The Parasitic Mistletoe

Skyrail Nature Diary: November 2010

You may know of the mistletoe as a decoration you place above your doorway at Christmas time. In the rainforest, mistletoes are a parasitic plant species; a plant that takes from its host tree by penetrating its roots and taking the tree's nutrients. Despite this, mistletoes are not completely parasitic and are still an important part of rainforest ecosystems.

Mistletoes are related to sandalwoods, which are root parasites and grow mostly as dense clumps in the tree tops. They belong to five distantly related families, two of which make up 98% of the mistletoe population; the Loranthaceae (940 species, Gondwanan origins) and the Viscaceae (350 species).

Having semi-succulent leaves, mistletoes are perfectly able to carry out photosynthesis - a process by which plants produce sugars from sunlight and carbon dioxide. Water, minerals and up to 60% of carbohydrates are taken from the host tree, which affects the mistletoe's life cycle significantly. This leads to extended flowering and abundant fruits throughout the year.

Mistletoes are a keystone resource for many plant and animal species. Many birds and insects feed off the nectar and fruits. Mammals are also known to eat both the leaves and the fruits of mistletoes. Bird species from 50 different families use mistletoes as nesting sites or as material to build their nests.

The life cycle of the mistletoe is relatively simple. The flowers are shaped like small hollowed out horns, coloured red, orange or yellow. Nectar is produced in large quantities and when the fruits ripen, they are immediately eaten by several species of birds. Some birds, like the Mistletoebird (Dicaeum hirundinaceum), are believed to be reliant on mistletoe berries. Each mistletoe fruit has a seed with a sticky thread attached to it, which sticks to the bird's tail feathers. To rid themselves of this nuisance, the bird rubs its rump on a branch, causing the sticky thread to catch on the tree. If the conditions are right, the seed will soon germinate and drill into the branch with a peculiar parasitic root system known as a haustorium.

A number of studies have been carried out recently regarding the specific host trees of mistletoe varieties. Here in Australia, most research has been done in eucalypt forests in the south. From the research conducted, there does not appear to be any absolute reliability on any single host. Some eucalypts have up to three different species of mistletoe infecting them.

This being said, there are noticeable mistletoe hosts here in the Wet Tropics (Tropical North Queensland, Australia). There is at least three or four species of mistletoe growing in Barron Gorge National Park, distinguished by their different flowers. Identifying them to the level of species has not been attempted as yet. However, some trees do appear to be more prone to mistletoe infestation than others. The Blood Mistletoe (Amyema sanguineum: Loranthaceae) appears to target a small number of eucalypts and the Queensland Silver Ash (Flindersia bourjotiana: Rutaceae) appears to be infested with a noticeably red leafed mistletoe. These mistletoes are very large and may well be used for nesting sites by some birds, although this hasn't been observed so far.