The Dispersal, Establishment and Management of Ammophila arenaria Seed in Coastal Foredunes
This thesis examines the seed ecology of the coastal dune grass, Ammophila arenaria (L.) Link (marram grass) in a densely vegetated foredune (Type-1, after Hesp, 2002) in southern New Zealand. A. arenaria is widely recognised as a threat to the biological and landform diversity of temperate dune systems around the world, yet the characteristics of seed production, seed dispersal, and seed storage on A. arenaria-dominated foredunes is not fully understood. An understanding of these processes will assist greatly in the future management and eradication of this highly invasive species. Through a series of empirical field-based investigations at Mason Bay, Stewart Island and Oreti Beach, Southland, this thesis examines: (i) the relationship between seed production and the foredune landscape; (ii) the primary and secondary dispersal of A. arenaria seed and the influence of the vegetation canopy on dispersal in the foredune; (iii) the effect of the foredune vegetation canopy on A. arenaria seed germination and seedling survival; and (iv) the relative density of the A. arenaria seed bank in relation to spatial seed distribution and burial depths within the foredune and deflation zone. The implications of these findings in relation to the eradication of A. arenaria at Mason Bay, Stewart Island are discussed. The size and variability of A. arenaria seed production on the Mason Bay foredune, was examined by surveying four transects using a stratified random sampling procedure. Flowering density was significantly higher on the stoss face, probably due to the provision of nutrients through salt spray, sea foam, and organic material. The primary and secondary dispersal of A. arenaria seed on the foredune was studied through a ground surface survey and seed trapping experiment at Mason Bay, and a release and tracking investigation at Oreti Beach. Primary dispersal predominantly occurred across short distances (20–30 m) under moderate to high wind speeds, and secondary dispersal was limited to a few centimetres due to the dense vegetation canopy and rapid burial by sand. Seeds and seedlings were introduced to the Oreti Beach and Mason Bay foredunes to investigate the influence of the foredune vegetation canopy on germination and seedling survivorship. Both seed germination and seedling survival were significantly limited by the vegetation canopy. The soil seed bank was examined by extracting core samples from the foredune and deflation zone at Mason Bay and testing seeds for viability. Seeds were found to the maximum depth of sampling (340 cm) in the foredune, and retained a high viability at all depths (69.4%). Little seed was found in the deflation zone. The findings of this study suggest that the foredune environment at Mason Bay provides a highly efficient system in terms of producing A. arenaria seed, dispersing seed within this landform, and storing seed as a soil seed bank. Most, if not all seed remains within the foredune due to the dense plant canopy that restricts dispersal (although long-distance dispersal into the dune hinterland cannot be ruled out), and the germination of seed is inhibited by the vegetation canopy and the restriction of light penetration. This contributes to the development of the highly persistent and extensive soil seed bank within the foredune at Mason Bay. Two scenarios are proposed for managing A. arenaria at Mason Bay, Scenario one involves the ‘natural mobilisation’ of the foredune, by eradicating the A. arenaria population and allowing the dune to naturally erode, whilst reactively eliminating seedlings that emerge from the soil seed bank. Scenario two has a more proactive approach, ‘large-scale destabilisation’, which entails the removal of the A. arenaria canopy, and the subsequent excavation and remobilisation of the foredune with earth-moving equipment. Sand would most likely be deposited inland due to the greater volume of sand that could be transported, and follow-up spraying would be required. The management of the foredune soil seed bank is an inevitable stage in achieving control over, and eventually, full eradication of A. arenaria in the Mason Bay dune system.
Advisor: Hilton, Mike; Konlechner, Teresa; Lord, Janice
Degree Name: Master of Applied Science
Degree Discipline: Environmental Management
Publisher: University of Otago
Keywords: Ammophila arenaria; Seed production; Seed dispersal; Seed germination; Seed bank; Eradication; Foredune; Stewart Island; New Zealand
Research Type: Thesis