Morphodynamics of short-lived wetlands of coastal dune slacks, Manawatū, New Zealand

Abstract

While coastal sand dunes are common in New Zealand, dune slack wetlands are one of the country's most rare and threatened ecosystems, containing a number of rare turf species of seasonally aquatic habitats. To understand changes in such wetlands, historical aerial images were analysed for three study sites along the prograding Manawatū coast and mapped using GIS. Morphometrics, times of origin, turnover and longevity of wetlands were derived. Dune slack wetlands are located amongst depressions in parabolic dune fields, about every 250 m along the coast, have a grand mean of 6130 m2 for area of open water, with a maximum of 8 ha, and are 2.3 times longer than wide, with the long axis mostly oriented 113 degrees from the strandline. They maintain seasonally wet areas for an average of 20–30 years, before being gradually infilled by aeolian sand. New wetlands can recur at the same place as previous wetlands, but adjacent plantations of exotic pines appear to alter their morphodynamics. These results indicate that Manawatū's dune slack wetlands form very dynamic, short-lived ecosystems, which may be internationally unique. They need to be better understood to formulate adequate management initiatives, essential for the perpetuation of the rare plant communities that inhabit them.