TRANSPIRATION INTENSITY OF SILVER BIRCH PREPARATORY STAND (BETULA PENDULA ROTH) COMPARED TO A HOMOGENEOUS AND EVEN-AGE NORWAY SPRUCE STAND (PICEA ABIES (L.) KARST.) IN THE POLE STAND GROWING STAGE

Two different forest types at the pole stand growing stage were compared regarding their transpiration during the peak of the growing season: a pure, artificially established even-aged Norway spruce (Picea abies (L.) Karst.) pole stand and a pure, naturally regenerated silver birch (Betula pendula Roth) preparatory pole stand. Both sites were comparable, as evidenced by the non-significant differences between daily averages of potential evapotranspiration on both sites. Both forest stands had been managed in the past and were reduced to 1,800 and 900 trees/ha for Norway spruce and silver birch, respectively. The forest transpiration of the silver birch preparatory pole stand was significantly higher than that of the Norway spruce pole stand, representing a transpiration rate over 1.4 times higher. Over a 56-day period, the cumulative transpiration was 127 mm for the silver birch stand and 90 mm for the Norway spruce stand. When transpiration was correlated to the potential of transpiration for particular sites, it was found that the portion was over 71% for silver birch and 42% for Norway spruce stand, respectively. Significant correlation of transpiration with the potential of evapotranspiration was established for both forest types, with this relationship being more distinct for the Norway spruce stand, indicating that its transpiration rate is more under control.