The vegetation sequence at Whangapoua Estuary, Great Barrier Island, New Zealand

Abstract

One hundred and eight vegetation plots were measured along six transects, running from estuarine mudflats to freshwater swamp in Whangapoua Estuary. These were analysed to define the patterns of plant communities, and evaluate their relationships with environmental gradients. TWINSPAN classification of the vegetation plots and species suggests that the three main vegetation zones (mangroves, salt meadow/marsh, and freshwater swamp) can be divided into six broad vegetation communities named as follows: (A) Avicennia marina, (B) Juncus kraussii sea rush, (C) Leptocarpus similis salt meadow, (D) Baumea juncea sedges, (E) Leptospermum scoparium shrubland, (F) Typha orientalis/Cordyline australis swamp forest. DECORANA ordination reveals a gradient of communities from mangrove through salt meadow to shrubland then to swamp forest. The pattern of species distribution from salt to freshwater results from the interaction between species and the physical constraints of the salinity and freshwater inundation gradients. This pattern accounts for c. 77% of the current floristic variation in the modern vegetation, indicated by the different vegetation communities exhibiting similar distribution patterns along the estuarine to freshwater gradient.