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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix B: Coastline Processes

Appendix B8 - Dune Vegetation

1 INTRODUCTION

The understanding of coastal dune vegetation, in particular knowledge of the species which are present on the dunes of NSW, their distribution both along the coast and in different dune environments, and of the characteristic associations in which they are found, is critical to the effective management of dunes and their role in coastal processes and coastline hazard management.

Vegetation is the key factor in dune stability and it is the vulnerability of dune vegetation that makes the dunes sensitive to impact. Examples of human activities that can damage dunes are: bush clearing; stock grazing; road building; winning of borrow materials; stockpiling building materials; earthworks in general; construction; fuel storage; housing; most recreational activities; pedestrian and vehicular traffic; brush cutting and bushfires.

Information required to properly manage dune vegetation includes knowledge of species present, the locations of species, and processes by which species persist. Detailed information on dune vegetation and dune management is found in references by Clarke (1989a,b), Chapman (1989) and the Soil Conservation Service (1990). This appendix is based on these references.

2 DUNE VEGETATION : ROLE AND VALUES

For the purposes of coastal dune management, it is convenient to divide dune vegetation into three groups based on performance, growth habit and zone of colonisation. These groups are primary colonising species, (grasses and vines); secondary shrubs and transient species; and tertiary species (enduring trees and understorey) - see Figure B8.1.

Figure B8.1

Figure B8.1 Coastal Sand Dune Vegetation - Niches and Function. (After Chapman, 1989).

Dune vegetation is highly adapted to the salt laden winds of the coast, and maintains the foredunes by holding the sand already in the dune, trapping sand blown up from the beach, and aiding in the repair of damage inflicted on the dune either by natural phenomena or by human impact. The combination of dune height, dune shape and intact vegetation creates a protective system which directs salt-laden winds upwards and over the dune crest, (see Figure B8.1). As a result, salt sensitive vegetation communities, (including littoral rainforests), can establish in close proximity to the beach.

However, while the sequence of herbaceous forms followed by shrub and tree forms is true in the general sense, the sequence illustrated in Figure B8.1 is an oversimplification of the patterns that exist. A variety of forms can be found within each of the dune physiographic units, and at many sites the sequence has been truncated by disturbance or by lack of a foredune ridge.

Dune vegetation not only stabilises the dune, but also has habitat, educational and recreational values. The beach and the foredune are used extensively for active and passive recreation, vegetation contributes to recreational and aesthetic values. Indirectly, the stabilising ability of vegetation limits the amount of wind blown sand in the beach environment and thus enhances its recreational utility. Similarly, healthy vegetation significantly contributes to beach user comfort by reducing reflected radiation from the foredune.

The aesthetic values of dune vegetation are generally neglected and it would probably be safe to say that few beach users are aware of them until the vegetation has been degraded or lost. Zonal patterns of natural vegetation on the beach and foredune provide an important visual resource in terms of change in texture, colour and form. The wind sculpted aeroform of coastal shrubs and trees enhances the visual quality of the beach and provides contrast for the stark, rigid, angular shapes of man made structures.

Beyond the foredune, taller hinddune woodlands and forests afford protection from onshore winds and provide ideal opportunities for passive recreation such as picnics, informal camps, and backpack activities. These pursuits are not necessarily incompatible with the dune environment but nevertheless require careful management so as not to degrade the very features which make them attractive.

3 VEGETATION AND COASTAL PROCESSES

Vegetation plays an important role in the stabilisation and formation of coastal dunes. Foreshore vegetation impacts on several of the sand transport pathways, and therefore influences the rate of shoreline recession and dune rebuilding.

The strandline and beach vegetation is generally transient, being removed during storm events. Nevertheless these pioneer plants trap and hold windblown sand so that it does not damage plants on the relatively more stable foredune. The beach vegetation is mainly dominated by the grasses, Spinifex sericeus and Festuca littoralis, which aid in the creation of beach mounds and ridges, (incipient foredunes), under prograding conditions.

