13G FOREST INFLUENCES. 



Trauf^piralion. 



All vegetatiou takes up a certain amount of water, a part of wbicli 

 is consumed in building- up its body, and a still larger part returned to 

 the atmospliere by transpiration during the process of growth. 



The factor of dissipation having been fully discussed on pp. 9G, 130 of 

 this bulletin, it need not be further considered here, except to recall the 

 conclusion that forest growth transpu^es considerably less than other 

 kinds of vegetatiou. 



Since this water is given off again to the atmosphere in the locality 

 where it has fallen — thus enriching the atmospheric moisture — and is 

 therefore only diverted temporarily for the i^urpose of doing diity in 

 producing useful substance and retaining it in the locality where it has 

 fallen for a longer time, transpiration may even be considered as an- 

 element of conserA^ation. 



There is still to be considered a certain amount of moisture which is 

 retained and stored up in the body of the plant, partly as a necessary 

 permanent constituent, partly as a temporary constituent, being evap- 

 orated when the plant dies or the wood is seasoned. The amounts thus 

 retained vary considerably according to age, cax)acity for transpiration, 

 site, soil, climate, density, slow or rapid growth, weather, seasons, and 

 even the time of the day. It is therefore almost impossible to give 

 anything but very rough approximations, especially as also the differ- 

 ent parts of the tree vary considerably in the amounts of water 

 present. 



The wafer which euters into chemical composition of the wood substance repre- 

 sents about 50 per cent of the weight of dry substance. 



The water hygroscopically retained in the living tree varies within the wide range 

 of from 18.6 to 51.8 per cent in the wood, while the leaves contain as much as 54 to 

 65, and some even over 70 per cent while living; when dry, still 10 to 12 per cent. 

 The wood of deciduous hard woods, like oak, ash, elm, birch, beech, contain in the 

 average 38 to 45 per cent; soft deciduous trees 45 to 55 per cent, and the conifers 52 

 to 65 per cent. White pine when young may show as high as 77 per cent of its 

 weight as water, while larch, of all conifers, has the smallest water capacity, 

 namely, 45 to 55 per cent, ranking with the deciduous soft woods. 



This hygroscopic water is reduced by seasoning to 10 or 12 per cent ; this amount 

 being retained even in well seasoned woods. 



Given the entire mass of wood and foliage on an acre of forest, an 

 approximative calculation of the total quantity of water contained in 

 the trees will show that 56 to 60 per cent of the weight of the forest 

 must be attributed to water, while only 44 to 40 per cent is repre- 

 sented by dry substance. In agricultural crops it is known that the 

 amounts of water are still larger, reaching sometimes 95 per cent of 

 the whole weight. The production of dry substance in a well-kept 

 dense timber forest may amount annually to from 2,500 to 3,000 pounds 

 per acre, leaving, then, for the hygroscopic water, 3,750 pounds, and 

 the chemically fixed water, say, 1,250 pounds ; so that for this factor 

 of dissipation 5,000 pounds in round numbers as a maximum will 

 suffice. 



