Friday, November 6, 2009

Do Mistletoes Kill Trees?

The definitive answer seems to be a simple “no”.
That they do is certainly a fondly-held belief in some quarters. We have seen it raise its head again with the current debate about the trees lining “Cathedral Drive” at Hampton. One writer to a newspaper has expressed the opinion that the trees might as well be killed, because they’re “infested” with mistletoe.
True, the trees are going to die one day. So am I, but I hope nobody uses this as a reason why I “might as well” be killed at once.
The trees are not going to die from mistletoe infestation, however.
Mistletoes don’t poison trees. Nothing passes from the mistletoe to the tree.
Variable mistletoe Amyema congener.  
This plant is growing on a boonaree (Alectryon diversifolium) in Franke Scrub.

This tolerant mistletoe grows on many different host shrubs and trees, including introduced ones like oleanders and crepe myrtles. Some local native hosts are Auranticarpa rhombifolia, Geijera salicifolia, lillypillies and callistemons.

Mistletoes are only partially parasitic. They make their own food, by photosynthesis, just like any other green plant.
What they take from their hosts is water. The amount they take does exceed what any other little branch on the same tree is taking, as Mistletoe leaves lose slightly more water than the tree’s own leaves - but there’s not much in it. A very small tree carrying a big mistletoe will have its growth rate slowed down, but the effect on a large tree is negligible.
Mistletoes are accused of killing branches, and this is true of some mistletoe species. They hijack the branch, preventing water from travelling past the point of the mistletoe's attachment. Branches killed are usually only very small at the time the "hijack" occurs, and the mistletoe replaces any small gap thus created in the canopy, with its own foliage.

Lucas’s Mistletoe
Amyema lucasii

This is a mistletoe which only grows on Flindersias, such as crows ash and leopard ash. This specimen was also in Franke Scrub.

Mistletoes are relatively short-lived plants. Trees can outlive generations of them. Some of the mistletoe skeletons we see on old dead trees would be of plants which died before the tree did, and only became obvious when the tree itself came to the end of its life, shedding its concealing canopy.
Trees under moisture stress have the ability to restrict their water loss to some extent by restricting the flow to their outer branches. This also kills off any mistletoes which were living there, so a drought-stressed tree actually has the ability to rid itself of some of its passengers.

Russet Mistletoe, Amyema miquellii restricts itself to Eucalypts.
A well-grown plant is 2m tall, so needs a large host tree.

Mistletoe “infestation” meanwhile can be a bit of an illusion. Mistletoes are forest edge plants, so we see a lot of them along roadsides, where humans have created long strips of their natural "edge" habitat. Only a few metres into the surrounding bush, the mistletoe population is always much smaller.
A lone tree, left in a paddock is a special case. Left in isolation after clearing, it becomes a “forest edge” all of its own and attracts the mistletoe birds which replant as they feed. It only takes 20 minutes for a mistletoe berry to pass through their little systems and for the seed to be deposited on a handy branch. The birds tend to spend longer in isolated trees before moving on, so these trees do get more than their fair share of mistletoe seeds.
Meanwhile, these lone trees in paddocks, often the source of the “mistletoes kill trees” myth, are already doing it tough.

Detail: Russet Mistletoe, Amyema miquellii. This highly ornamental mistletoe has red flowers, but its showy red spring leaves are an even more showy feature.

There are two reasons why paddock trees seem to die at a higher than normal rate.
One is that livestock collect under them in the heat of the day, and compact the soil with their hooves. Australia has no native hoofed animals. Livestock create a hard soil, unnaturally low in microorganisms - NOT a healthy environment for the poor trees' roots. In addition, stock crowded under trees add high levels of fertilisers to the soil. This is particularly hard on native trees which are adapted to Australia's relatively infertile soils. Even non-natives, quite unblessed by mistletoes, can be killed by excessive fertilising of the sort that paddock trees are subjected to.
A second reason is that some farmers, when they cleared, left the large trees for stock shelter but have never made any provision for their replacement as the trees die of old age. This happening at an increasing rate now, more than a century after the original clearing was done. No tree will live forever!
While it is possible that sick and dying trees are less able to fight off mistletoes so carry a heavier than natural burden, it’s a bit tough to blame the mistletoes for the death of these mistreated, elderly, paddock trees.
So we should stop blaming the mistletoes, and relax and enjoy them instead. They are beautiful plants in their own right, and deserve to be planted in mature gardens for their ornamental qualities as well as their environmental ones.

Brush Mistletoe, Amylotheca dictyophleba

This one grows on introduced trees as well as a variety of natives. We commonly see it around Toowoomba in winter, where is revealed on the London plane trees as they lose their leaves. It gives these trees no trouble, and provides a good source of fruit for native birds.

At least 41 species of native Australian birds are known to feed on mistletoe fruits. Honeyeaters visit them for their nectar. And a great many small bird species nest in their dense foliage. These are great bird-attracting plants!
There are also some beautiful butterflies which are disappearing from our suburbs because they can only breed on mistletoes. We need more of them!
If you’re worried that a too vigorous mistletoe might damage the tree that is the pride of your garden (though it probably won’t) you can prune it just like any other plant. Reducing its size will give the tree a chance to recover if indeed it was feeling the stress.
(Note: the particular mistletoe species shown here are members of the Loranthaceae family. The majority of Australian  mistletoes belong to this old Gondwanan family, and are characterised by their showy flowers and edible fruits.)


Irey said...

Does anyone ever collect seed of the Brush mistletoe I would love to grow some in my garden.
Cheers Irey

Patricia Gardner said...

I think the seed would have to be very fresh to be viable, so it is a matter of finding a plant for yourself and keeping an eye on it till the fruit ripens. These are quite common plants around Toowoomba. Find some by checking out the plane trees this winter when they're leafless, and then keep an eye on them until they flower (November) and fruit (February). Best of luck!