A spatial analysis of the relationship between mangrove (Avicennia marina var australasica) physiognomy and sediment accretion in the Hauraki Plains, New Zealand

The relationship between mangrove (Avicennia marina var. australasica) physiognomy and forest bed accretion was quantified using experimental and field measurements at the Piako river mouth, Firth of Thames, New Zealand. Sediment accretion under six densities of artificial pneumatophores was monitored for 13 weeks in an experimental plot, while accretion was simultaneously monitored for 5 months at 102 locations along two parallel 500-m transects. Experimental results demonstrated that accretion increases with pneumatophore density. This was supported by bivariate analyses of cumulative accretion and Avicennia marina stem density, basal area and pneumatophore density along the transects. Only the latter is significantly correlated with accretion. Geostatistical analyses showed that variation in sediment accretion occurs at the same spatial scale as pneumatophore density. Accretion and pneumatophore density are spatially autocorrelated over distances <70 m and have fractal dimensions of 1.5-1.6. It is proposed that Avicennia marina grow pneumatophores at a density that balances the benefits of aeration with the drawbacks of sediment accretion. These results are compared with the concept of positive-feedback switches in plant communities. (C) 1996 Academic Press Limited