Abstract
Understanding the effects of seedling density on sediment accretion, biogeochemistry and belowground biomass in mangrove systems can help explain ecological functioning and inform appropriate planting densities during restoration or climate change mitigation programs. The objectives of this study were to examine: 1) impacts of mangrove seedling density on surface sediment accretion, texture, belowground biomass and biogeochemistry, and 2) origins of the carbon (C) supplied to the mangroves in Palakuda, Puttalam Lagoon, Sri Lanka. Rhizophora mucronata propagules were planted at densities of 6.96, 3.26, 1.93 and 0.95 seedlings m−2 along with an unplanted control (0 seedlings m−2). The highest seedling density generally had higher sediment accretion rates, finer sediments, higher belowground biomass, greatest number of fine roots and highest concentrations of C and nitrogen (N) (and the lowest C/N ratio). Sediment accretion rates, belowground biomass (over 1370 days), and C and N concentrations differed significantly between seedling densities. Fine roots were significantly greater compared to medium and coarse roots across all plantation densities. Sulphur and carbon stable isotopes did not vary significantly between different density treatments. Isotope signatures suggest surface sediment C (to a depth of 1 cm) is not derived predominantly from the trees, but from seagrass adjacent to the site.
Highlights
Mangrove forests trap sediment within their complex aboveground root structures and can influence soil surface elevation; healthy forests with adequate sediment supplies can, contribute to vertical land building (Wolanski et al, 1992; Furukawa et al, 1997; Krauss et al., 2003; Huxham et al, 2010; Kumara et al, 2010)
We examine the origins of the sediment C and N supplied to the mangroves in a study site in the Palakuda, Puttalam Lagoon on the west coast of Sri Lanka
The natural sediment accretion of the site measured on the Control Plots was more than doubled to a level of 16.2 ± 1.3 mm yr-1under the highest planting density (6.96 m-2)
Summary
Mangrove forests trap sediment within their complex aboveground root structures and can influence soil surface elevation; healthy forests with adequate sediment supplies can, contribute to vertical land building (Wolanski et al, 1992; Furukawa et al, 1997; Krauss et al., 2003; Huxham et al, 2010; Kumara et al, 2010). Mangrove belowground biomass studies can help in understanding nutrient turnover and the potential to store C; information on belowground stock is scant (Tamooh et al., 2008). Belowground standing stock generally ranges between 0.72 to 30.62 kg m-2 worldwide (Komiyama et al, 2008; Tamooh et al, 2008) Cormier et al (2015) report belowground biomass as low as 0.45 to 2.64 kg m-2 for mangroves in Micronesia. Variations in species diversity, tree density, forest age, level of disturbances and tree growth rates all contribute to this large variance among different mangrove forests. Studying processes involving root dynamics is important in understanding forest functions, in mangroves that may invest proportionately large amounts of productivity into their root systems (Saintilan, 1997a,b; McKee et al, 2007). Fine roots are the principal pathway for nutrient absorption for plants, understanding their distribution in the soil can increase understanding of soil nutrient distribution
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