Abstract
This thesis investigates the potential for a sand-dune plant, Ammophila arenaria (L.) Link (marram grass, European beach grass) to disperse and establish new colonies from pieces of its rhizome system. Such dispersal threatens the natural character of temperate dune-systems globally. Four questions were examined: 1) how readily does A. arenaria regenerate from rhizome fragments; 2) how many propagules does the rhizome system of A. arenaria produce; 3) how far can rhizomes be dispersed; and 4) how does the stranding environment affect establishment? These questions were addressed through a series of comparative glasshouse-based experiments and empirical field-based measurements.
The regenerative potential of A. arenaria rhizomes was assessed by measuring the ability of fragments of rhizome to produce tillers under ideal growing conditions, and following exposure to abiotic stress (salinity, desiccation, burial). The density of dormant meristems (the bud bank) of A. arenaria was sampled at three dune-systems in southern New Zealand to determine propagule density. The distance that A. arenaria can disperse was inferred by determining the time that a fragment of rhizome could remain both buoyant and viable when in seawater. To examine the effect of the stranding environment on establishment, the effect of increasing levels of abiotic stress on tiller survival was measured. The focus in all components of this thesis was on identifying those biological aspects relating to the reproduction of A. arenaria from rhizome that may limit establishment, or make establishment more successful in one environment compared to another.
A. arenaria regenerates readily from fragments of rhizome. Between 45 – 100% of fragments were viable. A decrease in fragment length, and obtaining fragments from the horizontal rhizome system, from low-vigour populations, and during spring/summer months was correlated with a reduction in fragment viability. The bud bank of A. arenaria is large — of the order of 102 to 103 buds m-3 depending on annual rates of sand accretion, the population vigour, and the height of the foredune. The rhizomes of A. arenaria are capable of being dispersed over long periods of time— they remain buoyant and viable for at least 70 days. An increase in water temperature and a decrease in node number were correlated with a decrease in dispersability. Finally, A. arenaria possesses a high tolerance to most of the stresses it would experience when establishing from stranded rhizome on the back-beach. Of those examined, only desiccation and wave activity are likely to regularly limit establishment.
The principal factors limiting dispersal appear to be extrinsic variables relating to the processes involved in the transportation and stranding of rhizome stranding of rhizomes in a suitable location for growth, rather than the inability of fragments to regenerate. It was concluded that A. arenaria produces large numbers of propagules capable of dispersing over long distances. The dispersal of rhizomes in the sea allows A. arenaria to invade remote dune-systems considerable distances from existing populations. Few dune-systems are likely to be sufficiently remote that A. arenaria cannot arrive. Managing the spread of A. arenaria from marine-dispersed rhizomes is now critical for dune conservation in many regions.