Variation in shoot water status of Picea abies (L.) Karst. trees with different life histories

Daily dynamics of the shoot water potential (ψ) in Norway spruce Picea abies (L.) Karst.] depending on soil water availability and atmospheric evaporative demand was studied on originally freely-grown trees and suppressed trees exposed to full sunlight after clear-cutting. The base water potential (ψ_b) was significantly correlated (R² = 0.73) with the available soil water storage (W_tr) at a depth of 20–40 cm. ψ_b was relatively constant at high soil water status and decreased sharply if W_tr dropped below 14 mm. Among the atmospheric factors observed, the vapour pressure deficit (VPD) proved to be the most relevant variable in predicting diurnal changes in ψ. The leaf water status was more sensitive to VPD at sufficient soil water storage. With soil drying, the daily amplitude of ψ diminished, testifying to the more efficient stomatal control under soil drought. For predicting the daily course of ψ from VPD and ψ_b (taken as a measure of the soil water availability) an empirical model was derived, which described 78–80% of the total variatin in the shoot water potential. In contrast to values of ψ_b, the daily values of ψ differed significantly (P < 0.05) between the trees with different life histories. The newly exposed trees demonstrated substantially lower values of ψ with increasing atmospheric evaporative demand at the same soil water availability. Dynamic water stress is considered one of the main causes of the continuing growth retardation in suppressed spruce trees after their release from overstorey.