Changes in phosphorus concentration in needles of adult Norway spruce - nutrient re-translocation or dilution effect?

Regarding the low availability of phosphorus in soil, tree internal cycling of phosphorus through re-translocation among needles would be a good strategy for conifers to cope with soil phosphorus deficiency and to support new needles in annual growth. Therefore, the relationship between the amount of plant-available phosphorus in the soil and the differences in concentrations of phosphorus among first-and second-year needles (P-difference_{Ny1 − Ny2}) of adult Norway spruce was examined. No significant correlation could be detected between the stocks of available phosphorus extracted using citric acid and P-difference_{Ny1 − Ny2}, even for trees with deficient nutritional status. The temporal variations of P-difference_{Ny1 − Ny2} at single plots showed the same order of magnitude as the variability between plots. The typically lower concentrations of phosphorus in second-year needles result mainly from an increase in needle weight of older needles. The net phosphorus re-translocation into younger needles appears to be of minor importance.