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 56 days, 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. A 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.
Although the birch stand had half the number of individuals per unit area compared to the spruce stand, its transpiration was significantly higher, as was the ratio of birch transpiration to potential evapotranspiration (PET) compared to spruce (see Fig. 4). This is a general characteristic of broadleaved trees, which is particularly typical of silver birch due to its high proportion of sapwood and its pioneering growth strategy. On the other hand, a closer correlation between potential evapotranspiration (PET) and stand transpiration was found for Norway spruce, which better regulated its transpiration loss by stomatal conductance in relation to the actual potential evapotranspiration (PET). Furthermore, our study suggests that there was no significant reduction in soil water availability during the given period, i.e. there was no pronounced drought stress period, as evidenced by the absence of stagnation of increment, which can also be used as an indicator of drought stress.
The work benefits from a grant from Norway and Technology Agency of the Czech Republic within the KAPPA Programme (TO01000345)