Four-year-old seedlings of Picea sitchensis (Bong.) Carr. were grown in sand culture throughout 1989 and irrigated with nutrient solutions containing either 1.0 mol N m−3 (Low N) or 6.0 mol N m−3 (High N), to precondition their growth and capacity for N storage. During 1990 N enriched with 15N was supplied, either from 15 March to 27 June, or 28 June to 20 November. Recovery of unlabelled N was used to determine the storage and immobilization of N for foliage growth, and the partitioning of labelled N taken up during the two periods was measured.
Initial growth of trees in 1990 was unaffected by the current N supply and determined only by the N supplied the previous year. High N throughout increased the number of needles grown in 1990 compared to low N-treated trees, but had little effect on the dry weight of individual needles. When preconditioned with High N, trees responded to Low N in 1990 by a reduction in needle dry weight, without altering the number of needles produced. Low N trees supplied with High N in 1990 responded by increasing both needle numbers and dry weight, compared with trees supplied with Low N throughout. The amount of unlabelled N remobilized to foliage growth in 1990 was unaffected by the current N supply but reflected the amount of N in store, as determined by the N supply the previous year. The majority of N was remobilized from the 1989 foliage and none from roots. Partitioning of labelled N taken up during 1990 altered during the year, with a greater proportion of N taken up after 28 June recovered in the roots in all treatments, due to root growth as opposed to allocation of N to storage during the autumn, since root N concentrations fell between 17 June and the final harvest on 20 November.