PHOTOSYNTHESIS AND GROWTH OF POPLAR TREES 269 



Table i 

 Daily assimilation across the stand, mg COg/g/day 



* In crown base; other figures in crown top. 



highest rate is in the northern border top. The top fohage of the stand 

 interior assimilates better, the closer the trees stand to the moister north 

 border. However, if one considers the assimilation of the base fohage, one 

 fmds a reverse arrangement, though much diminished, from the southern 

 border across the stand interior to the northern border. This is not at all 

 surprising, as here the hght factor dominates over the water factor. For one 

 thing the base fohage is always better supphed with water than the top 

 fohage, which leads to some diminution of the difference, and for another- 

 and this is the decisive fact-the hght reaching the base foliage of the stand 

 interior hes distinctly in the minimum range. 



As has already been mentioned the assimilation measurements were 

 carried out during and shortly after an extreme period of drought. Two 

 days after the beginning of the heavy rainfall in July the soil of the whole 

 stand area was totally saturated to a depth of 50 cm and in spite of that, still 

 one week after the beginning of the rainfall, the tops of the northern border 

 trees assimilated twice as strongly as those of the southern border ones. As 

 the loamy-sand soil of the site dried up rather quickly -in the humus- 

 bearing upper layer a field-capacity with a water-content of 1 5 % by weight 

 was measured, whereas in the layers of more than 25 cm soil depth the 

 percentage was only 7-8%-the southern border of the stand was for the 

 greater part of the year drier than the northern one, which affected not only 

 the metabohsm but also the vigour of growth. This is quite obvious in the 

 concluding table (Table 2). Not only in stem height but also in stem dia- 

 meter (measured one metre high) the trees of the north border are superior 

 to those of the south border. But while the trees increase continuously in 

 their stem height from the south border to the north border, the stem 



