Nutrient partitioning and storage in arid-zone forests of the mangroves Rhizophora stylosa and Avicennia marina

Abstract

Mangrove partitioning and storage of macronutrients and trace metals were examined in different arid coastal settings of Western Australia. Total living biomass in three Rhizophora stylosa forests, which ranged from 233 to 289 t DW ha -1 , was significantly greater than biomass in three Avicennia marina forests (range: 79-155 t DW ha -1 ). Although prop roots and stems were the largest single tree components for R. stylosa and A. marina, respectively, most nutrients were stored in leaves and living roots of both species. However, only a small fraction of the total nutrient pool was vested in tree biomass; the vast bulk was in soils. A large belowground pool of dead fine roots was identified at all stands, equivalent to 36-88% DW of total living tree biomass. The amount of Ca, S, Cl, Na, Si, Fe, Mn, Zn, B, Mo and Cu vested in dead roots of both species was greater than in the total living tree biomass. The proportion of Fe and S vested in live and dead roots was exceptionally large, consistent with previous evidence of metal plaques on mangrove roots. Sulphur, iron and zinc in dead roots of both species constituted the bulk of these metals. R. stylosa trees preferentially accumulated more Mg, S, Cl, Na, Si, Fe, Mn, B and Mo than A. marina trees. Proportionally greater storage of P, N, Ca, K, Cu and Zn occurred in two of the three A. marina forests. Foliar concentrations of Mg, S, Mn, B and Mo in mangrove leaves were at the high end of the range reported for other tropical trees, but other elemental concentrations were at the low or mid-range. Nitrogen limitation in these forests is implied by a positive correlation between total tree N and net canopy production and by a lower percentage of ecosystem N in tree biomass as compared with other forests. Unlike terrestrial forests where a large proportion of nutrient capital is vested in floor litter, most elements in these mangrove forests are stored in dead roots. A large reservoir of dead roots below the forest floor may serve as a conservation mechanism, particularly in such arid oligotrophic environments.

Carbon and Nitrogen Stocks and Below-Ground Allometry in Temperate Mangroves

“…Our values for root biomass and total sediment N stocks (15.4 ± 1.0 t N ha −1 ) are comparable with tropical A. marina growing in northern Australia (root biomass composed of 0.45-0.75% N, ∼0.08-0.3 t N ha −1 stored in root biomass, Table 4; Alongi et al, 2003). Our total sediment N stocks are also comparable with other tropical mangrove forests (Fujimoto et al, 1999;Ramos e Silva et al, 2007), yet higher than temperate K. obovata sediment in Japan (Khan et al, 2007; Table 4).…”

“…The total amount of C stored in above and below-ground biomass plus sediment was 117.1 ± 16.8 t C ha −1 . These values are within the range observed in other temperate mangrove systems (Howe et al, 2009;Saintilan et al, 2013), yet lower than those values reported from tropical mangrove forests (Table 4; Fujimoto et al, 1999;Alongi et al, 2003;Chmura et al, 2003). Lower C stocks in temperate mangrove forest are predominantly a result of lower mangrove biomass (as described above), as well as lower sediment C stocks.…”

Heterogeneity of elemental composition and natural abundance of stables isotopes of C and N in soils and leaves of mangroves at their southernmost West Atlantic range

“…In general the concentrations of the heavy metals (Fe, Mn, Cu, and Zn) appear to be within the lower range of concentrations reported for other mangrove sites in Brazil (Machado et al, 2002;Bernini et al, 2010) and elsewhere (Alongi et al, 2003;MacFarlane et al, 2007). The δ 13 C values separated S. densiflora from the rest because of its C4 photosynthesis.…”

“…Sulfur concentrations are well below concentrations reported for Avicennia and Rhizophora species elsewhere (Fry and Smith III 2002;Alongi et al, 2003;Bernini et al, 2006;Medina et al, 2010). We do not have an explanation for this fact but hypothesize that the mesic, humid climate where these communities are growing, reduces the stress of high level of available sulfate from marine waters through leaching brought about by rain and superficial run-off.…”

“…Besides, the N/P molar ratios are all below 30 for the mangrove species, and slightly above 30 in the associated species, well below the values reported from P-limited mangroves in the Caribbean . Sodium concentrations are lower than those reported for mangroves in drier climates (Medina et al, 2001;Alongi et al, 2003;Barboza et al, 2006) revealing the high fresh water availability of the communities studied. The Na/K molar relationships are larger than one, in the mangrove species and the grass S. densiflora, as expected for true halophytes (Medina, 1999).…”