124 M.G.STALFELT 



the rainy period. However, during the subsequent period of dry weather, 

 the values fell to 0-57 kg even after a few days, so that the consumption 

 became less than on the damp site. The difference between the values R 

 and 5 in Fig. 3 is due to such differences in water supply. 



The water consumption of trees on a dry site is thus dependent on the 

 current precipitation, and is hmited by it. Had more water been available, 

 the consumption would have been greater, and probably as great as that of 

 the trees on a damp site. 



The total quantity of water reaching the root zone of the trees-that is, 

 the sum of P and Q in Fig. 3 -is 289 mm, and the transpiration of the trees 

 211 mm. The difference, 78 mm, consists of water that has evaporated, and 

 possibly of the surface run-off on frozen ground as well. No excess water 

 capable of sinking to the ground-water is, on the other hand, present on the 

 dry site (distance from the water table 2-7 metres). It was possible, by means 

 of special measurements, to estabhsh that the precipitation that had sunk 

 through the moss and htter layer had remained cliiefly in the 20-cm thick 

 uppermost layer of the mineral soil. The water that sinks deeper (5 % during 

 the summer and 23% during the winter) reaches a maximal depth of 

 70-100 cm, but can be carried further by diffusion, and penetrate to a depth 

 of about 150 cm in the morainic soil. The water is subsequently returned to 

 the root zone, when the latter becomes dry. 



According to these measurements, interception is the largest debit item 

 in the water economy of woods. Partly on this account, the wood's need of 

 precipitation is probably greater than that of other plant communities. An 

 example has been given byRutter (1959) who found the water consumption 

 of an 18-year-old Pinus sylvestris plantation to be 15-36% greater than that 

 of grass. 



The many-layered structure characterising naturally-growing woods- as 

 well as their large leaf surface per unit of ground surface- gives these plant 

 communities a transpiration capacity that requires an ample water supply 

 to be fulfilled. Since the supply is diminished to such a great degree by 

 interception, it can be presumed that woods on the mainland have sufficient 

 water only on such occasions when precipitation is so plentiful, and so 

 frequent, that the soil is kept constantly damp. But if the precipitation 

 amounts to only 600-1000 mm a year-as in most regions of Scandinavia - 

 precipitation-free periods of one or several weeks are common. Under 

 such conditions, transpiration can take place without restriction only during 

 a few days after plentiful rain. During the rest of the time, consumption is 

 restricted by the lack of water, so that the transpiration regulators of the 

 plants come into action. 



