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 ∼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 ∼30%. Crown averaged stomatal conductance (g_s) was described by a Jarvis-type model. The addition of a stomatal response time constant (τ) 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 (K ₁ΔΨ) - was estimated from height, sapwood-to-leaf area ratio (A_s:A₁) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in g_s at reference conditions for trees across nutrient and [CO₂] treatments. Removal of ∼50% of A₁ from three trees yielded results suggesting that stomatal compensation (i.e., an increase in g_s) after pruning scales inversely with K₁ΔΨ, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in A_s:A₁.