The seasonal course of xylem sap parameters (electrical conductivity EC, potassium concentration [K+], and pH) of three conifers (Pinus cembra, Picea abies, and Larix decidua) growing at the alpine timberline was monitored. We also looked into possible effects of [K+] and pH on the difference in hydraulic conductivity (Δks). In all studied species, EC, [K+], and pH varied considerably over the year, with pH ranging between 7.3 (February) and 5.8 (June) and [K+] changing between 0.4 (January) and 2.5 mM (June). The Δks was overall low with positive values during winter (up to +20 %) and negative values in summer (-15 % in August). Samples perfused with alkaline solutions showed higher Δks. Xylem sap parameters in all conifers under study were surprisingly variable over the year thus indicating either effects upon seasonal changes in environmental factors or active adjustments, or both. Although Δks values over the year were minor, observed induction of Δks by high pH might indicate a role for hydraulic adjustment in harsh winter periods.
