Mistletoe – ‘Box Mistletoe’

Saturday, Apr 16, 2011 at 14:22

Stephen L (Clare) SA

Mistletoes are parasites. This diverse group of plants has over 1500 species found throughout the world. 91 species have been found in Australia and experts believe that there are more out there just waiting to be discovered. Of the 91 species found here in Australia, 66 species are endemic - that is found only in Australia. The remaining species are shared with New Guinea, New Caledonia, the Philippines and other parts of South East Asia.

Australian Mistletoes can be found from our Alpine Regions to the Deserts, the Tropics and all regions in between. They are found in all Australian mainland states, yet for reasons unknown to the experts and despite two hundred years of botanical exploration, Tasmania remains mistletoe free.

As parasites, Mistletoes take their water and nutrients directly from their host trees. However they manufacture their own carbohydrates using photosynthesis, a habit is known as hemiparasitism. Unlike most hemiparasites, mistletoes attach to their hosts above the ground and are distinguished by their growth form, being shrubby and often woody plants.

Mistletoes attach to their host via a specialised organ called a haustorium. This structure serves two purposes, firstly anchoring the mistletoe firmly to its host tree, and secondly tapping directly into the sap of the host tree, allowing water and dissolved nutrients to flow directly to the mistletoe.

The most diverse genus of mistletoe in Australia is the genus Amyema, with 39 of almost 100 species native to Australia.
Just like other flowering plants, mistletoes begin life as a seed. Unlike most plants that have a protective seed coat, mistletoe seeds are surrounded by a sticky translucent pulp known as viscin.

The seeds are eaten and distributed by birds. Eastern Spinebills, Painted Honeyeaters, Mistletoe Birds and many others are attracted to the coloured fruit. When the fruit is eaten the seed and its sticky coat is not digested in the gut of the bird. The birds defecate the undigested seed pulp, Mistletoe Birds taking as little as 14 minutes from consuming the fruit to deposition. The seed and pulp emerges in sticky strings that need various wiping and pecking manoeuvres to remove it from the feathers around the vent.

This sticky pulp is important for two reasons. Firstly it attaches the seed firmly to its host tree and secondly it acts like a sponge to absorb moisture from rain and dew for the developing embryo within the seed.

The act of breaking the fruit wall and removing the pulp triggers germination. Once the seed is removed from the plant, the embryo begins to grow and lengthen, with some species having an almost 100% germination rate. Even with such a high germination rates, not all seeds will survive, as many germinate in strange places including fence posts, power lines and buildings.

Once seed germinates and bonds to its host tree, the bond is permanent and the fate of the mistletoe depends solely on the fate of the host tree. It is in the best interest of the mistletoe to have a negligible effect on its host tree, so that the host and the mistletoe can both survive.

However when the host tree is infected by a large number of mistletoe plants, host vigour and survival can be compromised, leading to the decline and ultimately death of both the host tree and the mistletoe.

The growth of the mistletoe plant is at the expense of the infected host branch, which has reduced access to water and nutrients. This often results in the death of the outer portion of the branch of the host tree.

There are usually a number of factors keeping mistletoe numbers in check, but when these fail over a wide area there can be dire consequence to all vegetation in the ecosystem.

Mistletoe flowers have abundant nectar and nutritious fruit that attracts many types of birds. Each group of birds has a different effect on the fruit and flowers. Birds such as woodswallows, shrike-thrushes, trillers and lorikeets regularly feed on the nectar from mistletoe in full flower. Emus, butcherbirds, currawongs and ravens will gorge on the sticky fruit. Birds that keep the mistletoe in check include Superb Parrots and Swift Parrots as they will consume the complete flower and prevent any seed from forming. The impact of all of these birds feeding off the mistletoe is likely to reduce the successful seed dispersal, which in turn keeps the mistletoe in check.

Other native animals that feed on the mistletoe include gliders, fruit bats and possums. The gliders and the fruit bats consume the fruit and nectar, while possums consume the leaves and branches. Several species of kangaroo have been reported as eating mistletoe.

The world’s largest mistletoe tree, the Western Australian Christmas Tree, Nuytsia floribunda, is an important food source for the Black-gloved Wallaby and the Western Grey Kangaroo. Other non-native animals that also consume the leaves and branches, where the mistletoe is close to the ground and accesible include horses, sheep, goats, cattle, donkeys, camels and various deer. Mistletoe leaves and branches contain abundant water and are high in concentrations of metals in their tissues. It is for these very reasons that the mistletoe is highly sought after by herbivores.

