[S99- 



AMERICAN FORESTRY ASSOCIATION. 



281 



rocks to cover up the lower lands a 

 double disaster. The fertile soil of. the 

 higher lands is destroyed, the fertile soil 

 of the lower lands is buried under a waste 

 of debris. 



There is one other subject to consider 

 evaporation. Under this head will be in- 

 cluded transpiration from foliage. Tem- 

 perature and wind are the chief controlling 

 elements in evaporation. Woods lower 

 temperature and reduce the velocity of 

 the wind. It is to be expected, therefore, 

 that evaporation in woods would be much 

 smaller than in the open. Such is found 

 to be actually the case. The observations 

 of sixteen forest stations in Germany show 

 a marked saving effected by the woods. 

 Of the rainfall an average of 42 per cent, 

 was evaporated in the open and 24 per 

 cent, in the forest a clear saving of iS 

 per cent. The evaporation from water 

 surfaces in woods was found to be about 

 38 per cent, of that from water surfaces 

 in the open. 



As an offset to the saving in the evap- 

 oration comes the moisture transpired 

 through the foliage, and that retained in 

 the substance of the tree. The transpira- 

 tion computed by various observers ranges 

 fro.m an equivalent rainfall of one-quarter 

 inch per annum for four-year old Firs, up 

 to 15 inches for cereals and 37 inches for 

 grasses. Forests of mixed growth trans- 

 pire about 6 x / 2 inches. According to ob- 

 servations at the Austrian stations, decidu- 

 ous trees transpire during the period of 

 vegetation 500 to 1,000 pounds of water 

 per pound of dry leaves, and the conifer- 

 ous from 75 to 200 pounds. (This sug- 

 gests the natural selection of conifers for 

 our own mountain slopes.) 



One remark of Hohnel, regarding the 

 Austrian observations, is very suggestive. 

 He says: "A plant will transpire in pro- 

 portion to the amount of water which is 

 at its disposal." This remark serves to 

 illustrate the point that willows and other 

 water-loving growths along the streams 

 consume more water than they save. There 

 is a coincidence between the fall and rise 

 of the Los Angeles river and the budding 

 and fall of the willow leaves. 



It is estimated that a coniferous forest 



will transpire S per cent, of a total rain- 

 fall of 20 inches and a Beech forest 48 

 per cent. The amount of water annually 

 absorbed into the structure of the trees has 

 been estimated as ranging from 19 to 25 

 per cent, of the weight of the wood, and 

 54 to 65 per cent, of the weight of the 

 leaves. 



The hard wood deciduous trees absorb 

 38 to 45 per cent., the soft wood 45 to 55 

 per cent., and the conifers s 2 to 65 per 

 cent. These quantities are equivalent to 

 about 2 per cent, of the water required for 

 transpiration and are in addition thereto. 



On the basis of these figures a conifer- 

 ous forest, which of all forest makes the 

 .best showing, will give a net increase to 

 the ground storage of about 10 per cent, 

 of the rainfall, to say nothing of its effect 

 upon increased conductivity of the soil and 

 the storm water held back so that the earth 

 has better time to drink its fill, in them- 

 selves important items. 



The State of New Jersey has wisely ex- 

 pended large sums in measuring the flows 

 of its streams and in ascertaining the phvs- 

 ical elements controlling these flows. The 

 Engineer of that State, in language free 

 from hesitancy, says, after long labors and 

 study on the subject : 



" We believe it will be helpful to the 

 cause of forestry in the future if the ef- 

 fects of forests upon stream-flow are more 

 carefully and accurately stated. Their ef- 

 fect in holding and preserving the soil 

 upon slopes is very well known, and be- 

 sides this they create a mass of humus and 

 absorbent matter upon the surface which 

 has an effect upon stream-flow, and the 

 o-eneral evils resulting from deforestation 

 are a matter of careful observation and 

 record, so that too much stress cannot be 

 laid upon the desirability of preserving a 

 proper area of forest. 



"The study of the streams shows that 

 in every case, almost, it is the watershed 

 on which is the largest proportion of for- 

 est which shows the largest flow from 

 ground-water." This is particularly per- 

 tinent to the present discussion. 

 II. Hawgood, 



M. Inst. C. E., 

 Los Angeles, Cal. 



