PHOTOSYNTHESIS AND GROWTH OF A FIVE- 

 YEAR-OLD STAND OF POPLAR TREES IN 

 RELATION TO WATER ECONOMY OF THE SITE 



H.POLSTER 

 Institute of Forest Science, Tharandt, E. Germany 



It is generally known from studies of forest yield that the water supply of 

 trees in the stand profoundly affects growth and yield. The growth incre- 

 ment in times of drought is more or less impaired according to soil con- 

 ditions and site cHmate. There are many references to this subject in forest 

 hterature. Among others is Week (1948) who found out that the height 

 increment of spruces [Picea ahies) and "^mtsiPinus sylvestris) may decrease to 

 a third of the normal value (average of ten years or longer), as a con- 

 sequence of dry periods. Besides a very close correlation was to be seen 

 between growth capacity and rainfall over longer periods. While these 

 correlations are quite clear there is very Httle known about the correlation 

 of water supply and photosynthetic efficiency of trees in forest stands. The 

 first ecological investigations referring to this originate from Pisek and 

 TranquilUni (195 1, 1954), who determined the carbon dioxide exchange 

 (assimilation, respiration) as weU as the transpiration of 20-m high spruces 

 and beeches in forest stands. They found that the assimilation in the crowns 

 of their rather free-standing trees depended on the climate of the stand- 

 especially on hght, temperature and air humidity -as well as on the water 

 situation. The better the internal water balance of the tree, the less restricted 

 was assimilation intensity in periods of strong insolation or low air humidity. 

 Generally the trees' assimilation and transpiration reacted very sensitively 

 to disturbances of their water economy, although the top crowns main- 

 tained their metabolic activity longer than the rest of the tree. 



Together with my assistant, Dr. Neuwirth, I investigated for the first 

 time in 1957 the photosynthesis and growth of trees in relation to water 

 economy in a dense though rather small and relatively young tree stand. 

 For that purpose we used a planting of five-year-old black poplars, about 

 7 m high. In Fig. i can be seen a schematic diagram of the stand. The stand 

 area was 0-0075 ha and the trees were planted on a grid system with 6 trees 

 in the N.S. Hnes and 16 in the E.W. lines. For assimilation measurements we 

 used the infrared absorption recorder 'URAS'. As in previous studies the 

 single leaves to be investigated were enclosed in containers made of a very 