Many types of mistletoe resemble their host very closely. One species of mistletoe, the Buloke Mistletoe, has needle like foliage and dark coloured leaves making it almost impossible to locate when it is not flowering. This habit of mimicking its host is known as crypsis. Another example of host mimicry within the same species is seen in the Box Mistletoe which has leaves approaching 40cm in length when growing on Manna Gums. Yet the same species of mistletoe growing on various species of mallee has leaves less than 2 cm in length, a twenty fold difference in size.

Why this variation in form can occur in the same species has the experts baffled. One theory is that it could be a strategy to fool possums, gliders and other herbivores into not eating mistletoe leaves. Other experts have dispelled this theory, as laboratory trials have shown that possums and other nocturnal animals rely on their sense of smell and taste and not their vision to select leaves.

Mistletoes also provide an important site for many animals to call home, as dens, roosting and nesting sites, with 245 species of bird having been recorded as nesting in mistletoe clumps. The nestings can be in the form of building their nests inside the rigid framework of the branches, nesting atop of the stable platform created by the clump, or having hanging nests from the drooping branches.

Amyema miquelii – Box Mistletoe
The Box Mistletoe is one of Australia’s most prolific mistletoes and the most common species found west of the Great Dividing Range. It is found in all mainland states, but absent from the Cape York region. While Box Mistletoe has been recorded growing on 125 species of trees, across eight families, it is considered to be a mainly Eucalypt dependent parasite and has been recorded growing on 110 species of eucalypt. It is sometimes found growing on acacias, but is rarely found growing in acacia dominated habitats.

Box Mistletoe grows as a large pendulous mistletoe with shiny leaves and red flowers that are borne in groups of three on individual stalks. What makes this mistletoe easy to distinguish from its host tree is the bronze or yellow hue of the leaves when the plant is exposed to strong sunlight, yet plants that are shaded will have green leaves.

The Box Mistletoe is found in open forests and woodlands dominated by eucalypts. The red flowers are comprised of pinkish petals that open to reveal bright red stamens and style. A feature of the genus Amyema is that when the flower matures and starts to open, is has the characteristic ‘Chinese Lantern’ appearance. The fruit is smooth and mostly yellow – cream in colour, but sometimes retaining its green colour when ripe. Leaf size will vary according to the host tree that it is attached to. It has small 2 cm leaves on Mallee, but a massive 40 cm long leaves when growing on Manna Gums.

Being the most abundant Australian mistletoe, it is often found in very high numbers. There are numerous environmental reasons for this abundance that can also indicate that the natural habitat is out of balance. Factors such as the suppression of canopy fires, a reduction in herbivory by possums, and environmental change; can improve the conditions for mistletoe growth favouring its growth and dispersal.

Many studies of Mistletoe have been carried out in various areas of Eastern Australia, including my home state of South Australia. A survey of box mistletoe and its Eucalyptus hosts in reserves of the Mount Lofty Region revealed that almost a third of all gums were infected with box mistletoe. Even higher figures were recorded in the Clare Valley in the Mid North of South Australia. During the mid 1980’s up to 40% of eucalypts in the Clare region were infected so heavily that many trees were severely stressed and showed signs that they would not survive. A number of locals took matters into their own hands and started to prune mistletoe from the most affected trees. By the year 2000 a Mistletoe Action Group had been formed and was involved in scientifically based programs approved by the Native Vegetation Council. These included the management of Box Mistletoe in the Clare and Gilbert Valleys.

The creation of this group led to a better understanding of the ecology of mistletoe, with long term goals of restoring the ecological balance so that Box Mistletoe can be controlled naturally. Long term remedies to help fix the problem included:

Retain blocks of bushland and protect all existing native habitats with fencing and the management of weeds. An improved ecosystem puts less stress on individual trees. Many of the mature paddock trees are well over 300 years old, and retain hollows and provide alternative food plants for birds that eat mistletoe fruit, as well as for other natural control agents.

Fencing old trees from stock, which protects the trees by minimising nutrient build up from dung. Within the fenced off areas, free from stock, there is a natural regeneration of saplings. This allows for natural regrowth of tree seedlings and the understory. Also within the fenced off areas, there are plantings of local species to create an understory. The understorey creates a buffer habitat and in time helps to join up nearby areas.

The actions of this group has saved countless numbers of mature trees from death and with continued work by the group and landholders, the eucalyptus trees of the area are now looking healthy, and a healthy environment is a healthy community.

Smile like a Crocodile
BlogID: 2874
Views: 41983

Comments & Reviews(1)

Post a Comment
Blog Index

Sponsored Links