Abstract:
Studies conducted on tropical mangroves suggest that tree height is related to spatial variations of complex edaphic factors such as tidal duration, salinity, pH, redox potential and nutrient availability. Nutrient characteristics of mangrove forests are poorly studied for temperate regions compared to tropical areas. The aim of the study was to assess if tree and leaf characteristics of mangroves growing in the Firth of Thames (New Zealand) can be explained by physicochemical properties of the sediment. The specific objectives were to determine sediment physicochemical and chemical properties (pH, redox potential, salinity and nutrients) and to investigate effects of these parameters on tree characteristics (tree height, density, aboveground biomass and leaf nutrients). Three transects were established across a tidal inundation gradient of mangrove forest. Forest structure data, sediment and leaf samples were collected from 32 sampling sites along the three transects in the months of June and July 2011. Sediment samples (0-5 cm depth) were analysed for salinity, sediment water content, particle size distribution, pH, redox potential, available phosphorous, total carbon and total nitrogen. For leaf samples, total carbon, nitrogen and phosphorus were determined. Tree characteristics showed significant variations along and among transects. Sediment physicochemical parameters also exhibited variations along transects. Leaf nitrogen and phosphorus were correlated with sediment physicochemical parameters, salinity, pH and redox potential. Tidal inundation controls sediment pH and redox potential that in turn influence nutrient availability. No significant correlations were found between sediment nutrients (nitrogen and phosphorus) and leaf nutrients (nitrogen and phosphorus). This may be due to fertilization of mangrove forest sediments with nutrients from farming in the catchment area. Another possible explanation is the sediment layer depth examined in this study as nutrient availability varies with depth through interactions between sediment physicochemical parameters and difficulties involved in determining nutrients available for plant uptake.
