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H. POLSTER 



thin foil ('Klarzell'). The soil moisture was determined by means of the 

 thermal conductivity method of De Vries (1952) with special measuring 

 aggregates, constructed by Neuwirth. The water available for plants was 

 calculated from the observations in per cent of the available water maxi- 

 mum. Soil suction forces were measured by means of plaster blocks. 



North 

 border 



South 

 border 



5cm 



Fig. I . Schematic diagram of the test stand. 



During the vegetation period and especially in the course of the drought 

 period in the end of June and beginning of July , the measurements of soil 

 moisture and suction forces showed great differences between the northern 

 and southern border of the stand, and this appeared to affect the photo- 

 synthetic efficiency of the trees. As can be seen in Fig. 2, the southern border 

 was always much drier than the northern one, though the borders of the 

 stand were only 5 m apart. During the drought period the northern border 

 dried temporarily to 30 cm depth, whereas the southern border dried to a 

 depth of 60 cm and more. Before and after the drought period more than 

 50% of the north border's moisture content was available for plants, 

 whereas already by May the south border was nearly dry in its upper layers. 

 The heavy rainfall in the second half of July succeeded only in soaking the 

 southern border soil for about a fortnight, while the soil of the northern 

 border remained effectively water-saturated until the end of September. 



By means of six figures I want to show you now the influence of these 

 differences in soil moisture, surprisingly high for such a small stand, on the 

 photosynthesis of the trees. 



Under normal conditions of soil moisture, it was to be expected for a 

 stand of light-demanding poplars, that the trees at the south border which 



