Rare Plants at Awana, Great Barrier Island

BARRY SCOTT

As you travel north past the entrance to the Awana campground you drive alongside a road edge that is covered with an unstriking mixture of mānuka, Coprosma propinqua, swamp ribbon wood, rushes and sedges, kikuyu and a range of other weeds. So you could be forgiven for not realizing this small esplanade strip is a special habitat for two rare New Zealand native plants. If you stop and walk along the road edge you will find the rare dwarf mistletoe (Korthalsella salicornioides) growing on the exposed branches and branchlets of the mānuka. If you climb over the fence and look among the manuka you will find small patches of the rare ground plant, Leptinella tenella. But tread carefully! Although a relatively neglected area, in the words of Jeremy Warden “This is an ecologically significant site that needs considerably more protection and restoration.” Both plants are at risk primarily because of habitat destruction. 

Korthalsella salicornioides is one of three endemic species of Korthalsella, also known as Pygmy Mistletoes, found in New Zealand(1). It is classified as ‘Threatened – Nationally Critical’ by the New Zealand Plant Conservation Network (NZPCN). The Māori name for this mistletoe is pirita(2). All are hemiparasitic as they still have chlorophyll so can photosynthesize even though they extract nutrients from their host.

They colonise their host by way of haustoria (modified roots) that encircle the stele of the host. They break out through the bark to form the visible aerial tissues. K. salicornioides is distinguished from K. clavata and K lindsayi by its highly segmented, circular aerial tissue. It forms a dense mass of green to reddish-yellow beaded succulent stems and lacks true leaves. The genus is named after the Dutch botanist, Pieter Willem Korthals, and the species by the resemblance to the common glasswort (Salicornia europaea), a segmented succulent dicot that grows in salt marshes and on beaches. Although K. salicornioides has been report-ed on up to 26 taxa, 96% of the total records are from mānuka (Leptospermum scoparium) and kānuka (Kunzea ericoides)(3), suggesting it is highly host specific. Flowers are borne at the tip of the internodes in the axils of rudimentary leaves and are just 0.4 to 0.7 mm across in size. 

They form tiny transparent oval berries that contain a sticky seed that is forcibly ejected once mature. Leptinella tenella/delicate button daisy (Asteraceae) is a small creeping herb that usually grows on stream margins where they enter estuaries(4). Growth of this ‘Threatened – Nationally Vulnerable’ species is favoured on sites that are kept open through periodic disturbance from high tides and flooding(4). It is intolerant of too much shading and competition from grasses such as kikuyu. The open mānuka/rush/sedge vegetation of the Awana stream margin is therefore ideal for the growth of this rare plant. The bright green finely toothed leaves are a very distinctive feature of this herb. The plant is monoecious (separate male and female flowers on the same plant). In order to protect the habitat of this species a section of the Awana esplanade reserve land was fenced off in 2014 by Council to prevent livestock access to the stream(5).

A program of weed control was also carried out, especially control of kikuyu which is highly invasive and will readily out compete most native plant regrowth. A survey of the fenced off area with Jeremy Warden (Auckland Council) and Thomas Daly (Envirokiwi) in September of 2022 found that L. tenella was flourishing in large patches of growth among the twig rushes, Machaerina juncea and M. articulata, that were surrounded by a sparse canopy of manuka, Coprosma propinqua and swamp ribbonwood (Plagianthus divaricatus), highlighting the value of the fencing and weed control. However, there are still large fenced off areas at this site in need of further weed control and plant restoration to protect this rare herb and the dwarf mistletoe. 

To further protect and help restore this site, this last winter I initiated a project of weed control and native planting on the Council esplanade strips at the northern and southern end of this site. Mexican devil and a number of other weeds have been removed and seedlings of mānuka, cabbage trees and flax planted. Enrichment of these sites with Coprosma propinqua and swamp ribbonwood (Plagianthus divaricatus) is needed so seed of both have been collected and sown with the hope they germinate this year. It is surprising that neither species is available on the island given how important they are in the estuaries across the island. However, there is still one large area of land that is DoC managed that would significantly add to the area under protection. Up until this last winter the site has been regularly grazed with cattle but I now have agreement from the owners of the cattle to cease grazing to allow restoration of this site. A survey of this site identified large patches of L. tenella, although significantly trodden and weedy, as well as some of the finest specimens of K. salicornioides (see photographs) on the remnant mānuka. Planting of this paddock will hopefully help protect these two rare plant species and maybe provide additional habitat for the threatened pāteke. 

References:

1. https://www.nzpcn.org.nz/flora/species/korthalsella-salicornioides/ 

2. Taylor R (1855). Te Ika a Maui. New Zealand and its inhabitants. Werthheim and MacIntosh, London. 

3. Sultan A, Tate JA, de Lange PJ, Glenny D, Ladley JJ, Heenan P & Robertson A (2018). Host range, host specificity, regional host preferences and genetic variability of Korthalsella Tiegh. (Viscaceae) mistletoes in New Zealand. New Zealand Journal of Botany 56:127-162. 

4. https://www.nzpcn.org.nz/flora/species/leptinella-tenella/ 

5. Woolly J (2017). Leptinella tenella conservation at Awana – project update. Report prepared for the Great Barrier Island Local Board.