Narrabeen Dunes Restoration

Toolijooa managed dune revegetation and restoration works within thirteen dune bays along Narrabeen Beach between 2005 and 2008. Pre-European vegetation communities within the project area include Spinifex Spinifex sericeus Grassland and Coastal Wattle Acacia longifolia subsp. sophorae Heath. Where they remain along Narrabeen Beach, these dune systems and vegetation communities have been substantially altered and heavily degraded by coastal development. This degradation has impacted both upon coastal biodiversity and the structural resilience of coastal ecosystems to large sea conditions and tidal surges following major weather events. This project aimed to reduce the cover and influence of invasive exotic species, raise native plant species diversity and restore a vegetation and dune structure more indicative of intact coastal dune systems. These goals intended to restore habitat for a number of coastal fauna species, improve the site’s public amenity and increase dune stability.

The project site was split into four Management Strategy Areas: the fore dune, shrub dune areas, open dune areas and lawn / hind dune. MSAs were designed to correspond with dune position and existing vegetation structure, and subsequently the type of management they require.

The initial project aim was to control weeds to a level sufficient to allow the installation of 40,000 provenance tubestock as soon as possible.

Major invasive exotic species that were targeted for control / removal included Asparagus aethiopicus, Chrysanthemoides monilifera subsp. rotundata, Coprosma repens, Cynodon dactylon, Gazania rigens, Hydrocotyle bonariensis, Pennisetum clandestinum and Stenotaphrum secundatum, while a suite of typical exotic annual species were also present throughout. Of particular concern amongst these was Gazania rigens. This species is particularly difficult to eradicate do to its rhizomatous growth which makes complete manual removal difficult and its waxy leaf cuticle which leads to low herbicide uptake and poor herbicide efficancy.

Sand dunes are intrinsically highly erosive and it was of primary concern over the entire project to integrate erosion control measures into all our work strategies. Strategies to minimise dune erosion during initial weed control and revegetation works included:

  • retention of standing and fallen dead wood and laying of dead wood that required removal to facilitate weed control. Dead wood material also provide habitat and feeding sites for a variety of insects, reptiles and mammals,
  • short-term retention of patches of exotic species on hillocks and ridges in order to continue capturing sand until a replacement native species cover had established,
  • avoiding over-clearance of dune vegetation for revegetation works by retaining existing weed cover in rows parallel to the prevailing wind / shoreline and revegetating in patches. This technique was particularly important on open dune surfaces. The retained weed cover was progressively removed during the maintenance period, with the rate of removal dictated by growth and expansion of revegetated native species and remnant vegetation,
  • retaining less tenacious weed species that provide good ground cover, such as
    S. secundatum
    and P. clandestinum, until as late as possible in the contract period.

A variety of treatment techniques were utilised to control exotic species according to their growing situation. The density and extent of weed cover throughout the site necessitated the use of herbicides. Given the concerns of using herbicides on sands, extreme care was taken to maintain accurate spray rates for each weed species in order to minimise herbicide output. Wherever possible, control of weeds by cut/scrape-painting and manual removal were prioritised, however, use of these techniques had to also be balanced with exacerbating erosion of the dune.

Due to the intensity of labour, timeframe and level of mechanical disturbance required for control of Hydrocotyle bonariensis,as well as its role in binding the dunes in some areas,this species was generally not targeted for removal. H. bonariensis was removed only as deemed necessary/appropriate during the final maintenance period. It is hoped that this species will to some extent be shaded / crowded upon establishment of a dense native species vegetation cover.

It was ensured that secure fencing was in place prior to revegetation works and properly maintained during the plant establishment phase in particular. Tubestock distribution was arranged to mimic broad gradations in dune vegetation structure and composition and care was taken to not install taller shrub species in areas where they would obscure views once mature. Wherever possible, tubestock were installed within and amongst existing niches, such as depressions, hillocks, established plants and dead wood, which were considered favourable to plant establishment.

Following revegetation works, the first year of maintenance initiated removal of the remaining weed cover in a staged manner that corresponded with the growth and expansion of planted native species and remnant vegetation. Subsequent maintenance works focussed upon maintaining low weed levels in an ongoing effort to eradicate invasive exotic species from the site entirely.

Overall, native species richness and abundance vastly improved over the course of the project, while the decreases in richness and abundance of exotic weed species has been marked over the entire site. Regeneration of planted species including Actites megalocarpa, Carpobrotus glaucescens, Leucopogon  parviflorus Pelargonium australe and Scaevola calendulacea is occurring in all dune bays, while Calystegia soldanella has shown significant regeneration throughout, competing with some of the more vigorous exotic species such as Acetosa sagittata and Hydrocotyle bonariensis. Over the course of the contract, Chamaesyce psammogeton, a listed Endangered species under the NSW Threatened Species Conservation Act 1995, was found at the site by field crews. Identification of this species, previously unknown to occur within Narrabeen dunes, was verified and its distribution mapped.

Populations of Acetosa sagittata, Gazania rigens and Hydrocotyle bonariensis now occur only in isolated patches. However, due to their rhizomatous / tuberous growth and poor uptake of herbicides, these species, predictably, will require ongoing maintenance for some time.