Seasonal dynamics of major cations in xylem sap and needles of Norway spruce (Picea abies L. Karst.) in stands with different soil solution chemistry

Soil solution, xylem sap and needles of mature trees were sampled in three spruce stands over one vegetation period and analysed for major cations. Investigations of nutrient distribution between these three pools and evaluations of seasonal dynamics give the following results: The highest nutrient concentrations in the xylem sap occur at the time of bud break and become gradually lower during the vegetation period. The trees show similar trends of xylem sap concentrations with time for potassium, calcium and magnesium regardless of the nutritional status of the plots. No coupling of xylem sap to soil solution composition can be observed in spite of a high variability of soil solution chemistry in time. The major cations in the current needles exhibit a significantly different trend with time. No time-based correlations for nutrient contents could be found for the needles from the previous year. Despite mobilisation of storage pools in the deficient stand, trees are not able to increase the Ca and Mg contents in the needles up to the level of the other stands. Potassium could be retranslocated in sufficient extent for nutrition of current needles. Due to seasonal variability and dependence upon internal processes, such as retranslocation and mobilisation of nutrients, xylem sap does not seem to be a good tool for the estimation of the nutritional status of forest sites. It was concluded that only minor transport into new foliage via xylem sap will proceed after nutrient flush during the bud break and the nutrient content in the new biomass will be governed by dilution due to biomass growth or by nutrient transport by other means than xylem sap.