Marram grass seed ecology: the nature of the seed bank and secondary dispersal

Abstract

Marram Grass (Ammophila arenaria), represents a significant threat to the natural values of dune systems in southern New Zealand. Following introduction, marram grass’ primary mode of invasive spread is vegetative. Key studies suggest that sexual reproduction is of lesser importance (Huiskes, 1979; Hertling and Lubke, 1999; Krajnyk and Maun, 1982). In southern New Zealand, observational evidence suggests that seed may play a much more important role in the invasion of marram grass than has been previously thought. This study therefore has two objectives relating to marram grass seed ecology: 1) to investigate the role of secondary seed dispersal in marram invasion in transgressive dune systems; and 2) to determine if marram grass forms viable seed banks in southern New Zealand. The investigation of secondary seed dispersal was investigated along three lines of enquiry.

Seed production was estimated for flowering populations of marram grass in six depositional environments at Mason Bay, Stewart Island. This aimed to identify those environments that are important sources of invasion and the potential for invasion via seed. The greatest seed production was found in the exposed populations of the northern and backdune nabkha (17,382.5 and 17,772 seeds m-2 respectively). The lowest production was found in the semi-stabilised and moribund populations on the trailing arms and coppice dunes (196.5 and 39.4 seeds m-2 respectively). Seed production may be related to the input of fresh sand.

Understanding which factors affect germination and emergence in marram grass seed was a key factor in the first objective of the study. The effects of light and darkness and burial appeared to play a major role in determining germination. Marram grass seed was shown to be highly sensitive to light and failed to emerge from depths greater than 5 cm.The restriction of light penetration with increasing burial may be the main factor inhibiting germination in marram grass seed.

Marram grass seedling distribution data was compared against dune environments at Mason Bay. Previous literature suggests that the deflation surfaces (i.e. stonefields) would provide ideal microsites for emergence of seedlings by increasing soil moisture and protecting from micro-erosion. Analysis of seedling distribution however showed a greater proportion of seedlings concentrated outside of deflation surfaces (68.26%) compared to within (31.74%). This finding may be due to the high rainfall experienced at Mason Bay, the eroding nature of the deflation surfaces or the greater overall area of sandsheets.

A unique opportunity was taken to age the marram grass seed bank in two dune systems in southern New Zealand. Viable seed recovered from St Kilda and Allan’s Beach was aged between 0 to 21+ years old. The longest previous estimate of marram grass seed bank longevity is ‘at least 9 years’, making this result particularly significant. The marram grass seed bank may therefore represent a significant threat of re-invasion following control on the foredune.

The final outcomes of this research are a conceptual model of invasion by marram grass seed and management recommendations for ongoing and future marram grass control operations. The entire invasion process is expected to take between six to nine years. Empirical information relating to some aspects of marram grass invasion ecology is currently unavailable and is therefore a key area of future research. Future control operations need to monitor the foredune at least 21 years following eradication and bi-annual systematic searches are required to prevent the establishment and flowering of any new seedlings.