This article was published in The Fringe, October 2014
New Zealand's Mistletoes - Pirita
Every Christmas we receive cards depicting snowy vistas with mistletoe hanging over doorways. Traditionally you can kiss anyone passing underneath the mistletoe twig, but where would you go to find a sprig in New Zealand?
Mistletoes elsewhere in the world have evolved in the presence of tree-climbing herbivores, but New Zealand mistletoe species had never been browsed by mammals or marsupials until the Australian possum was introduced in 1837. Our native mistletoes are like ice-cream for possums and the nine species of mistletoe have become increasingly rare. In fact one is now declared officially extinct. Trilepidea adamsii was found in the wider Auckland region but has not been seen since 1954.
The remaining eight species are spread throughout New Zealand, but Auckland has only one species left, the green mistletoe, Ileostylus micranthus. Thanks to the possum control of the last thirty years it is becoming more common and can be found at an increasing number of sites in Auckland and beyond. Within Auckland, the Waitakere Ranges has probably the greatest number of plants, mostly growing on totara as the host tree, but they are also found on Coprosma grandifolia, and at Miranda on Coprosma propinqua. On the South Kaipara peninsula the green mistletoe parasitizes both Coprosma propinqua and kanuka (Kunzea ericoides).
The fact that the possum has probably been the main factor in the extinction of one of our mistletoe species and that the others are at risk or declining, made me wonder how Australian mistletoe has survived. There are more than 90 mistletoe species across the Tasman, spread throughout Australia except for Tasmania which seems to have had them 50 million years ago, but according to pollen records, by the time of the Miocene around 15 million years ago all the Tasmanian species of mistletoe had died out. Could the possum have devoured them all? Ironically, now that possums and koalas are endangered in Australia, the mistletoe in some areas is reaching plague proportions, and there is now concern that the host trees are being adversely affected.
We are fortunate that the more attractive mistletoes such as the Peraxilla species with its bright scarlet flowers, have survived the onslaught of possum. Although still endangered, they generally occur as hemi-parasitic bushes on beech trees. Another factor in their slow dispersal is the reduction in the number of fruit-eating birds such as bellbirds. Possums and rats are implicated in the reduction of bird numbers as well. Without birds, few seeds are spread beyond the parent tree, and so recovery of small populations takes a long time.
Once a bird has been attracted to the bright orange fruit of Ileostylus, the chances of dispersal are good as the seed is surrounded by a gooey viscous fruit that sticks to everything it touches - first the bird's beak, then its foot as the bird tries to remove the fruit, and then the branch upon which the bird is perching as it attempts to clean its beak on the tree's bark. If there is a suitable crevice, the seed will be caught in it, and still surrounded by glue, it adheres, firmly attached until the seed germinates producing a root that extends out and eventually penetrates through the bark of the host's branch and establishes a direct 'pipe-line' into the host-plant's sap system. Utilising some of the nutrients from the host, but manufacturing some of its own from the green chlorophyll of its leaves, the mistletoe is regarded as half a parasite (hence the term 'hemi-parasite'). The point of attachment is called a 'haustorium' and it can swell into a contorted mass as the growing parasite needs more food from the host plant.
Only by reducing possum numbers will we succeed in preserving these fascinating plants for the future. And who would want a Christmas without mistletoe?