Modern pollen-vegetation relationships along transects on the Whangapoua Estuary, Great Barrier Island, northern New Zealand

Abstract

Aim To quantify pollen-vegetation relationships from saline to fresh water in an estuarine gradient from surface samples of the modern pollen rain, to allow more accurate interpretations of the stratigraphic palynological record.

Location Whangapoua Estuary, Great Barrier Island, northern New Zealand.

Methods Six transects were laid out along a vegetation sequence running from estuarine mud to freshwater swamp. One hundred and eight plots were sampled for vegetation and surface sediments from wet sand, mud, plant litter or moss, along these transect lines (sand and mud sites are inundated by most tides, other sites less frequently). All sediment samples were analysed for pollen. The relationships between plant species frequency and pollen representation were examined at a community scale using TWINSPAN and ordination analyses and for individual species using fidelity and dispersibility indices, regression, and boxplot analyses.

Results The quantitative relationships between source taxon vegetation frequency and its pollen representation varied between species due to differential pollen production and dispersal. TWINSPAN of the surface pollen samples suggests five vegetation types: (A) mangrove (Avicennia marina), (C) Leptocarpus similis salt meadow, (D) Baumea sedges, (E) Leptospermum shrubland and (F) Typha/Cordyline swamp forest. The (B) Juncus kraussii community is not represented palynologically owing to the destruction of its delicate pollen grains during acetolysis of samples. Detrended Correspondence Analysis places these communities on an estuarine to freshwater gradient. However, pollen assemblages at the seaward end of the salinity gradient are less clearly representative of the associated vegetation than those at the landward end probably because the open vegetation at the former allows the influx of wind- and water-dispersed pollen from surrounding vegetation.

Main conclusions The vegetation pattern (zonation) at Whangapoua is reflected in the pollen rain. When the long-distance and over-represented pollen types are excluded, five out of six of the broad vegetation communities can be identified by their pollen spectra. Species with high fidelity and low to moderate dispersibility indices can be used to identify the vegetation types in the sedimentary sequences. The more open vegetation types at the ‘marine end’ of the sequence tend to be ‘overwhelmed’ by regional pollen, but the nature of the sediments and the presence of discriminatory species (e.g. Avicennia marina, Plagianthus divaricatus, Cordyline australis) even in small amounts will allow correct identification of the local vegetation represented in sedimentary palynological sequences. A boxplot analysis indicates that the pollen types Avicennia marina (mangroves), Sarcocornia quinqueflora (salt meadow), Plagianthus divaricatus (sedges), Gleichenia (shrubland) and Cordyline australis (swamp forest) are highly discriminatory in relation to vegetation type. These discriminatory pollen types help with the interpretation of stratigraphic pollen studies. However, salt marsh vegetation communities in the sediments must be interpreted with caution as the marine sediments are easily affected by erosion, bioturbation, and tidal inundation effects.

Keywords

Modern pollen rain, modern pollen-vegetation relationships, pollen dispersal, pollen representation, salinity gradient, estuaries, New Zealand.