d15N during leaf senescence in the mangrove, Kandelia candel (L.) Druce

Kandelia candel (L.) Druce is the dominant mangrove species in two nature reserves in Northern Taiwan. We characterized the nutrient quality of the leaf litter originating from K. candel and attempted to suggest the extent to which the observed decline of nutrient concentrations in senescing leaves could be attributed to the withdrawal of these nutrients into the main plant before abscission. On a dry weight basis, senescent leaves and leaf litter had significantly lower concentrations of N, P and K and higher concentrations of Na than did younger ones. At one site Ca and Mg contents rose with senescence; at the second site there was no change with age of leaf. Leaf d13C and C: N ratios suggest that withdrawal into the main plant may not be the major cause for the observed decline of C and N during leaf senescence. The d13C values of mature leaves ranged from -26‰ to -30‰; senescent leaves and leaf litter were 0.6‰ more negative for d13C than fresh leaves at the most seaward site. We attribute this decline of d13C to loss of stomatal control during senescence. d15N ranged from +4.3‰ to +9.8‰ for individual samples with no significant differences among ages of leaves, indicating no important gaseous N losses during senescence. P was probably not limiting to growth at these sites.