PRECIPITATION IN A SPRUCE STAND 123 



each day. The daily transpiration was calculated from these values. The 

 quantity of needles (in kilogrammes) per tree was also measured at the end 

 of the experimental period. The number of experimental days was 42, 

 distributed over the months of May- August during 4 years. The days were 

 chosen irrespective of the weather, except that measurements were not 

 made on days when it rained for more than 4 hours. 



The total transpiration during the 4 months of May- August was calcu- 

 lated from the mean values of the daily measurements. For trees on dry sites 

 it amounted to 21 1 mm, and for trees on damp sites to 378 mm, calculated 

 as the precipitation on the root zone of the trees under the crowns and in the 

 gaps. 



Naturally, I am aware that the method has its deficiencies. The greatest 

 error is probably that the transpiration measurements were made on 

 samples taken only from the lower third of the crowns, and that the samples 

 were allowed to transpire at a level of merely 1-2 m above the ground. The 

 trees were about 20 m high. At the natural site of the samples -higher up 

 in the crowns of the trees -there are greater opportunities of evaporation. 

 This applies particularly to the upper half of the crowns. This error can be 

 expected to have a one-sided effect on the experiments, resulting in the 

 values for transpiration being too low. Obviously, extrapolation of short- 

 term values to longer periods, such as days and months, is an additional 

 source of error. Even so, I consider that the deficiencies and difficulties of 

 the momentan method are less than those inherent in other methods for 

 measuring the water consumption of plants under natural conditions. 



A comparison between the water consumption of the trees and the supply 

 of precipitation to the roots is shown in Fig. 3. It can be seen that the trees 

 on a dry site [R in Fig. 3) utilise a quantity of water corresponding to the 

 precipitation supphed to their roots during the summer and, in addition, 

 a considerable part of the precipitation stored in the ground during the 

 autumn and winter. On a damp site (5 in Fig 3) -where the roots of the 

 trees reach the ground-water-the quantity of water consumed represents 

 the entire annual precipitation in the root zone, as well as part of the 

 ground-water. Thus, at damp sites, the trees have a draining effect on the soil. 



How the water supply to the root zone influences the consumption of the 

 trees can be seen from a comparison made between the trees on the two 

 sites during a rainy and a dry period, respectively. It was found that the 

 trees on the damp site transpired about as much (0-73 and 0-74 kg of water 

 per kg of needles per day) during both periods. Their water consumption 

 was thus practically independent of the current precipitation. The water 

 consumption of the trees on the dry site was about as large (0-69 kg) during 



