Foliar nitrogen dynamics in Liquidambar styraciflua saplings:: response to nitrogen fertilization

The nitrogen (N) content of mature leaves and recently abscised leaves representing different crown positions of sweetgum (Liquidambar styraciflua L.) saplings was measured to (i) quantify the effects of N fertilization on seasonal distribution of foliar N, (ii) determine if N availability affected the relationship between leaf N per unit leaf area (N-A) and leaf light exposure, and (iii) project the proportion of N incorporated into developing leaves that could be supplied by N reabsorbed from leaves senescing during the growing season. Fertilizer approximately doubled the foliar N pool and increased N-A at all crown levels. The N-A generally increased from the base to the top of the tree and acropetally along the branch. Fertilization elevated the N-A for a given light exposure level and increased the slope of the N-A to light relationship. Litter N-A increased during the season across all plots. Fertilized trees reabsorbed more N from senescing foliage than controls. Whereas N reabsorbed from leaves falling prior to July 14 could supply only 17% of the N needed for new leaf development during that period, all of the N needed for the small amount of foliage produced after mid-July could have been obtained through retranslocation from senescing foliage.