i64 WALTER TRANQUILLINI 



decrease is not greater is due to the fact that water absorption is possible 

 from the snow-covered soil at least at times. 



The water balance of comparable plants which are completely covered 

 with snow is different. Beginning at the time when they are snowed-up the 

 water content of their needles rises slowly but steadily. Therefore the o.v. 

 falls continuously under the snow. Plants do not lose water by transpiration 

 because the air within the cavities of snow is saturated with water vapour. 

 Hence they are able to balance slowly their water deficit existing at the 

 beginning of the snow cover, first by absorption by the needles, later on by 

 absorption from the soil. For this reason shortly before plants melt out of 

 the snow their o.v. is as low as in the summer. 



Adult trees also have a favourable water balance because they are able 

 to compensate greatly the water loss of their crowns by absorption from 

 the unfrozen soil. Their o.v. does not rise over 28 atm, and the water 

 content of the needles decreases only by 13% of that at the begimiing of 

 November. That their needles dry up less than those of smaller plants on 

 comparable places is due to the bigger water storage in their branches and 

 stems. 



In snowless locations above the timber-line young plants have the worst 

 water balance. In winter they lose 52%, that is somewhat more than one 

 half of the water content in autumn. Their water content decreases to 78% 

 of the dry weight, and their o.v. increases to 42 atm. 



We have to compare tliis dehydration with the aimual trend of the 

 resistance to dehydration, measured by Pisek and Larcher (1954), of several 

 plants in the sub-alpine region, to decide whether the critical Hmit at which 

 plants begin to suffer injuries was exceeded or not (Fig. 8). 



From Fig. 8 can be seen that only in winter are plants dried severely 

 and that this dehydration has its maximum between February and April. 



We see also, that during tliis time the water content of needles of grown 

 up pine trees remains well above the level of dehydration injury. 



Larcher (1957) has collected all measurements of water content which 

 Pisek and co-workers carried out near Innsbruck above the timber-line 

 during several years from January to April. From this collection we can 

 determine the mean and the maximum drought stress (Hofler, Migsch and 

 Rottenburg, 1941; Larcher, i960) (Table 3). This shows that this tree 

 species is extremely adapted to drought. 



Smaller pine trees are stressed much more because they have in their 

 stems less water than taller trees. 



In snowless sites in the zone between timber-line and tree-line the water 

 content of pine plants decreases to the critical hmit (Fig. 7). 



