Expansion dynamics of monospecific, temperate mangroves and sedimentation in two embayments of a barrier-enclosed lagoon, Tauranga harbour, New Zealand

Abstract

In recent years, mangrove expansion has become a coastal management issue in the North Island of New Zealand. Little is known about the spatial evolution and forest structure of temperate mangrove forests in New Zealand or about the associated rates of sedimentation. The extent of mangrove colonization in both a large (Waikareao Estuary) and small (Welcome Bay) embayment within Tauranga Harbour were documented. Forest structure and growth rates were described using tree height, stem density, pneumatophore density, and seedling establishment. Surface elevation changes within mangrove habitat were determined from erosion pin measurements and, on bare intertidal flats, using buried baseplates. Sediment texture and availability was also assessed using sediment traps. Results show that in 1943, mangrove vegetation covered <1% of either embayment, with ongoing expansion resulting in mangrove habitat occupying 9% of Welcome Bay and 6% of Waikareao Estuary in 2003. Mean plant heights were all <1.5 m, with vertical plant growth of 3 cm y−¹ to 7.5 cm y−¹. Pneumatophore densities ranged from 200 per m² to 540 per m², and seedling densities of <1 per m² to 14 per m², with seasonal variation, were documented. Surface elevation changes within mangrove habitat ranged from 0 to +21 mm y−¹, compared with −16 to +15 mm y−¹ on adjacent bare flats. Surface sediment within the mangrove habitat was mostly muddy, whereas bare flats were dominated by fine sand. Sediment-trap results provided accumulation rates of up to 32,000 g m−² mo−¹ on bare flats, and close to 29,000 g m−² mo−¹ within the fringing mangrove vegetation. In this study, mangrove vegetation was found to successfully trap and hold muddy sediments, resulting in increased surface elevation. Rising surface elevation from fine sand deposition is also occurring on bare tidal flats in front of mangrove vegetation at some sites.