Fertilization effects on mean stomatal conductance are mediated through changes in the hydraulic attributes of mature Norway spruce trees

Stomatal conductance was quantified with sap flux sensors and whole-tree chambers in mature Norway spruce (Picea abies (L.) Karst.) trees after 3 years of exposure to elevated CO2 concentration ([CO2]) in a 13-year nutrient optimization experiment. The long-term nutrient optimization treatment increased tree height by 3.7 m (67%) and basal diameter by 8 cm (68%); the short-term elevated [CO2] exposure had no effect on tree size or allometry. Nighttime transpiration was estimated as similar to 7% of daily transpiration in unchambered trees; accounting for the effect of nighttime flux on the processing of sap flux signals increased estimated daily water uptake by similar to 30%. Crown averaged stomatal conductance (g(s) was described by a Jarvis-type model. The addition of a stomatal response time constant (tau) and total capacitance of stored water (C-tot) improved the fit of the model. Model estimates for C-tot scaled with sapwood volume of the bole in fertilized trees. Hydraulic support-defined as a lumped variable of leaf-specific hydraulic conductivity and water potential gradient (K1 Delta Psi) -was estimated from height, sapwood-to-leaf area ratio (A(s):A(1)) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in g, at reference conditions for trees across nutrient and [CO2] treatments. Removal of similar to 50% of A, from three trees yielded results suggesting that stomatal compensation (i.e., an increase in g(s) after pruning scales inversely with K1 Delta Psi, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in A(s):A(1).