Nutrient-use efficiency in arid-zone forests of the mangroves Rhizophora stylosa and Avicennia marina

Abstract

Nutrient-use efficiency (NUE) within forests of the mangroves Rhizophora stylosa and Avicennia marina was estimated in arid Western Australia using litter fall rates and rates of leaf CO 2 exchange. Litter fall rates ranged from 9.8 to 34.4 t DW ha −1 y −1 but equated to only 13–41% (mean = 30%) of net canopy primary production. Foliar N:P ratios were in most instances ≥16, suggesting P limitation. NUE for N based on litter fall rates were significantly less (NUE L = 167–322 g DW g −1 N) than those based on photosynthesis measurements (NUE P = 234–448 g DW g −1 N), suggesting that NUE estimates for nitrogen based on litter fall data are underestimates. NUE P estimates for N were significantly greater for R. stylosa than for A. marina. NUE for P were not significantly different, with NUE L ranging from 2905 to 5053 g DW g −1 P and NUE P ranging from 1632 to 4992 g DW g −1 P. Both sets of NUE are at the higher end of the range of estimates calculated for most other forests and equivalent to those for wet tropical mangroves. These arid-zone trees live in low-nutrient habitats, but it appears that selection on components of NUE (i.e. traits that reduce nutrient loss) rather than on NUE itself equates to a lack of clear patterns in NUE between different environments, emphasizing the flexible nature of nutrient allocation in woody plants. NUE in R. stylosa correlated inversely with mature leaf N and P content, implying that NUE in this species is maximized by the synthesis of low-nutrient leaves, i.e. a nutrient retention strategy, whereas such does not appear to be the case for A. marina. This strategy translates into a direct advantage in terms of net primary productivity for R. stylosa. This idea is supported by evidence of longer nutrient residence times for R. stylosa than for A. marina.