get as nnn^h precipitation as 'those which 

 are in the open fields. Generally the rain gauges are alike in the two 

 stations. Mucli dilference is caused by the position in which the for- 

 est rain gauge is placed, whether under a crown of dense foliage or 

 near its edge, or under the lighter fidiage of smaller or scattered trees. 

 Very little informntion is given on this ])oint. The rain actniilly caught 

 aiinnnlly under trees in the German service varies from 8!> i^er cent of 

 that in the open (at Carlsberg) to 52 per cent (at St. Johauu). The 

 average is 75 per cent, or three-quarters; and ten of the sixteen sta- 

 tions do uot vary 5 per cent either way from this mean. A twelve- 

 years series of observations gives, for the three Swiss stations, 90, 84, 



Fig. 54. — Vercontane of pieri]iitation, received uurtcr trees (all kind.s) — German oli.sei-vatioiis. 



and 77 per cent, giving a mean of 84 per cent, considerably larger 

 than that given by the German stations. 



The station at Bellefontaine, near I^Tancy, also gives 84 per cent as 

 the mean of eleven years of observation. 



The proportion of the precii^itation which passes through the foliage 

 varies slightly with the season. Fig. 54 shows the relative amounts 

 (percentages of ontside precipitation) for the individual months. The 

 percentage withheld is somewhat greater in the warmer season, thongh 

 the difference is not great. Fig. 55 distingiiislies 1)etween the ever- 

 green and decidnous trees. Tlie values for the evergreens are rep- 

 resented by the broken line. Tlicy :ipi)ear very even, witli only a slight 

 tendency downwards in the warm season. Tlie dotted line is the curve 

 of the percentages for deciduous trees. It is less even than the other. 

 The dip downward is decidedly more marked, but it is not great. It 

 would seem that the advent of the foliage would have a much greater 

 effect than that represented by the slight turn downwards of the curve 



