LEAF DEMOGRAPHY AND THE SEASONAL INTERNAL CYCLING OF NITROGEN IN SYCAMORE (ACER-PSEUDOPLATANUS L) SEEDLINGS IN RELATION TO NITROGEN SUPPLY

The dynamics of the seasonal internal cycling of nitrogen (N) in Acer pseudoplatanus were studied in relation to N supply and leaf demography. Trees were grown in sand culture, and throughout 1988, supplied with either 1.0 mol N m-3 (Low N) or 6.0 mol N m-3 (High N), labelled with N-15 to 5.0 atom %, in order to precondition their growth and capacity for N storage over the winter period. In 1989, the initial spring growth of leaves was independent of the current N supply at either 1.0 or 6.0 mol N m-3, and depended only upon the N supplied in 1988. A net loss of N-15 from perennial tissues and recovery of N-15 in the growing leaves confirmed that the current N supply had no effect on the remobilization of N from winter stores in the roots and stem. After 22 May 1989, remobilization of stored N for leaf growth had ceased and leaf demography was influenced by the current N supply. Plants preconditioned to 6.0 mol N m-3 in 1988, but receiving only 1.0 mol N m-3 in 1989, were unable to sustain previous leaf growth after 22 May and there was a decrease in leaf mass and area due to leaf abscission. In contrast, trees preconditioned with 1.0 mol N m-3 in 1988, but receiving 6.0 mol N m-3 in 1989 greatly increased their leaf growth after 22 May. This was a consequence of the stimulation by N uptake of growth of many small leaves on axillary shoots at the base of the canopy. Although leaf number increased, mean area per leaf decreased. The trees were inefficient at withdrawing N from senescing leaves in both years, allowing most of the N to fall from the tree with the leaves. The spring remobilization of N, therefore, was dependent largely upon the direct uptake of N into the stem and roots, in late summer and autumn, for storage over the winter period. The results are discussed in relation to leaf demography and the nitrogen use efficiency of deciduous trees.