Daily dynamics of leaf and soil-to-branch hydraulic conductance in silver birch (Betula pendula) measured in situ

Daily dynamics of leaf (KL) and soil-to-branch hydraulic conductance (KS–B) was investigated in silver birch (Betula pendula Roth.) using evaporative flux method in situ: water potential drop was measured with a pressure chamber and evaporative flux was estimated as sap flux density measured with sap flow gauges. Canopy position had a significant (P < 0.001) effect on both KL and KS–B. Upper-canopy leaves exhibited 1.7 and soil-to-branch pathway 2.3 times higher hydraulic efficiency than those for lower-canopy. KL varied significantly with time of day: KL for both upper- and lower-canopy leaves was lowest in the morning and rose gradually achieving maximal values in late afternoon (4.75 and 3.38 mmol m−2 s−1 MPa−1, respectively). Relevant environmental factors affecting KL were photosynthetic photon flux density (QP), air relative humidity (RH) and air temperature (TA). KS–B started rising in the morning and reached maximum in the lower canopy (1.44 mmol m−2 s−1 MPa−1) at 1300 h and in the upper canopy (2.52 mmol m−2 s−1 MPa−1) at 1500 h, decreasing afterwards. Environmental factors controlling KS–B were ΨS and QP. The diurnal patterns of KL reflect a combination of environmental factors and endogenous rhythms. The temporal pattern of KS–B refers to daily up- and down-regulation of hydraulic conductance of water transport pathway from soil–root interface to leaves with respect to changing irradiance.