The foredune vegetation proper is usually composed of semi-permanent populations of herbs, shrubs and trees which stabilise the foredune sand mass. Sand trapped in the foredune acts as a reservoir of sand for the beach during periods of wave erosion and to a certain extent, by the development of soil concretions and a dense root web, the foredune vegetation also buffers the effects of storm erosion.

A model showing the relationship between dune vegetation, and other coastal processes is shown in Figure B8.2.

Figure B8.2

Figure B8.2 A Schematic Model of Physiography, Environmental Gradients, and Distribution of Species Across a Typical Foredune on the Coast of New South Wales. (After Clarke, Examples of the "artificial" degradation of dune vegetation, by human activities, are listed in the introduction to this appendix.

A Population dynamics for the three principal dune zones under stable geomorphological conditions. Life cycles are drawn as circles of increasing size to represent length of life of plants; herbs which are long lived owing to clonal perennisation are denoted by spirals. Solid lines and arrows represent directional change.

B Distribution and abundance (as amplitude) of dominant species shown as hypothetical Gaussian curves, some of which are skewed due to abrupt environmental thresholds and competition.

C Typical cross section of the physiography of a stable coastal dune.

D An indication of the environmental gradients which affect the distribution of dune species. Arrows indicate direction of increasing magnitude of factor.

Many dune plant communities have special value arising from previous studies which allow them to be used as temporal reference points. In this regard they can reflect the changing physical nature of the coastline. Shoreline recession and progradation for example, may be detected from vegetative evidence.

Changes in climatic conditions are also frequently reflected in vegetative distributions, for example, changes in the spatial distribution of Spinifex and Festuca could well be related to changes in atmospheric and ocean temperature patterns. Qualitative evidence from old photographic records indicates that Spinifex has replaced Festuca on the central and southern coast of NSW, perhaps suggesting warming of temperature, or nutrient enrichment.

4 VEGETATION DEGRADATION

The foredune complex is a dynamic, resilient landform essential to the long-term stability of the dune system. However, dune vegetation may be degraded or damaged through both natural and artificial processes. Natural instability may be initiated by discrete high energy storm events capable of breaching or scarping the foredune. Failure of the dune vegetation to recover from storm wave attack may result in dune erosion due to the effects of onshore winds mobilising bare sand in the form of blowouts. Movement of sand landward of the foredune complex in blowouts constitutes erosion of the beach/dune system, since sand removed from the swept prism becomes emplaced within the terrestrial environment. The blowouts that induce this mode of erosion may in turn initiate further degradation within landward dunes as drifting sand buries and kills vegetation in its path.

Perhaps the most important "natural"factor responsible for altering the structure of stable vegetation is fire, the effect of which is to reduce the presence of mature shrubs and trees, resulting in a grassland/forbland formation that in some locations is maintained by repeated firing.

Once degraded, natural healing of dune vegetation may be inhibited by:

a) Breakdown of the vegetation canopy which, when intact, acts like a storm shutter (Fig. B8.1), deflecting winds over it. Entry of salt-laden wind to the internal structure of the vegetation can expose sensitive vegetation elements (especially rainforest species) to salt-burn and resulting in irreversible degeneration of the vegetation binding the foredune.

b) Continued pedestrian and vehicular traffic and other disturbance factors at a level sufficient to prevent recolonisation of the exposed surfaces.

In order to restore the "health"and function of the dune vegetation, active dune management programs must be implemented. These are dealt with in Section 5.2.3 and Appendix D5.

5 REFERENCES

Chapman, D.M., (1989). "Coastal Dunes of New South Wales Status and Management".University of Sydney. Coastal Studies Unit Technical Report No: 89/3.

Clarke, P.J. (1989a). "Coastal Dune Vegetation of New South Wales".University of Sydney. Coastal Studies Unit Technical Report No: 89/1.

Clarke, P.J. (1989b). "Coastal Dune Plants of New South Wales".University of Sydney. Coastal Studies Unit Technical Report No: 89/4.

SCS (1990). "Coastal Dune Management - A Manual of Coastal Dune Management and Rehabilitation Techniques".Eds P.A. Conacher, D.W.B. Joy, R.J. Stanley, and P.T. Treffry. Soil Conservation Service of NSW, 1990.