No abstract

Sap flux density (F LA ) in branches of Betula pendula trees was examined with respect to environmental variables and canopy position. F LA of branches sampled within the basal and top thirds of crowns was analysed with respect to stomatal (g s ), canopy (g c ) and soil-to-leaf hydraulic conductance (K T ). The canopy positions differed significantly (P < 0Ð001) by F LA : the day-time values in the upper canopy were on average 1Ð8 times greater than in the lower canopy (59Ð2 vs 33Ð2 g m 2 h 1 ), associated with greater g s , g c and K T observed in the upper canopy. The differences in g c among the canopy positions resulted primarily from a ¾10-fold greater aerodynamic boundary-layer conductance (g bl ) in the upper canopy (mean g bl was 1695 vs 161 mmol m 2 s 1 , respectively) in the day time. The asymptotic nature of g s versus g c relationship observed in the lower canopy suggests that stomata limit transpiration from the shade foliage at higher g bl . The linear relationship (R 2 D 0Ð49-0Ð58, P < 0Ð001) between K T and F LA alludes to the role of hydraulic capacity in the control of sap fluxes. The dominant environmental factor affecting F LA in the day time was irradiance, and air relative humidity at night. The strong dependence of F LA on atmospheric evaporative demand in the night time proved that nocturnal branch sap flow indeed represents transpiration, but not tissue water recharge. Nocturnal F LA was about 24Ð7 and 19Ð5% of the respective mean day-time rates for the upper and lower canopy. Owing to short northern nights in summer, nocturnal transpiration constituted only 6-8% of the daily total water loss. The variation in branch-level sap flow is determined not merely by environmental gradients existing within the forest canopy, but depends largely on trees' internal properties, including stomatal conductance and plant hydraulic properties.